Provided by: parallel_20161222-1_all bug

NAME

       parallel - build and execute shell command lines from standard input in parallel

SYNOPSIS

       parallel [options] [command [arguments]] < list_of_arguments

       parallel [options] [command [arguments]] ( ::: arguments | :::+ arguments | ::::
       argfile(s) | ::::+ argfile(s) ) ...

       parallel --semaphore [options] command

       #!/usr/bin/parallel --shebang [options] [command [arguments]]

DESCRIPTION

       GNU parallel is a shell tool for executing jobs in parallel using one or more computers. A
       job can be a single command or a small script that has to be run for each of the lines in
       the input. The typical input is a list of files, a list of hosts, a list of users, a list
       of URLs, or a list of tables. A job can also be a command that reads from a pipe. GNU
       parallel can then split the input into blocks and pipe a block into each command in
       parallel.

       If you use xargs and tee today you will find GNU parallel very easy to use as GNU parallel
       is written to have the same options as xargs. If you write loops in shell, you will find
       GNU parallel may be able to replace most of the loops and make them run faster by running
       several jobs in parallel.

       GNU parallel makes sure output from the commands is the same output as you would get had
       you run the commands sequentially. This makes it possible to use output from GNU parallel
       as input for other programs.

       For each line of input GNU parallel will execute command with the line as arguments. If no
       command is given, the line of input is executed. Several lines will be run in parallel.
       GNU parallel can often be used as a substitute for xargs or cat | bash.

   Reader's guide
       Start by watching the intro videos for a quick introduction:
       http://www.youtube.com/playlist?list=PL284C9FF2488BC6D1

       Then look at the EXAMPLEs after the list of OPTIONS. That will give you an idea of what
       GNU parallel is capable of.

       Then spend an hour walking through the tutorial (man parallel_tutorial). Your command line
       will love you for it.

       Finally you may want to look at the rest of this manual if you have special needs not
       already covered.

       If you want to know the design decisions behind GNU parallel, try: man parallel_design.
       This is also a good intro if you intend to change GNU parallel.

OPTIONS

       command  Command to execute.  If command or the following arguments contain replacement
                strings (such as {}) every instance will be substituted with the input.

                If command is given, GNU parallel solve the same tasks as xargs. If command is
                not given GNU parallel will behave similar to cat | sh.

                The command must be an executable, a script, a composed command, or a function.

                Bash functions: export -f the function first or use env_parallel.

                Bash aliases: Use env_parallel.

                Ksh functions and aliases: Use env_parallel.

                Zsh functions and aliases: Use env_parallel.

                Fish functions and aliases: Use env_parallel.

                Ksh functions and aliases: Use env_parallel.

                Pdksh functions and aliases: Use env_parallel.

                The command cannot contain the character \257 (macron: X).

       {}       Input line. This replacement string will be replaced by a full line read from the
                input source. The input source is normally stdin (standard input), but can also
                be given with -a, :::, or ::::.

                The replacement string {} can be changed with -I.

                If the command line contains no replacement strings then {} will be appended to
                the command line.

       {.}      Input line without extension. This replacement string will be replaced by the
                input with the extension removed. If the input line contains . after the last /
                the last . till the end of the string will be removed and {.} will be replaced
                with the remaining. E.g. foo.jpg becomes foo, subdir/foo.jpg becomes subdir/foo,
                sub.dir/foo.jpg becomes sub.dir/foo, sub.dir/bar remains sub.dir/bar. If the
                input line does not contain . it will remain unchanged.

                The replacement string {.} can be changed with --er.

                To understand replacement strings see {}.

       {/}      Basename of input line. This replacement string will be replaced by the input
                with the directory part removed.

                The replacement string {/} can be changed with --basenamereplace.

                To understand replacement strings see {}.

       {//}     Dirname of input line. This replacement string will be replaced by the dir of the
                input line. See dirname(1).

                The replacement string {//} can be changed with --dirnamereplace.

                To understand replacement strings see {}.

       {/.}     Basename of input line without extension. This replacement string will be
                replaced by the input with the directory and extension part removed. It is a
                combination of {/} and {.}.

                The replacement string {/.} can be changed with --basenameextensionreplace.

                To understand replacement strings see {}.

       {#}      Sequence number of the job to run. This replacement string will be replaced by
                the sequence number of the job being run. It contains the same number as
                $PARALLEL_SEQ.

                The replacement string {#} can be changed with --seqreplace.

                To understand replacement strings see {}.

       {%}      Job slot number. This replacement string will be replaced by the job's slot
                number between 1 and number of jobs to run in parallel. There will never be 2
                jobs running at the same time with the same job slot number.

                The replacement string {%} can be changed with --slotreplace.

                To understand replacement strings see {}.

       {n}      Argument from input source n or the n'th argument. This positional replacement
                string will be replaced by the input from input source n (when used with -a or
                ::::) or with the n'th argument (when used with -N). If n is negative it refers
                to the n'th last argument.

                To understand replacement strings see {}.

       {n.}     Argument from input source n or the n'th argument without extension. It is a
                combination of {n} and {.}.

                This positional replacement string will be replaced by the input from input
                source n (when used with -a or ::::) or with the n'th argument (when used with
                -N). The input will have the extension removed.

                To understand positional replacement strings see {n}.

       {n/}     Basename of argument from input source n or the n'th argument.  It is a
                combination of {n} and {/}.

                This positional replacement string will be replaced by the input from input
                source n (when used with -a or ::::) or with the n'th argument (when used with
                -N). The input will have the directory (if any) removed.

                To understand positional replacement strings see {n}.

       {n//}    Dirname of argument from input source n or the n'th argument.  It is a
                combination of {n} and {//}.

                This positional replacement string will be replaced by the dir of the input from
                input source n (when used with -a or ::::) or with the n'th argument (when used
                with -N). See dirname(1).

                To understand positional replacement strings see {n}.

       {n/.}    Basename of argument from input source n or the n'th argument without extension.
                It is a combination of {n}, {/}, and {.}.

                This positional replacement string will be replaced by the input from input
                source n (when used with -a or ::::) or with the n'th argument (when used with
                -N). The input will have the directory (if any) and extension removed.

                To understand positional replacement strings see {n}.

       {=perl expression=}
                Replace with calculated perl expression. $_ will contain the same as {}. After
                evaluating perl expression $_ will be used as the value. It is recommended to
                only change $_ but you have full access to all of GNU parallel's internal
                functions and data structures. A few convenience functions and data structures
                have been made:

                 Q(string)     shell quote a string

                 pQ(string)    perl quote a string

                 total_jobs()  number of jobs in total

                 $job->slot()  slot number of job

                 $job->seq()   sequence number of job

                 @arg          the arguments

                Example:

                  seq 10 | parallel echo {} + 1 is {= '$_++' =}
                  parallel csh -c {= '$_="mkdir ".Q($_)' =} ::: '12" dir'
                  seq 50 | parallel echo job {#} of {= '$_=total_jobs()' =}

                See also: --rpl --parens

       {=n perl expression=}
                Positional equivalent to {=perl expression=}. To understand positional
                replacement strings see {n}.

                See also: {=perl expression=} {n}.

       ::: arguments
                Use arguments from the command line as input source instead of stdin (standard
                input). Unlike other options for GNU parallel ::: is placed after the command and
                before the arguments.

                The following are equivalent:

                  (echo file1; echo file2) | parallel gzip
                  parallel gzip ::: file1 file2
                  parallel gzip {} ::: file1 file2
                  parallel --arg-sep ,, gzip {} ,, file1 file2
                  parallel --arg-sep ,, gzip ,, file1 file2
                  parallel ::: "gzip file1" "gzip file2"

                To avoid treating ::: as special use --arg-sep to set the argument separator to
                something else. See also --arg-sep.

                If multiple ::: are given, each group will be treated as an input source, and all
                combinations of input sources will be generated. E.g. ::: 1 2 ::: a b c will
                result in the combinations (1,a) (1,b) (1,c) (2,a) (2,b) (2,c). This is useful
                for replacing nested for-loops.

                ::: and :::: can be mixed. So these are equivalent:

                  parallel echo {1} {2} {3} ::: 6 7 ::: 4 5 ::: 1 2 3
                  parallel echo {1} {2} {3} :::: <(seq 6 7) <(seq 4 5) \
                    :::: <(seq 1 3)
                  parallel -a <(seq 6 7) echo {1} {2} {3} :::: <(seq 4 5) \
                    :::: <(seq 1 3)
                  parallel -a <(seq 6 7) -a <(seq 4 5) echo {1} {2} {3} \
                    ::: 1 2 3
                  seq 6 7 | parallel -a - -a <(seq 4 5) echo {1} {2} {3} \
                    ::: 1 2 3
                  seq 4 5 | parallel echo {1} {2} {3} :::: <(seq 6 7) - \
                    ::: 1 2 3

       :::+ arguments
                Like ::: but linked like --link to the previous input source.

                Contrary to --link, values do not wrap: The shortest input source determines the
                length.

                Example:

                  parallel echo ::: a b c :::+ 1 2 3 ::: X Y :::+ 11 22

       :::: argfiles
                Another way to write -a argfile1 -a argfile2 ...

                ::: and :::: can be mixed.

                See -a, ::: and --link.

       ::::+ argfiles
                Like :::: but linked like --link to the previous input source.

                Contrary to --link, values do not wrap: The shortest input source determines the
                length.

       --null
       -0       Use NUL as delimiter.  Normally input lines will end in \n (newline). If they end
                in \0 (NUL), then use this option. It is useful for processing arguments that may
                contain \n (newline).

       --arg-file input-file
       -a input-file
                Use input-file as input source. If you use this option, stdin (standard input) is
                given to the first process run.  Otherwise, stdin (standard input) is redirected
                from /dev/null.

                If multiple -a are given, each input-file will be treated as an input source, and
                all combinations of input sources will be generated. E.g. The file foo contains 1
                2, the file bar contains a b c.  -a foo -a bar will result in the combinations
                (1,a) (1,b) (1,c) (2,a) (2,b) (2,c). This is useful for replacing nested for-
                loops.

                See also --link and {n}.

       --arg-file-sep sep-str
                Use sep-str instead of :::: as separator string between command and argument
                files. Useful if :::: is used for something else by the command.

                See also: ::::.

       --arg-sep sep-str
                Use sep-str instead of ::: as separator string. Useful if ::: is used for
                something else by the command.

                Also useful if you command uses ::: but you still want to read arguments from
                stdin (standard input): Simply change --arg-sep to a string that is not in the
                command line.

                See also: :::.

       --bar    Show progress as a progress bar. In the bar is shown: % of jobs completed,
                estimated seconds left, and number of jobs started.

                It is compatible with zenity:

                  seq 1000 | parallel -j30 --bar '(echo {};sleep 0.1)' \
                    2> >(zenity --progress --auto-kill) | wc

       --basefile file
       --bf file
                file will be transferred to each sshlogin before a jobs is started. It will be
                removed if --cleanup is active. The file may be a script to run or some common
                base data needed for the jobs.  Multiple --bf can be specified to transfer more
                basefiles. The file will be transferred the same way as --transferfile.

       --basenamereplace replace-str
       --bnr replace-str
                Use the replacement string replace-str instead of {/} for basename of input line.

       --basenameextensionreplace replace-str
       --bner replace-str
                Use the replacement string replace-str instead of {/.} for basename of input line
                without extension.

       --bg     Run command in background thus GNU parallel will not wait for completion of the
                command before exiting. This is the default if --semaphore is set.

                See also: --fg, man sem.

                Implies --semaphore.

       --bibtex
       --citation
                Print the BibTeX entry for GNU parallel and silence citation notice.

                If it is impossible for you to run --bibtex you can use --will-cite.

                If you use --will-cite in scripts to be run by others you are making it harder
                for others to see the citation notice.  The development of GNU parallel is
                indirectly financed through citations, so if your users do not know they should
                cite then you are making it harder to finance development. However, if you pay
                10000 EUR, you should feel free to use --will-cite in scripts.

       --block size
       --block-size size
                Size of block in bytes to read at a time. The size can be postfixed with K, M, G,
                T, P, k, m, g, t, or p which would multiply the size with 1024, 1048576,
                1073741824, 1099511627776, 1125899906842624, 1000, 1000000, 1000000000,
                1000000000000, or 1000000000000000, respectively.

                GNU parallel tries to meet the block size but can be off by the length of one
                record. For performance reasons size should be bigger than a two records. GNU
                parallel will warn you and automatically increase the size if you choose a size
                that is too small.

                If you use -N, --block-size should be bigger than N+1 records.

                size defaults to 1M.

                When using --pipepart a negative block size is not interpreted as a blocksize but
                as the number of blocks each jobslot should have. So this will run 10*5 = 50 jobs
                in total:

                  parallel --pipepart -a myfile --block -10 -j5 wc

                This is an efficient alternative to --round-robin because data is never read by
                GNU parallel, but you can still have very few jobslots process a large amount of
                data.

                See --pipe and --pipepart for use of this.

       --cat    Create a temporary file with content. Normally --pipe/--pipepart will give data
                to the program on stdin (standard input). With --cat GNU parallel will create a
                temporary file with the name in {}, so you can do: parallel --pipe --cat wc {}.

                Implies --pipe unless --pipepart is used.

                See also --fifo.

       --cleanup
                Remove transferred files. --cleanup will remove the transferred files on the
                remote computer after processing is done.

                  find log -name '*gz' | parallel \
                    --sshlogin server.example.com --transferfile {} \
                    --return {.}.bz2 --cleanup "zcat {} | bzip -9 >{.}.bz2"

                With --transferfile {} the file transferred to the remote computer will be
                removed on the remote computer.  Directories created will not be removed - even
                if they are empty.

                With --return the file transferred from the remote computer will be removed on
                the remote computer.  Directories created will not be removed - even if they are
                empty.

                --cleanup is ignored when not used with --transferfile or --return.

       --colsep regexp
       -C regexp
                Column separator. The input will be treated as a table with regexp separating the
                columns. The n'th column can be access using {n} or {n.}. E.g. {3} is the 3rd
                column.

                --colsep implies --trim rl.

                regexp is a Perl Regular Expression: http://perldoc.perl.org/perlre.html

       --compress
                Compress temporary files. If the output is big and very compressible this will
                take up less disk space in $TMPDIR and possibly be faster due to less disk I/O.

                GNU parallel will try pzstd, lbzip2, pbzip2, zstd, pigz, lz4, lzop, plzip, lzip,
                lrz, gzip, pxz, lzma, bzip2, xz, clzip, in that order, and use the first
                available.

       --compress-program prg
       --decompress-program prg
                Use prg for (de)compressing temporary files. It is assumed that prg -dc will
                decompress stdin (standard input) to stdout (standard output) unless
                --decompress-program is given.

       --delimiter delim
       -d delim Input items are terminated by delim.  Quotes and backslash are not special; every
                character in the input is taken literally.  Disables the end-of-file string,
                which is treated like any other argument. The specified delimiter may be
                characters, C-style character escapes such as \n, or octal or hexadecimal escape
                codes.  Octal and hexadecimal escape codes are understood as for the printf
                command.  Multibyte characters are not supported.

       --dirnamereplace replace-str
       --dnr replace-str
                Use the replacement string replace-str instead of {//} for dirname of input line.

       -E eof-str
                Set the end of file string to eof-str.  If the end of file string occurs as a
                line of input, the rest of the input is not read.  If neither -E nor -e is used,
                no end of file string is used.

       --delay secs
                Delay starting next job secs seconds. GNU parallel will pause secs seconds after
                starting each job. secs can be less than 1 second.

       --dry-run
                Print the job to run on stdout (standard output), but do not run the job. Use -v
                -v to include the wrapping that GNU Parallel generates (for remote jobs, --tmux,
                --nice, --pipe, --pipepart, --fifo and --cat). Do not count on this literaly,
                though, as the job may be scheduled on another computer or the local computer if
                : is in the list.

       --eof[=eof-str]
       -e[eof-str]
                This option is a synonym for the -E option.  Use -E instead, because it is POSIX
                compliant for xargs while this option is not.  If eof-str is omitted, there is no
                end of file string.  If neither -E nor -e is used, no end of file string is used.

       --env var
                Copy environment variable var. This will copy var to the environment that the
                command is run in. This is especially useful for remote execution.

                In Bash var can also be a Bash function - just remember to export -f the
                function, see command.

                The variable '_' is special. It will copy all exported environment variables
                except for the ones mentioned in ~/.parallel/ignored_vars.

                To copy the full environment (both exported and not exported variables, arrays,
                and functions) use env_parallel.

                See also: --record-env.

       --eta    Show the estimated number of seconds before finishing. This forces GNU parallel
                to read all jobs before starting to find the number of jobs. GNU parallel
                normally only reads the next job to run.

                The estimate is based on the runtime of finished jobs, so the first estimate will
                only be shown when the first job has finished.

                Implies --progress.

                See also: --bar, --progress.

       --fg     Run command in foreground thus GNU parallel will wait for completion of the
                command before exiting.

                Implies --semaphore.

                See also --bg, man sem.

       --fifo   Create a temporary fifo with content. Normally --pipe and --pipepart will give
                data to the program on stdin (standard input). With --fifo GNU parallel will
                create a temporary fifo with the name in {}, so you can do: parallel --pipe
                --fifo wc {}.

                Beware: If data is not read from the fifo, the job will block forever.

                Implies --pipe unless --pipepart is used.

                See also --cat.

       --filter-hosts
                Remove down hosts. For each remote host: check that login through ssh works. If
                not: do not use this host.

                For performance reasons, this check is performed only at the start and every time
                --sshloginfile is changed. If an host goes down after the first check, it will go
                undetected until --sshloginfile is changed; --retries can be used to mitigate
                this.

                Currently you can not put --filter-hosts in a profile, $PARALLEL,
                /etc/parallel/config or similar. This is because GNU parallel uses GNU parallel
                to compute this, so you will get an infinite loop. This will likely be fixed in a
                later release.

       --gnu    Behave like GNU parallel. This option historically took precedence over --tollef.
                The --tollef option is now retired, and therefore may not be used. --gnu is kept
                for compatibility.

       --group  Group output. Output from each jobs is grouped together and is only printed when
                the command is finished. stderr (standard error) first followed by stdout
                (standard output). This takes some CPU time. In rare situations GNU parallel
                takes up lots of CPU time and if it is acceptable that the outputs from different
                commands are mixed together, then disabling grouping with -u can speedup GNU
                parallel by a factor of 10.

                --group is the default. Can be reversed with -u.

                See also: --line-buffer --ungroup

       --help
       -h       Print a summary of the options to GNU parallel and exit.

       --halt-on-error val
       --halt val
                When should GNU parallel terminate? In some situations it makes no sense to run
                all jobs. GNU parallel should simply give up as soon as a condition is met.

                val defaults to never, which runs all jobs no matter what.

                val can also take on the form of when,why.

                when can be 'now' which means kill all running jobs and halt immediately, or it
                can be 'soon' which means wait for all running jobs to complete, but start no new
                jobs.

                why can be 'fail=X', 'fail=Y%', 'success=X', or 'success=Y%' where X is the
                number of jobs that has to fail or succeed before halting, and Y is the
                percentage of jobs that has to fail or succeed before halting.

                Example:

                 --halt now,fail=1     exit when the first job fails. Kill running jobs.

                 --halt soon,fail=3    exit when 3 jobs fail, but wait for running jobs to
                                       complete.

                 --halt soon,fail=3%   exit when 3% of the jobs have failed, but wait for running
                                       jobs to complete.

                 --halt now,success=1  exit when a job succeeds. Kill running jobs.

                 --halt soon,success=3 exit when 3 jobs succeeds, but wait for running jobs to
                                       complete.

                 --halt now,success=3% exit when 3% of the jobs have succeeded. Kill running
                                       jobs.

                For backwards compability these also work:

                0      never

                1      soon,fail=1

                2      now,fail=1

                -1     soon,success=1

                -2     now,success=1

                1-99%  soon,fail=1-99%

       --header regexp
                Use regexp as header. For normal usage the matched header (typically the first
                line: --header '.*\n') will be split using --colsep (which will default to '\t')
                and column names can be used as replacement variables: {column name}, {column
                name/}, {column name//}, {column name/.}, {column name.}, {=column name perl
                expression =}, ..

                For --pipe the matched header will be prepended to each output.

                --header : is an alias for --header '.*\n'.

                If regexp is a number, it is a fixed number of lines.

       --hostgroups
       --hgrp   Enable hostgroups on arguments. If an argument contains '@' the string after '@'
                will be removed and treated as a list of hostgroups on which this job is allowed
                to run. If there is no --sshlogin with a corresponding group, the job will run on
                any hostgroup.

                Example:

                  parallel --hostgroups \
                    --sshlogin @grp1/myserver1 -S @grp1+grp2/myserver2 \
                    --sshlogin @grp3/myserver3 \
                    echo ::: my_grp1_arg@grp1 arg_for_grp2@grp2 third_arg@grp1+grp3

                my_grp1_arg may be run on either myserver1 or myserver2, third_arg may be run on
                either myserver1 or myserver3, but arg_for_grp2 will only be run on myserver2.

                See also: --sshlogin.

       -I replace-str
                Use the replacement string replace-str instead of {}.

       --replace[=replace-str]
       -i[replace-str]
                This option is a synonym for -Ireplace-str if replace-str is specified, and for
                -I {} otherwise.  This option is deprecated; use -I instead.

       --joblog logfile
                Logfile for executed jobs. Save a list of the executed jobs to logfile in the
                following TAB separated format: sequence number, sshlogin, start time as seconds
                since epoch, run time in seconds, bytes in files transferred, bytes in files
                returned, exit status, signal, and command run.

                For --pipe bytes transferred and bytes returned are number of input and output of
                bytes.

                To convert the times into ISO-8601 strict do:

                  perl -a -F"\t" -ne \
                    'chomp($F[2]=`date -d \@$F[2] +%FT%T`); print join("\t",@F)'

                See also --resume --resume-failed.

       --jobs N
       -j N
       --max-procs N
       -P N     Number of jobslots on each machine. Run up to N jobs in parallel.  0 means as
                many as possible. Default is 100% which will run one job per CPU core on each
                machine.

                If --semaphore is set, the default is 1 thus making a mutex.

       --jobs +N
       -j +N
       --max-procs +N
       -P +N    Add N to the number of CPU cores.  Run this many jobs in parallel.  See also
                --use-cpus-instead-of-cores.

       --jobs -N
       -j -N
       --max-procs -N
       -P -N    Subtract N from the number of CPU cores.  Run this many jobs in parallel.  If the
                evaluated number is less than 1 then 1 will be used.  See also
                --use-cpus-instead-of-cores.

       --jobs N%
       -j N%
       --max-procs N%
       -P N%    Multiply N% with the number of CPU cores.  Run this many jobs in parallel. See
                also --use-cpus-instead-of-cores.

       --jobs procfile
       -j procfile
       --max-procs procfile
       -P procfile
                Read parameter from file. Use the content of procfile as parameter for -j. E.g.
                procfile could contain the string 100% or +2 or 10. If procfile is changed when a
                job completes, procfile is read again and the new number of jobs is computed. If
                the number is lower than before, running jobs will be allowed to finish but new
                jobs will not be started until the wanted number of jobs has been reached.  This
                makes it possible to change the number of simultaneous running jobs while GNU
                parallel is running.

       --keep-order
       -k       Keep sequence of output same as the order of input. Normally the output of a job
                will be printed as soon as the job completes. Try this to see the difference:

                  parallel -j4 sleep {}\; echo {} ::: 2 1 4 3
                  parallel -j4 -k sleep {}\; echo {} ::: 2 1 4 3

                If used with --onall or --nonall the output will grouped by sshlogin in sorted
                order.

       -L max-lines
                When used with --pipe: Read records of max-lines.

                When used otherwise: Use at most max-lines nonblank input lines per command line.
                Trailing blanks cause an input line to be logically continued on the next input
                line.

                -L 0 means read one line, but insert 0 arguments on the command line.

                Implies -X unless -m, --xargs, or --pipe is set.

       --max-lines[=max-lines]
       -l[max-lines]
                When used with --pipe: Read records of max-lines.

                When used otherwise: Synonym for the -L option.  Unlike -L, the max-lines
                argument is optional.  If max-lines is not specified, it defaults to one.  The -l
                option is deprecated since the POSIX standard specifies -L instead.

                -l 0 is an alias for -l 1.

                Implies -X unless -m, --xargs, or --pipe is set.

       --line-buffer
       --lb     Buffer output on line basis. --group will keep the output together for a whole
                job. --ungroup allows output to mixup with half a line coming from one job and
                half a line coming from another job. --line-buffer fits between these two: GNU
                parallel will print a full line, but will allow for mixing lines of different
                jobs.

                --line-buffer takes more CPU power than than both --group and --ungroup, but can
                be faster than --group if the CPU is not the limiting factor.

                See also: --group --ungroup

       --xapply
       --link   Link input sources. Read multiple input sources like xapply. If multiple input
                sources are given, one argument will be read from each of the input sources. The
                arguments can be accessed in the command as {1} .. {n}, so {1} will be a line
                from the first input source, and {6} will refer to the line with the same line
                number from the 6th input source.

                Compare these two:

                  parallel echo {1} {2} ::: 1 2 3 ::: a b c
                  parallel --link echo {1} {2} ::: 1 2 3 ::: a b c

                Arguments will be recycled if one input source has more arguments than the
                others:

                  parallel --link echo {1} {2} {3} \
                    ::: 1 2 ::: I II III ::: a b c d e f g

                See also --header, :::+, ::::+.

       --load max-load
                Do not start new jobs on a given computer unless the number of running processes
                on the computer is less than max-load. max-load uses the same syntax as --jobs,
                so 100% for one per CPU is a valid setting. Only difference is 0 which is
                interpreted as 0.01.

       --controlmaster
       -M       Use ssh's ControlMaster to make ssh connections faster. Useful if jobs run remote
                and are very fast to run. This is disabled for sshlogins that specify their own
                ssh command.

       --xargs  Multiple arguments. Insert as many arguments as the command line length permits.

                If {} is not used the arguments will be appended to the line.  If {} is used
                multiple times each {} will be replaced with all the arguments.

                Support for --xargs with --sshlogin is limited and may fail.

                See also -X for context replace. If in doubt use -X as that will most likely do
                what is needed.

       -m       Multiple arguments. Insert as many arguments as the command line length permits.
                If multiple jobs are being run in parallel: distribute the arguments evenly among
                the jobs. Use -j1 or --xargs to avoid this.

                If {} is not used the arguments will be appended to the line.  If {} is used
                multiple times each {} will be replaced with all the arguments.

                Support for -m with --sshlogin is limited and may fail.

                See also -X for context replace. If in doubt use -X as that will most likely do
                what is needed.

       --memfree size
                Minimum memory free when starting another job. The size can be postfixed with K,
                M, G, T, P, k, m, g, t, or p which would multiply the size with 1024, 1048576,
                1073741824, 1099511627776, 1125899906842624, 1000, 1000000, 1000000000,
                1000000000000, or 1000000000000000, respectively.

                If the jobs take up very different amount of RAM, GNU parallel will only start as
                many as there is memory for. If less than size bytes are free, no more jobs will
                be started. If less than 50% size bytes are free, the youngest job will be
                killed, and put back on the queue to be run later.

       --minversion version
                Print the version GNU parallel and exit.  If the current version of GNU parallel
                is less than version the exit code is 255. Otherwise it is 0.

                This is useful for scripts that depend on features only available from a certain
                version of GNU parallel.

       --nonall --onall with no arguments. Run the command on all computers given with --sshlogin
                but take no arguments. GNU parallel will log into --jobs number of computers in
                parallel and run the job on the computer. -j adjusts how many computers to log
                into in parallel.

                This is useful for running the same command (e.g. uptime) on a list of servers.

       --onall  Run all the jobs on all computers given with --sshlogin. GNU parallel will log
                into --jobs number of computers in parallel and run one job at a time on the
                computer. The order of the jobs will not be changed, but some computers may
                finish before others.

                When using --group the output will be grouped by each server, so all the output
                from one server will be grouped together.

                --joblog will contain an entry for each job on each server, so there will be
                several job sequence 1.

       --output-as-files
       --outputasfiles
       --files  Instead of printing the output to stdout (standard output) the output of each job
                is saved in a file and the filename is then printed.

                See also: --results

       --pipe
       --spreadstdin
                Spread input to jobs on stdin (standard input). Read a block of data from stdin
                (standard input) and give one block of data as input to one job.

                The block size is determined by --block. The strings --recstart and --recend tell
                GNU parallel how a record starts and/or ends. The block read will have the final
                partial record removed before the block is passed on to the job. The partial
                record will be prepended to next block.

                If --recstart is given this will be used to split at record start.

                If --recend is given this will be used to split at record end.

                If both --recstart and --recend are given both will have to match to find a split
                position.

                If neither --recstart nor --recend are given --recend defaults to '\n'. To have
                no record separator use --recend "".

                --files is often used with --pipe.

                --pipe maxes out at around 1 GB/s input, and 100 MB/s output. If performance is
                important use --pipepart.

                See also: --recstart, --recend, --fifo, --cat, --pipepart, --files.

       --pipepart
                Pipe parts of a physical file. --pipepart works similar to --pipe, but is much
                faster.

                --pipepart has a few limitations:

                *  The file must be a normal file or a block device (technically it must be
                   seekable) and must be given using -a or ::::. The file cannot be a pipe or a
                   fifo as they are not seekable.

                   If using a block device with lot of NUL bytes, remember to set --recend ''.

                *  Record counting (-N) and line counting (-L/-l) do not work.

       --plain  Ignore any --profile, $PARALLEL, and ~/.parallel/config to get full control on
                the command line (used by GNU parallel internally when called with --sshlogin).

       --plus   Activate additional replacement strings: {+/} {+.} {+..} {+...} {..} {...} {/..}
                {/...} {##}. The idea being that '{+foo}' matches the opposite of '{foo}' and {}
                = {+/}/{/} = {.}.{+.} = {+/}/{/.}.{+.} = {..}.{+..} = {+/}/{/..}.{+..} =
                {...}.{+...} = {+/}/{/...}.{+...}

                {##} is the number of jobs to be run. It is incompatible with -X/-m/--xargs.

       --progress
                Show progress of computations. List the computers involved in the task with
                number of CPU cores detected and the max number of jobs to run. After that show
                progress for each computer: number of running jobs, number of completed jobs, and
                percentage of all jobs done by this computer. The percentage will only be
                available after all jobs have been scheduled as GNU parallel only read the next
                job when ready to schedule it - this is to avoid wasting time and memory by
                reading everything at startup.

                By sending GNU parallel SIGUSR2 you can toggle turning on/off --progress on a
                running GNU parallel process.

                See also --eta.

       --max-args=max-args
       -n max-args
                Use at most max-args arguments per command line.  Fewer than max-args arguments
                will be used if the size (see the -s option) is exceeded, unless the -x option is
                given, in which case GNU parallel will exit.

                -n 0 means read one argument, but insert 0 arguments on the command line.

                Implies -X unless -m is set.

       --max-replace-args=max-args
       -N max-args
                Use at most max-args arguments per command line. Like -n but also makes
                replacement strings {1} .. {max-args} that represents argument 1 .. max-args. If
                too few args the {n} will be empty.

                -N 0 means read one argument, but insert 0 arguments on the command line.

                This will set the owner of the homedir to the user:

                  tr ':' '\n' < /etc/passwd | parallel -N7 chown {1} {6}

                Implies -X unless -m or --pipe is set.

                When used with --pipe -N is the number of records to read. This is somewhat
                slower than --block.

       --max-line-length-allowed
                Print the maximal number of characters allowed on the command line and exit (used
                by GNU parallel itself to determine the line length on remote computers).

       --number-of-cpus
                Print the number of physical CPUs and exit (used by GNU parallel itself to
                determine the number of physical CPUs on remote computers).

       --number-of-cores
                Print the number of CPU cores and exit (used by GNU parallel itself to determine
                the number of CPU cores on remote computers).

       --no-keep-order
                Overrides an earlier --keep-order (e.g. if set in ~/.parallel/config).

       --nice niceness
                Run the command at this niceness. For simple commands you can just add nice in
                front of the command. But if the command consists of more sub commands (Like:
                ls|wc) then prepending nice will not always work. --nice will make sure all sub
                commands are niced - even on remote servers.

       --interactive
       -p       Prompt the user about whether to run each command line and read a line from the
                terminal.  Only run the command line if the response starts with 'y' or 'Y'.
                Implies -t.

       --parens parensstring
                Define start and end parenthesis for {= perl expression =}. The left and the
                right parenthesis can be multiple characters and are assumed to be the same
                length. The default is {==} giving {= as the start parenthesis and =} as the end
                parenthesis.

                Another useful setting is ,,,, which would make both parenthesis ,,:

                  parallel --parens ,,,, echo foo is ,,s/I/O/g,, ::: FII

                See also: --rpl {= perl expression =}

       --profile profilename
       -J profilename
                Use profile profilename for options. This is useful if you want to have multiple
                profiles. You could have one profile for running jobs in parallel on the local
                computer and a different profile for running jobs on remote computers. See the
                section PROFILE FILES for examples.

                profilename corresponds to the file ~/.parallel/profilename.

                You can give multiple profiles by repeating --profile. If parts of the profiles
                conflict, the later ones will be used.

                Default: config

       --quote
       -q       Quote command.  This will quote the command line so special characters are not
                interpreted by the shell. See the section QUOTING. Most people will never need
                this.  Quoting is disabled by default.

       --no-run-if-empty
       -r       If the stdin (standard input) only contains whitespace, do not run the command.

                If used with --pipe this is slow.

       --noswap Do not start new jobs on a given computer if there is both swap-in and swap-out
                activity.

                The swap activity is only sampled every 10 seconds as the sampling takes 1 second
                to do.

                Swap activity is computed as (swap-in)*(swap-out) which in practice is a good
                value: swapping out is not a problem, swapping in is not a problem, but both
                swapping in and out usually indicates a problem.

                --memfree may give better results, so try using that first.

       --record-env
                Record current environment variables in ~/.parallel/ignored_vars. This is useful
                before using --env _.

                See also --env.

       --recstart startstring
       --recend endstring
                If --recstart is given startstring will be used to split at record start.

                If --recend is given endstring will be used to split at record end.

                If both --recstart and --recend are given the combined string
                endstringstartstring will have to match to find a split position. This is useful
                if either startstring or endstring match in the middle of a record.

                If neither --recstart nor --recend are given then --recend defaults to '\n'. To
                have no record separator use --recend "".

                --recstart and --recend are used with --pipe.

                Use --regexp to interpret --recstart and --recend as regular expressions. This is
                slow, however.

       --regexp Use --regexp to interpret --recstart and --recend as regular expressions. This is
                slow, however.

       --remove-rec-sep
       --removerecsep
       --rrs    Remove the text matched by --recstart and --recend before piping it to the
                command.

                Only used with --pipe.

       --results name (alpha testing)
       --res name (alpha testing)
                Save the output into files.

                CSV file output

                If name ends in .csv/.tsv the output will be a CSV-file named name.

                .csv gives a comma separated value file. .tsv gives a TAB separated value file.

                -.csv/-.tsv are special: It will give the file on stdout (standard output).

                Dir structure output

                If name does not end in .csv/.tsv will be treated as a dir.

                The files will be stored in a directory tree rooted at name.  Within this
                directory tree, each command will result in two files: name/<ARGS>/stdout and
                name/<ARGS>/stderr, where <ARGS> is a sequence of directories representing the
                header of the input source (if using --header :) or the number of the input
                source and corresponding values.

                name can contain replacement strings.

                E.g:

                  parallel --header : --results foo echo {a} {b} \
                    ::: a I II ::: b III IIII

                will generate the files:

                  foo/a/I/b/III/stderr
                  foo/a/I/b/III/stdout
                  foo/a/I/b/IIII/stderr
                  foo/a/I/b/IIII/stdout
                  foo/a/II/b/III/stderr
                  foo/a/II/b/III/stdout
                  foo/a/II/b/IIII/stderr
                  foo/a/II/b/IIII/stdout

                and

                  parallel --results foo echo {1} {2} ::: I II ::: III IIII

                will generate the files:

                  foo/1/I/2/III/stderr
                  foo/1/I/2/III/stdout
                  foo/1/I/2/IIII/stderr
                  foo/1/I/2/IIII/stdout
                  foo/1/II/2/III/stderr
                  foo/1/II/2/III/stdout
                  foo/1/II/2/IIII/stderr
                  foo/1/II/2/IIII/stdout

                and

                  parallel --results foo-{1} echo {1} {2} ::: I II ::: III IIII

                will generate the files:

                  foo-I/1/I/2/IIII/seq
                  foo-I/1/I/2/IIII/stderr
                  foo-I/1/I/2/IIII/stdout
                  foo-I/1/I/2/III/seq
                  foo-I/1/I/2/III/stderr
                  foo-I/1/I/2/III/stdout
                  foo-II/1/II/2/IIII/seq
                  foo-II/1/II/2/IIII/stderr
                  foo-II/1/II/2/IIII/stdout
                  foo-II/1/II/2/III/seq
                  foo-II/1/II/2/III/stderr
                  foo-II/1/II/2/III/stdout

                If you do not want the dir structure, try --files --tag instead.

                See also --files, --tag, --header, --joblog.

       --resume Resumes from the last unfinished job. By reading --joblog or the --results dir
                GNU parallel will figure out the last unfinished job and continue from there. As
                GNU parallel only looks at the sequence numbers in --joblog then the input, the
                command, and --joblog all have to remain unchanged; otherwise GNU parallel may
                run wrong commands.

                See also --joblog, --results, --resume-failed, --retries.

       --resume-failed
                Retry all failed and resume from the last unfinished job. By reading --joblog GNU
                parallel will figure out the failed jobs and run those again. After that it will
                resume last unfinished job and continue from there. As GNU parallel only looks at
                the sequence numbers in --joblog then the input, the command, and --joblog all
                have to remain unchanged; otherwise GNU parallel may run wrong commands.

                See also --joblog, --resume, --retry-failed, --retries.

       --retry-failed
                Retry all failed jobs in joblog. By reading --joblog GNU parallel will figure out
                the failed jobs and run those again.

                --retry-failed ignores the command and arguments on the command line: It only
                looks at the joblog.

                    B<Differences between --resume, --resume-failed, --retry-failed>

                In this example exit {= $_%=2 =} will cause every other job to fail.

                  timeout -k 1 4 parallel --joblog log -j10 'sleep {}; exit {= $_%=2 =}' ::: {10..1}

                4 jobs completed. 2 failed:

                  Seq   [...]   Exitval Signal  Command
                  10    [...]   1       0       sleep 1; exit 1
                  9     [...]   0       0       sleep 2; exit 0
                  8     [...]   1       0       sleep 3; exit 1
                  7     [...]   0       0       sleep 4; exit 0

                --resume does not care about the Exitval, but only looks at Seq. If the Seq is
                run, it will not be run again. So if needed, you can change the command for the
                seqs not run yet:

                  parallel --resume --joblog log -j10 'sleep .{}; exit {= $_%=2 =}' ::: {10..1}

                  Seq   [...]   Exitval Signal  Command
                  [... as above ...]
                  1     [...]   0       0       sleep .10; exit 0
                  6     [...]   1       0       sleep .5; exit 1
                  5     [...]   0       0       sleep .6; exit 0
                  4     [...]   1       0       sleep .7; exit 1
                  3     [...]   0       0       sleep .8; exit 0
                  2     [...]   1       0       sleep .9; exit 1

                --resume-failed cares about the Exitval, but also only looks at Seq to figure out
                which commands to run. Again this means you can change the command, but not the
                arguments. It will run the failed seqs and the seqs not yet run:

                  parallel --resume-failed --joblog log -j10 'echo {};sleep .{}; exit {= $_%=3 =}' ::: {10..1}

                  Seq   [...]   Exitval Signal  Command
                  [... as above ...]
                  10    [...]   1       0       echo 1;sleep .1; exit 1
                  8     [...]   0       0       echo 3;sleep .3; exit 0
                  6     [...]   2       0       echo 5;sleep .5; exit 2
                  4     [...]   1       0       echo 7;sleep .7; exit 1
                  2     [...]   0       0       echo 9;sleep .9; exit 0

                --retry-failed cares about the Exitval, but takes the command from the joblog. It
                ignores any arguments or commands given on the command line:

                  parallel --retry-failed --joblog log -j10 this part is ignored

                  Seq   [...]   Exitval Signal  Command
                  [... as above ...]
                  10    [...]   1       0       echo 1;sleep .1; exit 1
                  6     [...]   2       0       echo 5;sleep .5; exit 2
                  4     [...]   1       0       echo 7;sleep .7; exit 1

                See also --joblog, --resume, --resume-failed, --retries.

       --retries n
                If a job fails, retry it on another computer on which it has not failed. Do this
                n times. If there are fewer than n computers in --sshlogin GNU parallel will re-
                use all the computers. This is useful if some jobs fail for no apparent reason
                (such as network failure).

       --return filename
                Transfer files from remote computers. --return is used with --sshlogin when the
                arguments are files on the remote computers. When processing is done the file
                filename will be transferred from the remote computer using rsync and will be put
                relative to the default login dir. E.g.

                  echo foo/bar.txt | parallel --return {.}.out \
                    --sshlogin server.example.com touch {.}.out

                This will transfer the file $HOME/foo/bar.out from the computer
                server.example.com to the file foo/bar.out after running touch foo/bar.out on
                server.example.com.

                  parallel -S server --trc out/./{}.out touch {}.out ::: in/file

                This will transfer the file in/file.out from the computer server.example.com to
                the files out/in/file.out after running touch in/file.out on server.

                  echo /tmp/foo/bar.txt | parallel --return {.}.out \
                    --sshlogin server.example.com touch {.}.out

                This will transfer the file /tmp/foo/bar.out from the computer server.example.com
                to the file /tmp/foo/bar.out after running touch /tmp/foo/bar.out on
                server.example.com.

                Multiple files can be transferred by repeating the option multiple times:

                  echo /tmp/foo/bar.txt | parallel \
                    --sshlogin server.example.com \
                    --return {.}.out --return {.}.out2 touch {.}.out {.}.out2

                --return is often used with --transferfile and --cleanup.

                --return is ignored when used with --sshlogin : or when not used with --sshlogin.

       --round-robin (alpha testing)
       --round (alpha testing)
                Normally --pipe will give a single block to each instance of the command. With
                --round-robin all blocks will at random be written to commands already running.
                This is useful if the command takes a long time to initialize.

                --keep-order will not work with --round-robin as it is impossible to track which
                input block corresponds to which output.

                --round-robin implies --pipe, except if --pipepart is given.

       --rpl 'tag perl expression'
                Use tag as a replacement string for perl expression. This makes it possible to
                define your own replacement strings. GNU parallel's 7 replacement strings are
                implemented as:

                  --rpl '{} '
                  --rpl '{#} 1 $_=$job->seq()'
                  --rpl '{%} 1 $_=$job->slot()'
                  --rpl '{/} s:.*/::'
                  --rpl '{//} $Global::use{"File::Basename"} ||= eval "use File::Basename; 1;"; $_ = dirname($_);'
                  --rpl '{/.} s:.*/::; s:\.[^/.]+$::;'
                  --rpl '{.} s:\.[^/.]+$::'

                The --plus replacement strings are implemented as:

                  --rpl '{+/} s:/[^/]*$::'
                  --rpl '{+.} s:.*\.::'
                  --rpl '{+..} s:.*\.([^.]*\.):$1:'
                  --rpl '{+...} s:.*\.([^.]*\.[^.]*\.):$1:'
                  --rpl '{..} s:\.[^/.]+$::; s:\.[^/.]+$::'
                  --rpl '{...} s:\.[^/.]+$::; s:\.[^/.]+$::; s:\.[^/.]+$::'
                  --rpl '{/..} s:.*/::; s:\.[^/.]+$::; s:\.[^/.]+$::'
                  --rpl '{/...} s:.*/::; s:\.[^/.]+$::; s:\.[^/.]+$::; s:\.[^/.]+$::'
                  --rpl '{##} $_=total_jobs()'

                If the user defined replacement string starts with '{' it can also be used as a
                positional replacement string (like {2.}).

                It is recommended to only change $_ but you have full access to all of GNU
                parallel's internal functions and data structures.

                Here are a few examples:

                  Is the job sequence even or odd?
                  --rpl '{odd} $_ = $job->seq() % 2 ? "odd" : "even"'
                  Pad job sequence with leading zeros to get equal width
                  --rpl '{0#} $f = "%0".int(1+log(total_jobs())/log(10))."d"; $_=sprintf($f,$job->seq())'
                  Job sequence counting from 0
                  --rpl '{#0} $_ = $job->seq() - 1'
                  Job slot counting from 2
                  --rpl '{%1} $_ = $job->slot() + 1'

                See also: {= perl expression =} --parens

       --max-chars=max-chars
       -s max-chars
                Use at most max-chars characters per command line, including the command and
                initial-arguments and the terminating nulls at the ends of the argument strings.
                The largest allowed value is system-dependent, and is calculated as the argument
                length limit for exec, less the size of your environment.  The default value is
                the maximum.

                Implies -X unless -m is set.

       --show-limits
                Display the limits on the command-line length which are imposed by the operating
                system and the -s option.  Pipe the input from /dev/null (and perhaps specify
                --no-run-if-empty) if you don't want GNU parallel to do anything.

       --semaphore
                Work as a counting semaphore. --semaphore will cause GNU parallel to start
                command in the background. When the number of jobs given by --jobs is reached,
                GNU parallel will wait for one of these to complete before starting another
                command.

                --semaphore implies --bg unless --fg is specified.

                --semaphore implies --semaphorename `tty` unless --semaphorename is specified.

                Used with --fg, --wait, and --semaphorename.

                The command sem is an alias for parallel --semaphore.

                See also man sem.

       --semaphorename name
       --id name
                Use name as the name of the semaphore. Default is the name of the controlling tty
                (output from tty).

                The default normally works as expected when used interactively, but when used in
                a script name should be set. $$ or my_task_name are often a good value.

                The semaphore is stored in ~/.parallel/semaphores/

                Implies --semaphore.

                See also man sem.

       --semaphoretimeout secs
       --st secs
                If secs > 0: If the semaphore is not released within secs seconds, take it
                anyway.

                If secs < 0: If the semaphore is not released within secs seconds, exit.

                Implies --semaphore.

                See also man sem.

       --seqreplace replace-str
                Use the replacement string replace-str instead of {#} for job sequence number.

       --shebang
       --hashbang
                GNU parallel can be called as a shebang (#!) command as the first line of a
                script. The content of the file will be treated as inputsource.

                Like this:

                  #!/usr/bin/parallel --shebang -r traceroute

                  qubes-os.org
                  debian.org
                  freenetproject.org

                --shebang must be set as the first option.

                On FreeBSD env is needed:

                  #!/usr/bin/env -S parallel --shebang -r traceroute

                  qubes-os.org
                  debian.org
                  freenetproject.org

                There are many limitations of shebang (#!) depending on your operating system.
                See details on http://www.in-ulm.de/~mascheck/various/shebang/

       --shebang-wrap
                GNU parallel can parallelize scripts by wrapping the shebang line. If the program
                can be run like this:

                  cat arguments | parallel the_program

                then the script can be changed to:

                  #!/usr/bin/parallel --shebang-wrap /the/original/parser --with-options

                E.g.

                  #!/usr/bin/parallel --shebang-wrap /usr/bin/python

                If the program can be run like this:

                  cat data | parallel --pipe the_program

                then the script can be changed to:

                  #!/usr/bin/parallel --shebang-wrap --pipe /the/original/parser --with-options

                E.g.

                  #!/usr/bin/parallel --shebang-wrap --pipe /usr/bin/perl -w

                --shebang-wrap must be set as the first option.

       --shellquote
                Does not run the command but quotes it. Useful for making quoted composed
                commands for GNU parallel.

       --shuf   Shuffle jobs. When having multiple input sources it is hard to randomize jobs.
                --shuf will generate all jobs, and shuffle them before running them. This is
                useful to get a quick preview of the results before running the full batch.

       --skip-first-line
                Do not use the first line of input (used by GNU parallel itself when called with
                --shebang).

       --sql DBURL (obsolete)
                Use --sqlmaster instead.

       --sqlmaster DBURL (alpha testing)
                Submit jobs via SQL server. DBURL must point to a table, which will contain the
                same information as --joblog, the values from the input sources (stored in
                columns V1 .. Vn), and the output (stored in columns Stdout and Stderr).

                If DBURL is prepended with '+' GNU parallel assumes the table is already made
                with the correct columns and appends the jobs to it.

                If DBURL is not prepended with '+' the table will be dropped and created with the
                correct amount of V-columns unless

                --sqlmaster does not run any jobs, but it creates the values for the jobs to be
                run. One or more --sqlworker must be run to actually execute the jobs.

                If --wait is set, GNU parallel will wait for the jobs to complete.

                The format of a DBURL is:

                  [sql:]vendor://[[user][:password]@][host][:port]/[database]/table

                E.g.

                  sql:mysql://hr:hr@localhost:3306/hrdb/jobs
                  mysql://scott:tiger@my.example.com/pardb/paralleljobs
                  sql:oracle://scott:tiger@ora.example.com/xe/parjob
                  postgresql://scott:tiger@pg.example.com/pgdb/parjob
                  pg:///parjob
                  sqlite3:///pardb/parjob

                It can also be an alias from ~/.sql/aliases:

                  :myalias mysql:///mydb/paralleljobs

       --sqlandworker DBURL (alpha testing)
                Shorthand for: --sqlmaster DBURL --sqlworker DBURL.

       --sqlworker DBURL (alpha testing)
                Execute jobs via SQL server. Read the input sources variables from the table
                pointed to by DBURL. The command on the command line should be the same as given
                by --sqlmaster.

                If you have more than one --sqlworker jobs may be run more than once.

       --ssh sshcommand
                GNU parallel defaults to using ssh for remote access. This can be overridden with
                --ssh. It can also be set on a per server basis (see --sshlogin).

       --sshdelay secs
                Delay starting next ssh by secs seconds. GNU parallel will pause secs seconds
                after starting each ssh. secs can be less than 1 seconds.

       -S [@hostgroups/][ncpu/]sshlogin[,[@hostgroups/][ncpu/]sshlogin[,...]]
       -S @hostgroup
       --sshlogin [@hostgroups/][ncpu/]sshlogin[,[@hostgroups/][ncpu/]sshlogin[,...]]
       --sshlogin @hostgroup
                Distribute jobs to remote computers. The jobs will be run on a list of remote
                computers.

                If hostgroups is given, the sshlogin will be added to that hostgroup. Multiple
                hostgroups are separated by '+'. The sshlogin will always be added to a hostgroup
                named the same as sshlogin.

                If only the @hostgroup is given, only the sshlogins in that hostgroup will be
                used. Multiple @hostgroup can be given.

                GNU parallel will determine the number of CPU cores on the remote computers and
                run the number of jobs as specified by -j.  If the number ncpu is given GNU
                parallel will use this number for number of CPU cores on the host. Normally ncpu
                will not be needed.

                An sshlogin is of the form:

                  [sshcommand [options]] [username@]hostname

                The sshlogin must not require a password (ssh-agent, ssh-copy-id, and sshpass may
                help with that).

                The sshlogin ':' is special, it means 'no ssh' and will therefore run on the
                local computer.

                The sshlogin '..' is special, it read sshlogins from ~/.parallel/sshloginfile

                The sshlogin '-' is special, too, it read sshlogins from stdin (standard input).

                To specify more sshlogins separate the sshlogins by comma, newline (in the same
                string), or repeat the options multiple times.

                For examples: see --sshloginfile.

                The remote host must have GNU parallel installed.

                --sshlogin is known to cause problems with -m and -X.

                --sshlogin is often used with --transferfile, --return, --cleanup, and --trc.

       --sshloginfile filename
       --slf filename
                File with sshlogins. The file consists of sshlogins on separate lines. Empty
                lines and lines starting with '#' are ignored. Example:

                  server.example.com
                  username@server2.example.com
                  8/my-8-core-server.example.com
                  2/my_other_username@my-dualcore.example.net
                  # This server has SSH running on port 2222
                  ssh -p 2222 server.example.net
                  4/ssh -p 2222 quadserver.example.net
                  # Use a different ssh program
                  myssh -p 2222 -l myusername hexacpu.example.net
                  # Use a different ssh program with default number of cores
                  //usr/local/bin/myssh -p 2222 -l myusername hexacpu
                  # Use a different ssh program with 6 cores
                  6//usr/local/bin/myssh -p 2222 -l myusername hexacpu
                  # Assume 16 cores on the local computer
                  16/:
                  # Put server1 in hostgroup1
                  @hostgroup1/server1
                  # Put myusername@server2 in hostgroup1+hostgroup2
                  @hostgroup1+hostgroup2/myusername@server2
                  # Force 4 cores and put 'ssh -p 2222 server3' in hostgroup1
                  @hostgroup1/4/ssh -p 2222 server3

                When using a different ssh program the last argument must be the hostname.

                Multiple --sshloginfile are allowed.

                GNU parallel will first look for the file in current dir; if that fails it look
                for the file in ~/.parallel.

                The sshloginfile '..' is special, it read sshlogins from ~/.parallel/sshloginfile

                The sshloginfile '.' is special, it read sshlogins from
                /etc/parallel/sshloginfile

                The sshloginfile '-' is special, too, it read sshlogins from stdin (standard
                input).

                If the sshloginfile is changed it will be re-read when a job finishes though at
                most once per second. This makes it possible to add and remove hosts while
                running.

                This can be used to have a daemon that updates the sshloginfile to only contain
                servers that are up:

                    cp original.slf tmp2.slf
                    while [ 1 ] ; do
                      nice parallel --nonall -j0 -k --slf original.slf \
                        --tag echo | perl 's/\t$//' > tmp.slf
                      if diff tmp.slf tmp2.slf; then
                        mv tmp.slf tmp2.slf
                      fi
                      sleep 10
                    done &
                    parallel --slf tmp2.slf ...

       --slotreplace replace-str
                Use the replacement string replace-str instead of {%} for job slot number.

       --silent Silent.  The job to be run will not be printed. This is the default.  Can be
                reversed with -v.

       --tty    Open terminal tty. If GNU parallel is used for starting an interactive program
                then this option may be needed. It will start only one job at a time (i.e. -j1),
                not buffer the output (i.e. -u), and it will open a tty for the job. When the job
                is done, the next job will get the tty.

                You can of course override -j1 and -u.

       --tag    Tag lines with arguments. Each output line will be prepended with the arguments
                and TAB (\t). When combined with --onall or --nonall the lines will be prepended
                with the sshlogin instead.

                --tag is ignored when using -u.

       --tagstring str
                Tag lines with a string. Each output line will be prepended with str and TAB
                (\t). str can contain replacement strings such as {}.

                --tagstring is ignored when using -u, --onall, and --nonall.

       --termseq sequence
                Termination sequence. When a job is killed due to --timeout, --memfree, --halt,
                or abnormal termination of GNU parallel, sequence determines how the job is
                killed. The default is:

                    TERM,200,TERM,100,TERM,50,KILL,25

                which sends a TERM signal, waits 200 ms, sends another TERM signal, waits 100 ms,
                sends another TERM signal, waits 50 ms, sends a KILL signal, waits 25 ms, and
                exits. GNU parallel discovers if a process dies before the waiting time is up.

       --tmpdir dirname
                Directory for temporary files. GNU parallel normally buffers output into
                temporary files in /tmp. By setting --tmpdir you can use a different dir for the
                files. Setting --tmpdir is equivalent to setting $TMPDIR.

       --tmux   Use tmux for output. Start a tmux session and run each job in a window in that
                session. No other output will be produced.

       --tmuxpane
                Use tmux for output but put output into panes in the first window.  Useful if you
                want to monitor the progress of less than 100 concurrent jobs.

       --timeout secs
                Time out for command. If the command runs for longer than secs seconds it will
                get killed with SIGTERM, followed by SIGTERM 200 ms later, followed by SIGKILL
                200 ms later.

                If secs is followed by a % then the timeout will dynamically be computed as a
                percentage of the median average runtime. Only values > 100% will make sense.

       --verbose
       -t       Print the job to be run on stderr (standard error).

                See also -v, -p.

       --transfer
                Transfer files to remote computers. Shorthand for: --transferfile {}.

       --transferfile filename
       --tf filename
                --transferfile is used with --sshlogin to transfer files to the remote computers.
                The files will be transferred using rsync and will be put relative to the default
                work dir. If the path contains /./ the remaining path will be relative to the
                work dir. E.g.

                  echo foo/bar.txt | parallel \
                    --sshlogin server.example.com --transferfile {} wc

                This will transfer the file foo/bar.txt to the computer server.example.com to the
                file $HOME/foo/bar.txt before running wc foo/bar.txt on server.example.com.

                  echo /tmp/foo/bar.txt | parallel \
                    --sshlogin server.example.com --transferfile {} wc

                This will transfer the file /tmp/foo/bar.txt to the computer server.example.com
                to the file /tmp/foo/bar.txt before running wc /tmp/foo/bar.txt on
                server.example.com.

                  echo /tmp/./foo/bar.txt | parallel \
                    --sshlogin server.example.com --transferfile {} wc {= s:.*/./:./: =}

                This will transfer the file /tmp/foo/bar.txt to the computer server.example.com
                to the file foo/bar.txt before running wc ./foo/bar.txt on server.example.com.

                --transferfile is often used with --return and --cleanup. A shorthand for
                --transferfile {} is --transfer.

                --transferfile is ignored when used with --sshlogin : or when not used with
                --sshlogin.

       --trc filename
                Transfer, Return, Cleanup. Shorthand for:

                --transferfile {} --return filename --cleanup

       --trim <n|l|r|lr|rl>
                Trim white space in input.

                n   No trim. Input is not modified. This is the default.

                l   Left trim. Remove white space from start of input. E.g. " a bc " -> "a bc ".

                r   Right trim. Remove white space from end of input. E.g. " a bc " -> " a bc".

                lr
                rl  Both trim. Remove white space from both start and end of input. E.g. " a bc "
                    -> "a bc". This is the default if --colsep is used.

       --ungroup
       -u       Ungroup output.  Output is printed as soon as possible and by passes GNU parallel
                internal processing. This may cause output from different commands to be mixed
                thus should only be used if you do not care about the output. Compare these:

                  seq 4 | parallel -j0 \
                    'sleep {};echo -n start{};sleep {};echo {}end'
                  seq 4 | parallel -u -j0 \
                    'sleep {};echo -n start{};sleep {};echo {}end'

                It also disables --tag. GNU parallel outputs faster with -u. Compare the speed of
                these:

                  parallel seq ::: 300000000 >/dev/null
                  parallel -u seq ::: 300000000 >/dev/null
                  parallel --line-buffer seq ::: 300000000 >/dev/null

                Can be reversed with --group.

                See also: --line-buffer --group

       --extensionreplace replace-str
       --er replace-str
                Use the replacement string replace-str instead of {.} for input line without
                extension.

       --use-cpus-instead-of-cores
                Count the number of physical CPUs instead of CPU cores. When computing how many
                jobs to run simultaneously relative to the number of CPU cores you can ask GNU
                parallel to instead look at the number of physical CPUs. This will make sense for
                computers that have hyperthreading as two jobs running on one CPU with
                hyperthreading will run slower than two jobs running on two physical CPUs. Some
                multi-core CPUs can run faster if only one thread is running per physical CPU.
                Most users will not need this option.

       -v       Verbose.  Print the job to be run on stdout (standard output). Can be reversed
                with --silent. See also -t.

                Use -v -v to print the wrapping ssh command when running remotely.

       --version
       -V       Print the version GNU parallel and exit.

       --workdir mydir
       --wd mydir
                Files transferred using --transferfile and --return will be relative to mydir on
                remote computers, and the command will be executed in the dir mydir.

                The special mydir value ... will create working dirs under ~/.parallel/tmp/ on
                the remote computers. If --cleanup is given these dirs will be removed.

                The special mydir value . uses the current working dir.  If the current working
                dir is beneath your home dir, the value . is treated as the relative path to your
                home dir. This means that if your home dir is different on remote computers (e.g.
                if your login is different) the relative path will still be relative to your home
                dir.

                To see the difference try:

                  parallel -S server pwd ::: ""
                  parallel --wd . -S server pwd ::: ""
                  parallel --wd ... -S server pwd ::: ""

                mydir can contain GNU parallel's replacement strings.

       --wait (alpha testing)
                Wait for all commands to complete.

                Used with --semaphore or --sqlmaster.

                See also man sem.

       -X       Multiple arguments with context replace. Insert as many arguments as the command
                line length permits. If multiple jobs are being run in parallel: distribute the
                arguments evenly among the jobs. Use -j1 to avoid this.

                If {} is not used the arguments will be appended to the line.  If {} is used as
                part of a word (like pic{}.jpg) then the whole word will be repeated. If {} is
                used multiple times each {} will be replaced with the arguments.

                Normally -X will do the right thing, whereas -m can give unexpected results if {}
                is used as part of a word.

                Support for -X with --sshlogin is limited and may fail.

                See also -m.

       --exit
       -x       Exit if the size (see the -s option) is exceeded.

EXAMPLE: Working as xargs -n1. Argument appending

       GNU parallel can work similar to xargs -n1.

       To compress all html files using gzip run:

         find . -name '*.html' | parallel gzip --best

       If the file names may contain a newline use -0. Substitute FOO BAR with FUBAR in all files
       in this dir and subdirs:

         find . -type f -print0 | parallel -q0 perl -i -pe 's/FOO BAR/FUBAR/g'

       Note -q is needed because of the space in 'FOO BAR'.

EXAMPLE: Reading arguments from command line

       GNU parallel can take the arguments from command line instead of stdin (standard input).
       To compress all html files in the current dir using gzip run:

         parallel gzip --best ::: *.html

       To convert *.wav to *.mp3 using LAME running one process per CPU core run:

         parallel lame {} -o {.}.mp3 ::: *.wav

EXAMPLE: Inserting multiple arguments

       When moving a lot of files like this: mv *.log destdir you will sometimes get the error:

         bash: /bin/mv: Argument list too long

       because there are too many files. You can instead do:

         ls | grep -E '\.log$' | parallel mv {} destdir

       This will run mv for each file. It can be done faster if mv gets as many arguments that
       will fit on the line:

         ls | grep -E '\.log$' | parallel -m mv {} destdir

EXAMPLE: Context replace

       To remove the files pict0000.jpg .. pict9999.jpg you could do:

         seq -w 0 9999 | parallel rm pict{}.jpg

       You could also do:

         seq -w 0 9999 | perl -pe 's/(.*)/pict$1.jpg/' | parallel -m rm

       The first will run rm 10000 times, while the last will only run rm as many times needed to
       keep the command line length short enough to avoid Argument list too long (it typically
       runs 1-2 times).

       You could also run:

         seq -w 0 9999 | parallel -X rm pict{}.jpg

       This will also only run rm as many times needed to keep the command line length short
       enough.

EXAMPLE: Compute intensive jobs and substitution

       If ImageMagick is installed this will generate a thumbnail of a jpg file:

         convert -geometry 120 foo.jpg thumb_foo.jpg

       This will run with number-of-cpu-cores jobs in parallel for all jpg files in a directory:

         ls *.jpg | parallel convert -geometry 120 {} thumb_{}

       To do it recursively use find:

         find . -name '*.jpg' | parallel convert -geometry 120 {} {}_thumb.jpg

       Notice how the argument has to start with {} as {} will include path (e.g. running convert
       -geometry 120 ./foo/bar.jpg thumb_./foo/bar.jpg would clearly be wrong). The command will
       generate files like ./foo/bar.jpg_thumb.jpg.

       Use {.} to avoid the extra .jpg in the file name. This command will make files like
       ./foo/bar_thumb.jpg:

         find . -name '*.jpg' | parallel convert -geometry 120 {} {.}_thumb.jpg

EXAMPLE: Substitution and redirection

       This will generate an uncompressed version of .gz-files next to the .gz-file:

         parallel zcat {} ">"{.} ::: *.gz

       Quoting of > is necessary to postpone the redirection. Another solution is to quote the
       whole command:

         parallel "zcat {} >{.}" ::: *.gz

       Other special shell characters (such as * ; $ > < | >> <<) also need to be put in quotes,
       as they may otherwise be interpreted by the shell and not given to GNU parallel.

EXAMPLE: Composed commands

       A job can consist of several commands. This will print the number of files in each
       directory:

         ls | parallel 'echo -n {}" "; ls {}|wc -l'

       To put the output in a file called <name>.dir:

         ls | parallel '(echo -n {}" "; ls {}|wc -l) >{}.dir'

       Even small shell scripts can be run by GNU parallel:

         find . | parallel 'a={}; name=${a##*/};' \
           'upper=$(echo "$name" | tr "[:lower:]" "[:upper:]");'\
           'echo "$name - $upper"'

         ls | parallel 'mv {} "$(echo {} | tr "[:upper:]" "[:lower:]")"'

       Given a list of URLs, list all URLs that fail to download. Print the line number and the
       URL.

         cat urlfile | parallel "wget {} 2>/dev/null || grep -n {} urlfile"

       Create a mirror directory with the same filenames except all files and symlinks are empty
       files.

         cp -rs /the/source/dir mirror_dir
         find mirror_dir -type l | parallel -m rm {} '&&' touch {}

       Find the files in a list that do not exist

         cat file_list | parallel 'if [ ! -e {} ] ; then echo {}; fi'

EXAMPLE: Composed command with multiple input sources

       You have a dir with files named as 24 hours in 5 minute intervals: 00:00, 00:05, 00:10 ..
       23:55. You want to find the files missing:

         parallel [ -f {1}:{2} ] "||" echo {1}:{2} does not exist ::: {00..23} ::: {00..55..5}

EXAMPLE: Calling Bash functions

       If the composed command is longer than a line, it becomes hard to read. In Bash you can
       use functions. Just remember to export -f the function.

         doit() {
           echo Doing it for $1
           sleep 2
           echo Done with $1
         }
         export -f doit
         parallel doit ::: 1 2 3

         doubleit() {
           echo Doing it for $1 $2
           sleep 2
           echo Done with $1 $2
         }
         export -f doubleit
         parallel doubleit ::: 1 2 3 ::: a b

       To do this on remote servers you need to transfer the function using --env:

         parallel --env doit -S server doit ::: 1 2 3
         parallel --env doubleit -S server doubleit ::: 1 2 3 ::: a b

       If your environment (aliases, variables, and functions) is small you can copy the full
       environment without having to export -f anything. See env_parallel.

EXAMPLE: Function tester

       To test a program with different parameters:

         tester() {
           if (eval "$@") >&/dev/null; then
             perl -e 'printf "\033[30;102m[ OK ]\033[0m @ARGV\n"' "$@"
           else
             perl -e 'printf "\033[30;101m[FAIL]\033[0m @ARGV\n"' "$@"
           fi
         }
         export -f tester
         parallel tester my_program ::: arg1 arg2
         parallel tester exit ::: 1 0 2 0

       If my_program fails a red FAIL will be printed followed by the failing command; otherwise
       a green OK will be printed followed by the command.

EXAMPLE: Log rotate

       Log rotation renames a logfile to an extension with a higher number: log.1 becomes log.2,
       log.2 becomes log.3, and so on. The oldest log is removed. To avoid overwriting files the
       process starts backwards from the high number to the low number.  This will keep 10 old
       versions of the log:

         seq 9 -1 1 | parallel -j1 mv log.{} log.'{= $_++ =}'
         mv log log.1

EXAMPLE: Removing file extension when processing files

       When processing files removing the file extension using {.} is often useful.

       Create a directory for each zip-file and unzip it in that dir:

         parallel 'mkdir {.}; cd {.}; unzip ../{}' ::: *.zip

       Recompress all .gz files in current directory using bzip2 running 1 job per CPU core in
       parallel:

         parallel "zcat {} | bzip2 >{.}.bz2 && rm {}" ::: *.gz

       Convert all WAV files to MP3 using LAME:

         find sounddir -type f -name '*.wav' | parallel lame {} -o {.}.mp3

       Put all converted in the same directory:

         find sounddir -type f -name '*.wav' | \
           parallel lame {} -o mydir/{/.}.mp3

EXAMPLE: Removing two file extensions when processing files

       If you have directory with tar.gz files and want these extracted in the corresponding dir
       (e.g foo.tar.gz will be extracted in the dir foo) you can do:

         parallel --plus 'mkdir {..}; tar -C {..} -xf {}' ::: *.tar.gz

EXAMPLE: Download 24 images for each of the past 30 days

       Let us assume a website stores images like:

         http://www.example.com/path/to/YYYYMMDD_##.jpg

       where YYYYMMDD is the date and ## is the number 01-24. This will download images for the
       past 30 days:

         parallel wget http://www.example.com/path/to/'$(date -d "today -{1} days" +%Y%m%d)_{2}.jpg' ::: $(seq 30) ::: $(seq -w 24)

       $(date -d "today -{1} days" +%Y%m%d) will give the dates in YYYYMMDD with {1} days
       subtracted.

EXAMPLE: Copy files as last modified date (ISO8601) with added random digits

         find . | parallel cp {} \
           '../destdir/{= $a=int(10000*rand); $_=`date -r "$_" +%FT%T"$a"`; chomp; =}'

       {= and =} mark a perl expression. date +%FT%T is the date in ISO8601 with time.

EXAMPLE: Digtal clock with "blinking" :

       The : in a digital clock blinks. To make every other line have a ':' and the rest a ' ' a
       perl expression is used to look at the 3rd input source. If the value modudo 2 is 1: Use
       ":" otherwise use " ":

         parallel -k echo {1}'{=3 $_=$_%2?":":" "=}'{2}{3} \
           ::: {0..12} ::: {0..5} ::: {0..9}

EXAMPLE: Aggregating content of files

       This:

         parallel --header : echo x{X}y{Y}z{Z} \> x{X}y{Y}z{Z} \
         ::: X {1..5} ::: Y {01..10} ::: Z {1..5}

       will generate the files x1y01z1 .. x5y10z5. If you want to aggregate the output grouping
       on x and z you can do this:

         parallel eval 'cat {=s/y01/y*/=} > {=s/y01//=}' ::: *y01*

       For all values of x and z it runs commands like:

         cat x1y*z1 > x1z1

       So you end up with x1z1 .. x5z5 each containing the content of all values of y.

EXAMPLE: Breadth first parallel web crawler/mirrorer

       This script below will crawl and mirror a URL in parallel.  It downloads first pages that
       are 1 click down, then 2 clicks down, then 3; instead of the normal depth first, where the
       first link link on each page is fetched first.

       Run like this:

         PARALLEL=-j100 ./parallel-crawl http://gatt.org.yeslab.org/

       Remove the wget part if you only want a web crawler.

       It works by fetching a page from a list of URLs and looking for links in that page that
       are within the same starting URL and that have not already been seen. These links are
       added to a new queue. When all the pages from the list is done, the new queue is moved to
       the list of URLs and the process is started over until no unseen links are found.

         #!/bin/bash

         # E.g. http://gatt.org.yeslab.org/
         URL=$1
         # Stay inside the start dir
         BASEURL=$(echo $URL | perl -pe 's:#.*::; s:(//.*/)[^/]*:$1:')
         URLLIST=$(mktemp urllist.XXXX)
         URLLIST2=$(mktemp urllist.XXXX)
         SEEN=$(mktemp seen.XXXX)

         # Spider to get the URLs
         echo $URL >$URLLIST
         cp $URLLIST $SEEN

         while [ -s $URLLIST ] ; do
           cat $URLLIST |
             parallel lynx -listonly -image_links -dump {} \; \
               wget -qm -l1 -Q1 {} \; echo Spidered: {} \>\&2 |
               perl -ne 's/#.*//; s/\s+\d+.\s(\S+)$/$1/ and do { $seen{$1}++ or print }' |
             grep -F $BASEURL |
             grep -v -x -F -f $SEEN | tee -a $SEEN > $URLLIST2
           mv $URLLIST2 $URLLIST
         done

         rm -f $URLLIST $URLLIST2 $SEEN

EXAMPLE: Process files from a tar file while unpacking

       If the files to be processed are in a tar file then unpacking one file and processing it
       immediately may be faster than first unpacking all files.

         tar xvf foo.tgz | perl -ne 'print $l;$l=$_;END{print $l}' | \
           parallel echo

       The Perl one-liner is needed to make sure the file is complete before handing it to GNU
       parallel.

EXAMPLE: Rewriting a for-loop and a while-read-loop

       for-loops like this:

         (for x in `cat list` ; do
           do_something $x
         done) | process_output

       and while-read-loops like this:

         cat list | (while read x ; do
           do_something $x
         done) | process_output

       can be written like this:

         cat list | parallel do_something | process_output

       For example: Find which host name in a list has IP address 1.2.3 4:

         cat hosts.txt | parallel -P 100 host | grep 1.2.3.4

       If the processing requires more steps the for-loop like this:

         (for x in `cat list` ; do
           no_extension=${x%.*};
           do_something $x scale $no_extension.jpg
           do_step2 <$x $no_extension
         done) | process_output

       and while-loops like this:

         cat list | (while read x ; do
           no_extension=${x%.*};
           do_something $x scale $no_extension.jpg
           do_step2 <$x $no_extension
         done) | process_output

       can be written like this:

         cat list | parallel "do_something {} scale {.}.jpg ; do_step2 <{} {.}" |\
           process_output

       If the body of the loop is bigger, it improves readability to use a function:

         (for x in `cat list` ; do
           do_something $x
           [... 100 lines that do something with $x ...]
         done) | process_output

         cat list | (while read x ; do
           do_something $x
           [... 100 lines that do something with $x ...]
         done) | process_output

       can both be rewritten as:

         doit() {
           x=$1
           do_something $x
           [... 100 lines that do something with $x ...]
         }
         export -f doit
         cat list | parallel doit

EXAMPLE: Rewriting nested for-loops

       Nested for-loops like this:

         (for x in `cat xlist` ; do
           for y in `cat ylist` ; do
             do_something $x $y
           done
         done) | process_output

       can be written like this:

         parallel do_something {1} {2} :::: xlist ylist | process_output

       Nested for-loops like this:

         (for colour in red green blue ; do
           for size in S M L XL XXL ; do
             echo $colour $size
           done
         done) | sort

       can be written like this:

         parallel echo {1} {2} ::: red green blue ::: S M L XL XXL | sort

EXAMPLE: Finding the lowest difference between files

       diff is good for finding differences in text files. diff | wc -l gives an indication of
       the size of the difference. To find the differences between all files in the current dir
       do:

         parallel --tag 'diff {1} {2} | wc -l' ::: * ::: * | sort -nk3

       This way it is possible to see if some files are closer to other files.

EXAMPLE: for-loops with column names

       When doing multiple nested for-loops it can be easier to keep track of the loop variable
       if is is named instead of just having a number. Use --header : to let the first argument
       be an named alias for the positional replacement string:

         parallel --header : echo {colour} {size} ::: colour red green blue ::: size S M L XL XXL

       This also works if the input file is a file with columns:

         cat addressbook.tsv | \
           parallel --colsep '\t' --header : echo {Name} {E-mail address}

EXAMPLE: Count the differences between all files in a dir

       Using --results the results are saved in /tmp/diffcount*.

         parallel --results /tmp/diffcount "diff -U 0 {1} {2} | \
           tail -n +3 |grep -v '^@'|wc -l" ::: * ::: *

       To see the difference between file A and file B look at the file '/tmp/diffcount/1/A/2/B'.

EXAMPLE: Speeding up fast jobs

       Starting a job on the local machine takes around 10 ms. This can be a big overhead if the
       job takes very few ms to run. Often you can group small jobs together using -X which will
       make the overhead less significant. Compare the speed of these:

         seq -w 0 9999 | parallel touch pict{}.jpg
         seq -w 0 9999 | parallel -X touch pict{}.jpg

       If your program cannot take multiple arguments, then you can use GNU parallel to spawn
       multiple GNU parallels:

         seq -w 0 999999 | parallel -j10 --pipe parallel -j0 touch pict{}.jpg

       If -j0 normally spawns 252 jobs, then the above will try to spawn 2520 jobs. On a normal
       GNU/Linux system you can spawn 32000 jobs using this technique with no problems. To raise
       the 32000 jobs limit raise /proc/sys/kernel/pid_max to 4194303.

EXAMPLE: Using shell variables

       When using shell variables you need to quote them correctly as they may otherwise be split
       on spaces.

       Notice the difference between:

         V=("My brother's 12\" records are worth <\$\$\$>"'!' Foo Bar)
         parallel echo ::: ${V[@]} # This is probably not what you want

       and:

         V=("My brother's 12\" records are worth <\$\$\$>"'!' Foo Bar)
         parallel echo ::: "${V[@]}"

       When using variables in the actual command that contains special characters (e.g. space)
       you can quote them using '"$VAR"' or using "'s and -q:

         V="Here  are  two "
         parallel echo "'$V'" ::: spaces
         parallel -q echo "$V" ::: spaces

EXAMPLE: Group output lines

       When running jobs that output data, you often do not want the output of multiple jobs to
       run together. GNU parallel defaults to grouping the output of each job, so the output is
       printed when the job finishes. If you want full lines to be printed while the job is
       running you can use --line-buffer. If you want output to be printed as soon as possible
       you can use -u.

       Compare the output of:

         parallel traceroute ::: qubes-os.org debian.org freenetproject.org
         parallel --line-buffer traceroute ::: qubes-os.org debian.org freenetproject.org
         parallel -u traceroute ::: qubes-os.org debian.org freenetproject.org

EXAMPLE: Tag output lines

       GNU parallel groups the output lines, but it can be hard to see where the different jobs
       begin. --tag prepends the argument to make that more visible:

         parallel --tag traceroute ::: qubes-os.org debian.org freenetproject.org

       --tag works with --line-buffer but not with -u:

         parallel --tag --line-buffer traceroute \
           ::: qubes-os.org debian.org freenetproject.org

       Check the uptime of the servers in ~/.parallel/sshloginfile:

         parallel --tag -S .. --nonall uptime

EXAMPLE: Keep order of output same as order of input

       Normally the output of a job will be printed as soon as it completes. Sometimes you want
       the order of the output to remain the same as the order of the input. This is often
       important, if the output is used as input for another system. -k will make sure the order
       of output will be in the same order as input even if later jobs end before earlier jobs.

       Append a string to every line in a text file:

         cat textfile | parallel -k echo {} append_string

       If you remove -k some of the lines may come out in the wrong order.

       Another example is traceroute:

         parallel traceroute ::: qubes-os.org debian.org freenetproject.org

       will give traceroute of qubes-os.org, debian.org and freenetproject.org, but it will be
       sorted according to which job completed first.

       To keep the order the same as input run:

         parallel -k traceroute ::: qubes-os.org debian.org freenetproject.org

       This will make sure the traceroute to qubes-os.org will be printed first.

       A bit more complex example is downloading a huge file in chunks in parallel: Some internet
       connections will deliver more data if you download files in parallel. For downloading
       files in parallel see: "EXAMPLE: Download 10 images for each of the past 30 days". But if
       you are downloading a big file you can download the file in chunks in parallel.

       To download byte 10000000-19999999 you can use curl:

         curl -r 10000000-19999999 http://example.com/the/big/file >file.part

       To download a 1 GB file we need 100 10MB chunks downloaded and combined in the correct
       order.

         seq 0 99 | parallel -k curl -r \
           {}0000000-{}9999999 http://example.com/the/big/file > file

EXAMPLE: Parallel grep

       grep -r greps recursively through directories. On multicore CPUs GNU parallel can often
       speed this up.

         find . -type f | parallel -k -j150% -n 1000 -m grep -H -n STRING {}

       This will run 1.5 job per core, and give 1000 arguments to grep.

EXAMPLE: Grepping n lines for m regular expressions.

       The simplest solution to grep a big file for a lot of regexps is:

         grep -f regexps.txt bigfile

       Or if the regexps are fixed strings:

         grep -F -f regexps.txt bigfile

       There are 3 limiting factors: CPU, RAM, and disk I/O.

       RAM is easy to measure: If the grep process takes up most of your free memory (e.g. when
       running top), then RAM is a limiting factor.

       CPU is also easy to measure: If the grep takes >90% CPU in top, then the CPU is a limiting
       factor, and parallelization will speed this up.

       It is harder to see if disk I/O is the limiting factor, and depending on the disk system
       it may be faster or slower to parallelize. The only way to know for certain is to test and
       measure.

   Limiting factor: RAM
       The normal grep -f regexs.txt bigfile works no matter the size of bigfile, but if
       regexps.txt is so big it cannot fit into memory, then you need to split this.

       grep -F takes around 100 bytes of RAM and grep takes about 500 bytes of RAM per 1 byte of
       regexp. So if regexps.txt is 1% of your RAM, then it may be too big.

       If you can convert your regexps into fixed strings do that. E.g. if the lines you are
       looking for in bigfile all looks like:

         ID1 foo bar baz Identifier1 quux
         fubar ID2 foo bar baz Identifier2

       then your regexps.txt can be converted from:

         ID1.*Identifier1
         ID2.*Identifier2

       into:

         ID1 foo bar baz Identifier1
         ID2 foo bar baz Identifier2

       This way you can use grep -F which takes around 80% less memory and is much faster.

       If it still does not fit in memory you can do this:

         parallel --pipepart -a regexps.txt --block 1M grep -F -f - -n bigfile |
           sort -un | perl -pe 's/^\d+://'

       The 1M should be your free memory divided by the number of cores and divided by 200 for
       grep -F and by 1000 for normal grep. On GNU/Linux you can do:

         free=$(awk '/^((Swap)?Cached|MemFree|Buffers):/ { sum += $2 }
                     END { print sum }' /proc/meminfo)
         percpu=$((free / 200 / $(parallel --number-of-cores)))k

         parallel --pipepart -a regexps.txt --block $percpu --compress grep -F -f - -n bigfile |
           sort -un | perl -pe 's/^\d+://'

       If you can live with duplicated lines and wrong order, it is faster to do:

         parallel --pipepart -a regexps.txt --block $percpu --compress grep -F -f - bigfile

   Limiting factor: CPU
       If the CPU is the limiting factor parallelization should be done on the regexps:

         cat regexp.txt | parallel --pipe -L1000 --round-robin --compress grep -f - -n bigfile |
           sort -un | perl -pe 's/^\d+://'

       The command will start one grep per CPU and read bigfile one time per CPU, but as that is
       done in parallel, all reads except the first will be cached in RAM. Depending on the size
       of regexp.txt it may be faster to use --block 10m instead of -L1000.

       Some storage systems perform better when reading multiple chunks in parallel. This is true
       for some RAID systems and for some network file systems. To parallelize the reading of
       bigfile:

         parallel --pipepart --block 100M -a bigfile -k --compress grep -f regexp.txt

       This will split bigfile into 100MB chunks and run grep on each of these chunks. To
       parallelize both reading of bigfile and regexp.txt combine the two using --fifo:

         parallel --pipepart --block 100M -a bigfile --fifo cat regexp.txt \
           \| parallel --pipe -L1000 --round-robin grep -f - {}

       If a line matches multiple regexps, the line may be duplicated.

   Bigger problem
       If the problem is too big to be solved by this, you are probably ready for Lucene.

EXAMPLE: Using remote computers

       To run commands on a remote computer SSH needs to be set up and you must be able to login
       without entering a password (The commands ssh-copy-id, ssh-agent, and sshpass may help you
       do that).

       If you need to login to a whole cluster, you typically do not want to accept the host key
       for every host. You want to accept them the first time and be warned if they are ever
       changed. To do that:

         # Add the servers to the sshloginfile
         (echo servera; echo serverb) > .parallel/my_cluster
         # Make sure .ssh/config exist
         touch .ssh/config
         cp .ssh/config .ssh/config.backup
         # Disable StrictHostKeyChecking temporarily
         (echo 'Host *'; echo StrictHostKeyChecking no) >> .ssh/config
         parallel --slf my_cluster --nonall true
         # Remove the disabling of StrictHostKeyChecking
         mv .ssh/config.backup .ssh/config

       The servers in .parallel/my_cluster are now added in .ssh/known_hosts.

       To run echo on server.example.com:

         seq 10 | parallel --sshlogin server.example.com echo

       To run commands on more than one remote computer run:

         seq 10 | parallel --sshlogin server.example.com,server2.example.net echo

       Or:

         seq 10 | parallel --sshlogin server.example.com \
           --sshlogin server2.example.net echo

       If the login username is foo on server2.example.net use:

         seq 10 | parallel --sshlogin server.example.com \
           --sshlogin foo@server2.example.net echo

       If your list of hosts is server1-88.example.net with login foo:

         seq 10 | parallel -Sfoo@server{1..88}.example.net echo

       To distribute the commands to a list of computers, make a file mycomputers with all the
       computers:

         server.example.com
         foo@server2.example.com
         server3.example.com

       Then run:

         seq 10 | parallel --sshloginfile mycomputers echo

       To include the local computer add the special sshlogin ':' to the list:

         server.example.com
         foo@server2.example.com
         server3.example.com
         :

       GNU parallel will try to determine the number of CPU cores on each of the remote
       computers, and run one job per CPU core - even if the remote computers do not have the
       same number of CPU cores.

       If the number of CPU cores on the remote computers is not identified correctly the number
       of CPU cores can be added in front. Here the computer has 8 CPU cores.

         seq 10 | parallel --sshlogin 8/server.example.com echo

EXAMPLE: Transferring of files

       To recompress gzipped files with bzip2 using a remote computer run:

         find logs/ -name '*.gz' | \
           parallel --sshlogin server.example.com \
           --transfer "zcat {} | bzip2 -9 >{.}.bz2"

       This will list the .gz-files in the logs directory and all directories below. Then it will
       transfer the files to server.example.com to the corresponding directory in $HOME/logs. On
       server.example.com the file will be recompressed using zcat and bzip2 resulting in the
       corresponding file with .gz replaced with .bz2.

       If you want the resulting bz2-file to be transferred back to the local computer add
       --return {.}.bz2:

         find logs/ -name '*.gz' | \
           parallel --sshlogin server.example.com \
           --transfer --return {.}.bz2 "zcat {} | bzip2 -9 >{.}.bz2"

       After the recompressing is done the .bz2-file is transferred back to the local computer
       and put next to the original .gz-file.

       If you want to delete the transferred files on the remote computer add --cleanup. This
       will remove both the file transferred to the remote computer and the files transferred
       from the remote computer:

         find logs/ -name '*.gz' | \
           parallel --sshlogin server.example.com \
           --transfer --return {.}.bz2 --cleanup "zcat {} | bzip2 -9 >{.}.bz2"

       If you want run on several computers add the computers to --sshlogin either using ',' or
       multiple --sshlogin:

         find logs/ -name '*.gz' | \
           parallel --sshlogin server.example.com,server2.example.com \
           --sshlogin server3.example.com \
           --transfer --return {.}.bz2 --cleanup "zcat {} | bzip2 -9 >{.}.bz2"

       You can add the local computer using --sshlogin :. This will disable the removing and
       transferring for the local computer only:

         find logs/ -name '*.gz' | \
           parallel --sshlogin server.example.com,server2.example.com \
           --sshlogin server3.example.com \
           --sshlogin : \
           --transfer --return {.}.bz2 --cleanup "zcat {} | bzip2 -9 >{.}.bz2"

       Often --transfer, --return and --cleanup are used together. They can be shortened to
       --trc:

         find logs/ -name '*.gz' | \
           parallel --sshlogin server.example.com,server2.example.com \
           --sshlogin server3.example.com \
           --sshlogin : \
           --trc {.}.bz2 "zcat {} | bzip2 -9 >{.}.bz2"

       With the file mycomputers containing the list of computers it becomes:

         find logs/ -name '*.gz' | parallel --sshloginfile mycomputers \
           --trc {.}.bz2 "zcat {} | bzip2 -9 >{.}.bz2"

       If the file ~/.parallel/sshloginfile contains the list of computers the special short hand
       -S .. can be used:

         find logs/ -name '*.gz' | parallel -S .. \
           --trc {.}.bz2 "zcat {} | bzip2 -9 >{.}.bz2"

EXAMPLE: Distributing work to local and remote computers

       Convert *.mp3 to *.ogg running one process per CPU core on local computer and server2:

         parallel --trc {.}.ogg -S server2,: \
           'mpg321 -w - {} | oggenc -q0 - -o {.}.ogg' ::: *.mp3

EXAMPLE: Running the same command on remote computers

       To run the command uptime on remote computers you can do:

         parallel --tag --nonall -S server1,server2 uptime

       --nonall reads no arguments. If you have a list of jobs you want run on each computer you
       can do:

         parallel --tag --onall -S server1,server2 echo ::: 1 2 3

       Remove --tag if you do not want the sshlogin added before the output.

       If you have a lot of hosts use '-j0' to access more hosts in parallel.

EXAMPLE: Using remote computers behind NAT wall

       If the workers are behind a NAT wall, you need some trickery to get to them.

       If you can ssh to a jump host, and reach the workers from there, then the obvious solution
       would be this, but it does not work:

         parallel --ssh 'ssh jumphost ssh' -S host1 echo ::: DOES NOT WORK

       It does not work because the command is dequoted by ssh twice where as GNU parallel only
       expects it to be dequoted once.

       So instead put this in ~/.ssh/config:

         Host host1 host2 host3
           ProxyCommand ssh jumphost.domain nc -w 1 %h 22

       It requires nc(netcat) to be installed on jumphost. With this you can simply:

         parallel -S host1,host2,host3 echo ::: This does work

   No jumphost, but port forwards
       If there is no jumphost but each server has port 22 forwarded from the firewall (e.g. the
       firewall's port 22001 = port 22 on host1, 22002 = host2, 22003 = host3) then you can use
       ~/.ssh/config:

         Host host1.v
           Port 22001
         Host host2.v
           Port 22002
         Host host3.v
           Port 22003
         Host *.v
           Hostname firewall

       And then use host{1..3}.v as normal hosts:

         parallel -S host1.v,host2.v,host3.v echo ::: a b c

   No jumphost, no port forwards
       If ports cannot be forwarded, you need some sort of VPN to traverse the NAT-wall. TOR is
       one options for that, as it is very easy to get working.

       You need to install TOR and setup a hidden service. In torrc put:

         HiddenServiceDir /var/lib/tor/hidden_service/
         HiddenServicePort 22 127.0.0.1:22

       Then start TOR: /etc/init.d/tor restart

       The TOR hostname is now in /var/lib/tor/hidden_service/hostname and is something similar
       to izjafdceobowklhz.onion. Now you simply prepend torsocks to ssh:

         parallel --ssh 'torsocks ssh' -S izjafdceobowklhz.onion \
           -S zfcdaeiojoklbwhz.onion,auclucjzobowklhi.onion echo ::: a b c

       If not all hosts are accessible through TOR:

         parallel -S 'torsocks ssh izjafdceobowklhz.onion,host2,host3' echo ::: a b c

       See more ssh tricks on
       https://en.wikibooks.org/wiki/OpenSSH/Cookbook/Proxies_and_Jump_Hosts

EXAMPLE: Parallelizing rsync

       rsync is a great tool, but sometimes it will not fill up the available bandwidth. This is
       often a problem when copying several big files over high speed connections.

       The following will start one rsync per big file in src-dir to dest-dir on the server
       fooserver:

         cd src-dir; find . -type f -size +100000 | \
           parallel -v ssh fooserver mkdir -p /dest-dir/{//}\; \
             rsync -s -Havessh {} fooserver:/dest-dir/{}

       The dirs created may end up with wrong permissions and smaller files are not being
       transferred. To fix those run rsync a final time:

         rsync -Havessh src-dir/ fooserver:/dest-dir/

       If you are unable to push data, but need to pull them and the files are called digits.png
       (e.g. 000000.png) you might be able to do:

         seq -w 0 99 | parallel rsync -Havessh fooserver:src-path/*{}.png destdir/

EXAMPLE: Use multiple inputs in one command

       Copy files like foo.es.ext to foo.ext:

         ls *.es.* | perl -pe 'print; s/\.es//' | parallel -N2 cp {1} {2}

       The perl command spits out 2 lines for each input. GNU parallel takes 2 inputs (using -N2)
       and replaces {1} and {2} with the inputs.

       Count in binary:

         parallel -k echo ::: 0 1 ::: 0 1 ::: 0 1 ::: 0 1 ::: 0 1 ::: 0 1

       Print the number on the opposing sides of a six sided die:

         parallel --link -a <(seq 6) -a <(seq 6 -1 1) echo
         parallel --link echo :::: <(seq 6) <(seq 6 -1 1)

       Convert files from all subdirs to PNG-files with consecutive numbers (useful for making
       input PNG's for ffmpeg):

         parallel --link -a <(find . -type f | sort) \
           -a <(seq $(find . -type f|wc -l)) convert {1} {2}.png

       Alternative version:

         find . -type f | sort | parallel convert {} {#}.png

EXAMPLE: Use a table as input

       Content of table_file.tsv:

         foo<TAB>bar
         baz <TAB> quux

       To run:

         cmd -o bar -i foo
         cmd -o quux -i baz

       you can run:

         parallel -a table_file.tsv --colsep '\t' cmd -o {2} -i {1}

       Note: The default for GNU parallel is to remove the spaces around the columns. To keep the
       spaces:

         parallel -a table_file.tsv --trim n --colsep '\t' cmd -o {2} -i {1}

EXAMPLE: Output to database

       GNU parallel can output to a database table and a CSV-file:

         DBURL=csv:///%2Ftmp%2Fmy.csv
         DBTABLEURL=$DBURL/mytable
         parallel --sqlandworker $DBTABLEURL seq ::: {1..10}

       It is rather slow and takes up a lot of CPU time because GNU parallel parses the whole CSV
       file for each update.

       A better approach is to use an SQLite-base and then convert that to CSV:

         DBURL=sqlite3:///%2Ftmp%2Fmy.sqlite
         DBTABLEURL=$DBURL/mytable
         parallel --sqlandworker $DBTABLEURL seq ::: {1..10}
         sql $DBURL '.headers on' '.mode csv' 'SELECT * FROM mytable;'

       This takes around a second per job.

       If you have access to a real database system, such as PostgreSQL, it is even faster:

         DBURL=pg://user:pass@host/mydb
         DBTABLEURL=$DBURL/mytable
         parallel --sqlandworker $DBTABLEURL seq ::: {1..10}
         sql $DBURL "COPY (SELECT * FROM mytable) TO stdout DELIMITER ',' CSV HEADER;"

       Or MySQL:

         DBURL=mysql://user:pass@host/mydb
         DBTABLEURL=$DBURL/mytable
         parallel --sqlandworker $DBTABLEURL seq ::: {1..10}
         sql -p -B $DBURL "SELECT * FROM mytable;" > mytable.tsv
         perl -pe 's/"/""/g;s/\t/","/g;s/^/"/;s/$/"/;s/\\\\/\\/g;s/\\t/\t/g;s/\\n/\n/g;' mytable.tsv

EXAMPLE: Output to CSV-file for R

       If you have no need for the advanced job distribution control that a database provides,
       but you simply want output into a CSV file that you can read into R or LibreCalc, then you
       can use --results:

         parallel --results my.csv seq ::: 10 20 30
         R
         > mydf <- read.csv("my.csv");
         > print(mydf[2,])
         > write(as.character(mydf[2,c("Stdout")]),'')

EXAMPLE: Run the same command 10 times

       If you want to run the same command with the same arguments 10 times in parallel you can
       do:

         seq 10 | parallel -n0 my_command my_args

EXAMPLE: Working as cat | sh. Resource inexpensive jobs and evaluation

       GNU parallel can work similar to cat | sh.

       A resource inexpensive job is a job that takes very little CPU, disk I/O and network I/O.
       Ping is an example of a resource inexpensive job. wget is too - if the webpages are small.

       The content of the file jobs_to_run:

         ping -c 1 10.0.0.1
         wget http://example.com/status.cgi?ip=10.0.0.1
         ping -c 1 10.0.0.2
         wget http://example.com/status.cgi?ip=10.0.0.2
         ...
         ping -c 1 10.0.0.255
         wget http://example.com/status.cgi?ip=10.0.0.255

       To run 100 processes simultaneously do:

         parallel -j 100 < jobs_to_run

       As there is not a command the jobs will be evaluated by the shell.

EXAMPLE: Processing a big file using more cores

       To process a big file or some output you can use --pipe to split up the data into blocks
       and pipe the blocks into the processing program.

       If the program is gzip -9 you can do:

         cat bigfile | parallel --pipe --recend '' -k gzip -9 > bigfile.gz

       This will split bigfile into blocks of 1 MB and pass that to gzip -9 in parallel. One gzip
       will be run per CPU core. The output of gzip -9 will be kept in order and saved to
       bigfile.gz

       gzip works fine if the output is appended, but some processing does not work like that -
       for example sorting. For this GNU parallel can put the output of each command into a file.
       This will sort a big file in parallel:

         cat bigfile | parallel --pipe --files sort |\
           parallel -Xj1 sort -m {} ';' rm {} >bigfile.sort

       Here bigfile is split into blocks of around 1MB, each block ending in '\n' (which is the
       default for --recend). Each block is passed to sort and the output from sort is saved into
       files. These files are passed to the second parallel that runs sort -m on the files before
       it removes the files. The output is saved to bigfile.sort.

       GNU parallel's --pipe maxes out at around 100 MB/s because every byte has to be copied
       through GNU parallel. But if bigfile is a real (seekable) file GNU parallel can by-pass
       the copying and send the parts directly to the program:

         parallel --pipepart --block 100m -a bigfile --files sort |\
           parallel -Xj1 sort -m {} ';' rm {} >bigfile.sort

EXAMPLE: Running more than 250 jobs workaround

       If you need to run a massive amount of jobs in parallel, then you will likely hit the
       filehandle limit which is often around 250 jobs. If you are super user you can raise the
       limit in /etc/security/limits.conf but you can also use this workaround. The filehandle
       limit is per process. That means that if you just spawn more GNU parallels then each of
       them can run 250 jobs. This will spawn up to 2500 jobs:

         cat myinput |\
           parallel --pipe -N 50 --round-robin -j50 parallel -j50 your_prg

       This will spawn up to 62500 jobs (use with caution - you need 64 GB RAM to do this, and
       you may need to increase /proc/sys/kernel/pid_max):

         cat myinput |\
           parallel --pipe -N 250 --round-robin -j250 parallel -j250 your_prg

EXAMPLE: Working as mutex and counting semaphore

       The command sem is an alias for parallel --semaphore.

       A counting semaphore will allow a given number of jobs to be started in the background.
       When the number of jobs are running in the background, GNU sem will wait for one of these
       to complete before starting another command. sem --wait will wait for all jobs to
       complete.

       Run 10 jobs concurrently in the background:

         for i in *.log ; do
           echo $i
           sem -j10 gzip $i ";" echo done
         done
         sem --wait

       A mutex is a counting semaphore allowing only one job to run. This will edit the file
       myfile and prepends the file with lines with the numbers 1 to 3.

         seq 3 | parallel sem sed -i -e 'i{}' myfile

       As myfile can be very big it is important only one process edits the file at the same
       time.

       Name the semaphore to have multiple different semaphores active at the same time:

         seq 3 | parallel sem --id mymutex sed -i -e 'i{}' myfile

EXAMPLE: Mutex for a script

       Assume a script is called from cron or from a web service, but only one instance can be
       run at a time. With sem and --shebang-wrap the script can be made to wait for other
       instances to finish. Here in bash:

         #!/usr/bin/sem --shebang-wrap -u --id $0 --fg /bin/bash

         echo This will run
         sleep 5
         echo exclusively

       Here perl:

         #!/usr/bin/sem --shebang-wrap -u --id $0 --fg /usr/bin/perl

         print "This will run ";
         sleep 5;
         print "exclusively\n";

       Here python:

         #!/usr/local/bin/sem --shebang-wrap -u --id $0 --fg /usr/bin/python

         import time
         print "This will run ";
         time.sleep(5)
         print "exclusively";

EXAMPLE: Start editor with filenames from stdin (standard input)

       You can use GNU parallel to start interactive programs like emacs or vi:

         cat filelist | parallel --tty -X emacs
         cat filelist | parallel --tty -X vi

       If there are more files than will fit on a single command line, the editor will be started
       again with the remaining files.

EXAMPLE: Running sudo

       sudo requires a password to run a command as root. It caches the access, so you only need
       to enter the password again if you have not used sudo for a while.

       The command:

         parallel sudo echo ::: This is a bad idea

       is no good, as you would be prompted for the sudo password for each of the jobs. You can
       either do:

         sudo echo This
         parallel sudo echo ::: is a good idea

       or:

         sudo parallel echo ::: This is a good idea

       This way you only have to enter the sudo password once.

EXAMPLE: GNU Parallel as queue system/batch manager

       GNU parallel can work as a simple job queue system or batch manager.  The idea is to put
       the jobs into a file and have GNU parallel read from that continuously. As GNU parallel
       will stop at end of file we use tail to continue reading:

         true >jobqueue; tail -n+0 -f jobqueue | parallel

       To submit your jobs to the queue:

         echo my_command my_arg >> jobqueue

       You can of course use -S to distribute the jobs to remote computers:

         true >jobqueue; tail -n+0 -f jobqueue | parallel -S ..

       If you keep this running for a long time, jobqueue will grow. A way of removing the jobs
       already run is by making GNU parallel stop when it hits a special value and then restart.
       To use --eof to make GNU parallel exit, tail also needs to be forced to exit:

         true >jobqueue;
         while true; do
           tail -n+0 -f jobqueue |
             (parallel -E StOpHeRe -S ..; echo GNU Parallel is now done;
              perl -e 'while(<>){/StOpHeRe/ and last};print <>' jobqueue > j2;
              (seq 1000 >> jobqueue &);
              echo Done appending dummy data forcing tail to exit)
           echo tail exited;
           mv j2 jobqueue
         done

       In some cases you can run on more CPUs and computers during the night:

         # Day time
         echo 50% > jobfile
         cp day_server_list ~/.parallel/sshloginfile
         # Night time
         echo 100% > jobfile
         cp night_server_list ~/.parallel/sshloginfile
         tail -n+0 -f jobqueue | parallel --jobs jobfile -S ..

       GNU Parallel discovers if jobfile or ~/.parallel/sshloginfile changes.

       There is a a small issue when using GNU parallel as queue system/batch manager: You have
       to submit JobSlot number of jobs before they will start, and after that you can submit one
       at a time, and job will start immediately if free slots are available.  Output from the
       running or completed jobs are held back and will only be printed when JobSlots more jobs
       has been started (unless you use --ungroup or --line-buffer, in which case the output from
       the jobs are printed immediately).  E.g. if you have 10 jobslots then the output from the
       first completed job will only be printed when job 11 has started, and the output of second
       completed job will only be printed when job 12 has started.

EXAMPLE: GNU Parallel as dir processor

       If you have a dir in which users drop files that needs to be processed you can do this on
       GNU/Linux (If you know what inotifywait is called on other platforms file a bug report):

         inotifywait -q -m -r -e MOVED_TO -e CLOSE_WRITE --format %w%f my_dir |\
           parallel -u echo

       This will run the command echo on each file put into my_dir or subdirs of my_dir.

       You can of course use -S to distribute the jobs to remote computers:

         inotifywait -q -m -r -e MOVED_TO -e CLOSE_WRITE --format %w%f my_dir |\
           parallel -S ..  -u echo

       If the files to be processed are in a tar file then unpacking one file and processing it
       immediately may be faster than first unpacking all files. Set up the dir processor as
       above and unpack into the dir.

       Using GNU Parallel as dir processor has the same limitations as using GNU Parallel as
       queue system/batch manager.

QUOTING

       GNU parallel is very liberal in quoting. You only need to quote characters that have
       special meaning in shell:

         ( ) $ ` ' " < > ; | \

       and depending on context these needs to be quoted, too:

         ~ & # ! ? space * {

       Therefore most people will never need more quoting than putting '\' in front of the
       special characters.

       Often you can simply put \' around every ':

         perl -ne '/^\S+\s+\S+$/ and print $ARGV,"\n"' file

       can be quoted:

         parallel perl -ne \''/^\S+\s+\S+$/ and print $ARGV,"\n"'\' ::: file

       However, when you want to use a shell variable you need to quote the $-sign. Here is an
       example using $PARALLEL_SEQ. This variable is set by GNU parallel itself, so the
       evaluation of the $ must be done by the sub shell started by GNU parallel:

         seq 10 | parallel -N2 echo seq:\$PARALLEL_SEQ arg1:{1} arg2:{2}

       If the variable is set before GNU parallel starts you can do this:

         VAR=this_is_set_before_starting
         echo test | parallel echo {} $VAR

       Prints: test this_is_set_before_starting

       It is a little more tricky if the variable contains more than one space in a row:

         VAR="two  spaces  between  each  word"
         echo test | parallel echo {} \'"$VAR"\'

       Prints: test two  spaces  between  each  word

       If the variable should not be evaluated by the shell starting GNU parallel but be
       evaluated by the sub shell started by GNU parallel, then you need to quote it:

         echo test | parallel VAR=this_is_set_after_starting \; echo {} \$VAR

       Prints: test this_is_set_after_starting

       It is a little more tricky if the variable contains space:

         echo test |\
           parallel VAR='"two  spaces  between  each  word"' echo {} \'"$VAR"\'

       Prints: test two  spaces  between  each  word

       $$ is the shell variable containing the process id of the shell. This will print the
       process id of the shell running GNU parallel:

         seq 10 | parallel echo $$

       And this will print the process ids of the sub shells started by GNU parallel.

         seq 10 | parallel echo \$\$

       If the special characters should not be evaluated by the sub shell then you need to
       protect it against evaluation from both the shell starting GNU parallel and the sub shell:

         echo test | parallel echo {} \\\$VAR

       Prints: test $VAR

       GNU parallel can protect against evaluation by the sub shell by using -q:

         echo test | parallel -q echo {} \$VAR

       Prints: test $VAR

       This is particularly useful if you have lots of quoting. If you want to run a perl script
       like this:

         perl -ne '/^\S+\s+\S+$/ and print $ARGV,"\n"' file

       It needs to be quoted like one of these:

         ls | parallel perl -ne '/^\\S+\\s+\\S+\$/\ and\ print\ \$ARGV,\"\\n\"'
         ls | parallel perl -ne \''/^\S+\s+\S+$/ and print $ARGV,"\n"'\'

       Notice how spaces, \'s, "'s, and $'s need to be quoted. GNU parallel can do the quoting by
       using option -q:

         ls | parallel -q  perl -ne '/^\S+\s+\S+$/ and print $ARGV,"\n"'

       However, this means you cannot make the sub shell interpret special characters. For
       example because of -q this WILL NOT WORK:

         ls *.gz | parallel -q "zcat {} >{.}"
         ls *.gz | parallel -q "zcat {} | bzip2 >{.}.bz2"

       because > and | need to be interpreted by the sub shell.

       If you get errors like:

         sh: -c: line 0: syntax error near unexpected token
         sh: Syntax error: Unterminated quoted string
         sh: -c: line 0: unexpected EOF while looking for matching `''
         sh: -c: line 1: syntax error: unexpected end of file

       then you might try using -q.

       If you are using bash process substitution like <(cat foo) then you may try -q and
       prepending command with bash -c:

         ls | parallel -q bash -c 'wc -c <(echo {})'

       Or for substituting output:

         ls | parallel -q bash -c \
           'tar c {} | tee >(gzip >{}.tar.gz) | bzip2 >{}.tar.bz2'

       Conclusion: To avoid dealing with the quoting problems it may be easier just to write a
       small script or a function (remember to export -f the function) and have GNU parallel call
       that.

LIST RUNNING JOBS

       If you want a list of the jobs currently running you can run:

         killall -USR1 parallel

       GNU parallel will then print the currently running jobs on stderr (standard error).

COMPLETE RUNNING JOBS BUT DO NOT START NEW JOBS

       If you regret starting a lot of jobs you can simply break GNU parallel, but if you want to
       make sure you do not have half-completed jobs you should send the signal SIGTERM to GNU
       parallel:

         killall -TERM parallel

       This will tell GNU parallel to not start any new jobs, but wait until the currently
       running jobs are finished before exiting.

ENVIRONMENT VARIABLES

       $PARALLEL_PID
                The environment variable $PARALLEL_PID is set by GNU parallel and is visible to
                the jobs started from GNU parallel. This makes it possible for the jobs to
                communicate directly to GNU parallel.  Remember to quote the $, so it gets
                evaluated by the correct shell.

                Example: If each of the jobs tests a solution and one of jobs finds the solution
                the job can tell GNU parallel not to start more jobs by: kill -TERM
                $PARALLEL_PID. This only works on the local computer.

       $PARALLEL_SHELL
                Use this shell the shell for the commands run by GNU Parallel:

                · $PARALLEL_SHELL. If undefined use:

                · The shell that started GNU Parallel. If that cannot be determined:

                · $SHELL. If undefined use:

                · /bin/sh

       $PARALLEL_SSH
                GNU parallel defaults to using ssh for remote access. This can be overridden with
                $PARALLEL_SSH, which again can be overridden with --ssh. It can also be set on a
                per server basis (see --sshlogin).

       $PARALLEL_SEQ
                $PARALLEL_SEQ will be set to the sequence number of the job running. Remember to
                quote the $, so it gets evaluated by the correct shell.

                Example:

                  seq 10 | parallel -N2 \
                    echo seq:'$'PARALLEL_SEQ arg1:{1} arg2:{2}

       $PARALLEL_TMUX
                Path to tmux. If unset the tmux in $PATH is used.

       $TMPDIR  Directory for temporary files. See: --tmpdir.

       $PARALLEL
                The environment variable $PARALLEL will be used as default options for GNU
                parallel. If the variable contains special shell characters (e.g. $, *, or space)
                then these need to be to be escaped with \.

                Example:

                  cat list | parallel -j1 -k -v ls
                  cat list | parallel -j1 -k -v -S"myssh user@server" ls

                can be written as:

                  cat list | PARALLEL="-kvj1" parallel ls
                  cat list | PARALLEL='-kvj1 -S myssh\ user@server' \
                    parallel echo

                Notice the \ in the middle is needed because 'myssh' and 'user@server' must be
                one argument.

DEFAULT PROFILE (CONFIG FILE)

       The global configuration file /etc/parallel/config, followed by user configuration file
       ~/.parallel/config (formerly known as .parallelrc) will be read in turn if they exist.
       Lines starting with '#' will be ignored. The format can follow that of the environment
       variable $PARALLEL, but it is often easier to simply put each option on its own line.

       Options on the command line take precedence, followed by the environment variable
       $PARALLEL, user configuration file ~/.parallel/config, and finally the global
       configuration file /etc/parallel/config.

       Note that no file that is read for options, nor the environment variable $PARALLEL, may
       contain retired options such as --tollef.

PROFILE FILES

       If --profile set, GNU parallel will read the profile from that file rather than the global
       or user configuration files. You can have multiple --profiles.

       Example: Profile for running a command on every sshlogin in ~/.ssh/sshlogins and prepend
       the output with the sshlogin:

         echo --tag -S .. --nonall > ~/.parallel/n
         parallel -Jn uptime

       Example: Profile for running every command with -j-1 and nice

         echo -j-1 nice > ~/.parallel/nice_profile
         parallel -J nice_profile bzip2 -9 ::: *

       Example: Profile for running a perl script before every command:

         echo "perl -e '\$a=\$\$; print \$a,\" \",'\$PARALLEL_SEQ',\" \";';" \
           > ~/.parallel/pre_perl
         parallel -J pre_perl echo ::: *

       Note how the $ and " need to be quoted using \.

       Example: Profile for running distributed jobs with nice on the remote computers:

         echo -S .. nice > ~/.parallel/dist
         parallel -J dist --trc {.}.bz2 bzip2 -9 ::: *

EXIT STATUS

       Exit status depends on --halt-on-error if one of these are used: success=X, success=Y%,
       fail=Y%.

       0     All jobs ran without error. If success=X is used: X jobs ran without error. If
             success=Y% is used: Y% of the jobs ran without error.

       1-100 Some of the jobs failed. The exit status gives the number of failed jobs. If Y% is
             used the exit status is the percentage of jobs that failed.

       101   More than 100 jobs failed.

       255   Other error.

       -1 (In joblog and SQL table)
             Killed by Ctrl-C, timeout, not enough memory or similar.

       -2 (In joblog and SQL table)
             $job->skip() was called in {= =}.

       -1000 (In SQL table)
             Job is ready to run (set by --sqlmaster).

       -1220 (In SQL table)
             Job is taken by worker (set by --sqlworker).

       If fail=1 is used, the exit status will be the exit status of the failing job.

DIFFERENCES BETWEEN GNU Parallel AND ALTERNATIVES

       There are a lot programs with some of the functionality of GNU parallel. GNU parallel
       strives to include the best of the functionality without sacrificing ease of use.

   SUMMARY TABLE
       The following features are in some of the comparable tools:

       Inputs
        I1. Arguments can be read from stdin
        I2. Arguments can be read from a file
        I3. Arguments can be read from multiple files
        I4. Arguments can be read from command line
        I5. Arguments can be read from a table
        I6. Arguments can be read from the same file using #! (shebang)
        I7. Line oriented input as default (Quoting of special chars not needed)

       Manipulation of input
        M1. Composed command
        M2. Multiple arguments can fill up an execution line
        M3. Arguments can be put anywhere in the execution line
        M4. Multiple arguments can be put anywhere in the execution line
        M5. Arguments can be replaced with context
        M6. Input can be treated as the complete command line

       Outputs
        O1. Grouping output so output from different jobs do not mix
        O2. Send stderr (standard error) to stderr (standard error)
        O3. Send stdout (standard output) to stdout (standard output)
        O4. Order of output can be same as order of input
        O5. Stdout only contains stdout (standard output) from the command
        O6. Stderr only contains stderr (standard error) from the command

       Execution
        E1. Running jobs in parallel
        E2. List running jobs
        E3. Finish running jobs, but do not start new jobs
        E4. Number of running jobs can depend on number of cpus
        E5. Finish running jobs, but do not start new jobs after first failure
        E6. Number of running jobs can be adjusted while running

       Remote execution
        R1. Jobs can be run on remote computers
        R2. Basefiles can be transferred
        R3. Argument files can be transferred
        R4. Result files can be transferred
        R5. Cleanup of transferred files
        R6. No config files needed
        R7. Do not run more than SSHD's MaxStartups can handle
        R8. Configurable SSH command
        R9. Retry if connection breaks occasionally

       Semaphore
        S1. Possibility to work as a mutex
        S2. Possibility to work as a counting semaphore

       Legend
        - = no
        x = not applicable
        ID = yes

       As every new version of the programs are not tested the table may be outdated. Please file
       a bug-report if you find errors (See REPORTING BUGS).

       parallel: I1 I2 I3 I4 I5 I6 I7 M1 M2 M3 M4 M5 M6 O1 O2 O3 O4 O5 O6 E1 E2 E3 E4 E5 E6 R1 R2
       R3 R4 R5 R6 R7 R8 R9 S1 S2

       xargs: I1 I2 -  -  -  -  - -  M2 M3 -  -  - -  O2 O3 -  O5 O6 E1 -  -  -  -  - -  -  -  -
       -  x  -  -  - -  -

       find -exec: -  -  -  x  -  x  - -  M2 M3 -  -  -  - -  O2 O3 O4 O5 O6 -  -  -  -  -  -  -
       -  -  -  -  -  -  -  -  - x  x

       make -j: -  -  -  -  -  -  - -  -  -  -  -  - O1 O2 O3 -  x  O6 E1 -  -  -  E5 - -  -  -
       -  -  -  -  -  - -  -

       ppss: I1 I2 -  -  -  -  I7 M1 -  M3 -  -  M6 O1 -  -  x  -  - E1 E2 ?E3 E4 - - R1 R2 R3 R4
       -  -  ?R7 ? ?  -  -

       pexec: I1 I2 -  I4 I5 -  - M1 -  M3 -  -  M6 O1 O2 O3 -  O5 O6 E1 -  -  E4 -  E6 R1 -  -
       -  -  R6 -  -  - S1 -

       xjobs, prll, dxargs, mdm/middelman, xapply, paexec, ladon, jobflow, ClusterSSH: TODO -
       Please file a bug-report if you know what features they support (See REPORTING BUGS).

   DIFFERENCES BETWEEN xargs AND GNU Parallel
       xargs offers some of the same possibilities as GNU parallel.

       xargs deals badly with special characters (such as space, \, ' and "). To see the problem
       try this:

         touch important_file
         touch 'not important_file'
         ls not* | xargs rm
         mkdir -p "My brother's 12\" records"
         ls | xargs rmdir
         touch 'c:\windows\system32\clfs.sys'
         echo 'c:\windows\system32\clfs.sys' | xargs ls -l

       You can specify -0, but many input generators are not optimized for using NUL as separator
       but are optimized for newline as separator. E.g head, tail, awk, ls, echo, sed, tar -v,
       perl (-0 and \0 instead of \n), locate (requires using -0), find (requires using -print0),
       grep (requires user to use -z or -Z), sort (requires using -z).

       GNU parallel's newline separation can be emulated with:

       cat | xargs -d "\n" -n1 command

       xargs can run a given number of jobs in parallel, but has no support for running number-
       of-cpu-cores jobs in parallel.

       xargs has no support for grouping the output, therefore output may run together, e.g. the
       first half of a line is from one process and the last half of the line is from another
       process. The example Parallel grep cannot be done reliably with xargs because of this. To
       see this in action try:

         parallel perl -e '\$a=\"1{}\"x10000000\;print\ \$a,\"\\n\"' '>' {} \
           ::: a b c d e f
         ls -l a b c d e f
         parallel -kP4 -n1 grep 1 > out.par ::: a b c d e f
         echo a b c d e f | xargs -P4 -n1 grep 1 > out.xargs-unbuf
         echo a b c d e f | \
           xargs -P4 -n1 grep --line-buffered 1 > out.xargs-linebuf
         echo a b c d e f | xargs -n1 grep 1 > out.xargs-serial
         ls -l out*
         md5sum out*

       xargs has no support for keeping the order of the output, therefore if running jobs in
       parallel using xargs the output of the second job cannot be postponed till the first job
       is done.

       xargs has no support for running jobs on remote computers.

       xargs has no support for context replace, so you will have to create the arguments.

       If you use a replace string in xargs (-I) you can not force xargs to use more than one
       argument.

       Quoting in xargs works like -q in GNU parallel. This means composed commands and
       redirection require using bash -c.

         ls | parallel "wc {} >{}.wc"
         ls | parallel "echo {}; ls {}|wc"

       becomes (assuming you have 8 cores)

         ls | xargs -d "\n" -P8 -I {} bash -c "wc {} >{}.wc"
         ls | xargs -d "\n" -P8 -I {} bash -c "echo {}; ls {}|wc"

   DIFFERENCES BETWEEN find -exec AND GNU Parallel
       find -exec offer some of the same possibilities as GNU parallel.

       find -exec only works on files. So processing other input (such as hosts or URLs) will
       require creating these inputs as files. find -exec has no support for running commands in
       parallel.

   DIFFERENCES BETWEEN make -j AND GNU Parallel
       make -j can run jobs in parallel, but requires a crafted Makefile to do this. That results
       in extra quoting to get filename containing newline to work correctly.

       make -j computes a dependency graph before running jobs. Jobs run by GNU parallel does not
       depend on eachother.

       (Very early versions of GNU parallel were coincidently implemented using make -j).

   DIFFERENCES BETWEEN ppss AND GNU Parallel
       ppss is also a tool for running jobs in parallel.

       The output of ppss is status information and thus not useful for using as input for
       another command. The output from the jobs are put into files.

       The argument replace string ($ITEM) cannot be changed. Arguments must be quoted - thus
       arguments containing special characters (space '"&!*)  may cause problems. More than one
       argument is not supported. File names containing newlines are not processed correctly.
       When reading input from a file null cannot be used as a terminator. ppss needs to read the
       whole input file before starting any jobs.

       Output and status information is stored in ppss_dir and thus requires cleanup when
       completed. If the dir is not removed before running ppss again it may cause nothing to
       happen as ppss thinks the task is already done. GNU parallel will normally not need
       cleaning up if running locally and will only need cleaning up if stopped abnormally and
       running remote (--cleanup may not complete if stopped abnormally). The example Parallel
       grep would require extra postprocessing if written using ppss.

       For remote systems PPSS requires 3 steps: config, deploy, and start. GNU parallel only
       requires one step.

       EXAMPLES FROM ppss MANUAL

       Here are the examples from ppss's manual page with the equivalent using GNU parallel:

       1 ./ppss.sh standalone -d /path/to/files -c 'gzip '

       1 find /path/to/files -type f | parallel gzip

       2 ./ppss.sh standalone -d /path/to/files -c 'cp "$ITEM" /destination/dir '

       2 find /path/to/files -type f | parallel cp {} /destination/dir

       3 ./ppss.sh standalone -f list-of-urls.txt -c 'wget -q '

       3 parallel -a list-of-urls.txt wget -q

       4 ./ppss.sh standalone -f list-of-urls.txt -c 'wget -q "$ITEM"'

       4 parallel -a list-of-urls.txt wget -q {}

       5 ./ppss config -C config.cfg -c 'encode.sh ' -d /source/dir -m 192.168.1.100 -u ppss -k
       ppss-key.key -S ./encode.sh -n nodes.txt -o /some/output/dir --upload --download ; ./ppss
       deploy -C config.cfg ; ./ppss start -C config

       5 # parallel does not use configs. If you want a different username put it in nodes.txt:
       user@hostname

       5 find source/dir -type f | parallel --sshloginfile nodes.txt --trc {.}.mp3 lame -a {} -o
       {.}.mp3 --preset standard --quiet

       6 ./ppss stop -C config.cfg

       6 killall -TERM parallel

       7 ./ppss pause -C config.cfg

       7 Press: CTRL-Z or killall -SIGTSTP parallel

       8 ./ppss continue -C config.cfg

       8 Enter: fg or killall -SIGCONT parallel

       9 ./ppss.sh status -C config.cfg

       9 killall -SIGUSR2 parallel

   DIFFERENCES BETWEEN pexec AND GNU Parallel
       pexec is also a tool for running jobs in parallel.

       EXAMPLES FROM pexec MANUAL

       Here are the examples from pexec's info page with the equivalent using GNU parallel:

       1 pexec -o sqrt-%s.dat -p "$(seq 10)" -e NUM -n 4 -c -- \
         'echo "scale=10000;sqrt($NUM)" | bc'

       1 seq 10 | parallel -j4 'echo "scale=10000;sqrt({})" | bc > sqrt-{}.dat'

       2 pexec -p "$(ls myfiles*.ext)" -i %s -o %s.sort -- sort

       2 ls myfiles*.ext | parallel sort {} ">{}.sort"

       3 pexec -f image.list -n auto -e B -u star.log -c -- \
         'fistar $B.fits -f 100 -F id,x,y,flux -o $B.star'

       3 parallel -a image.list \
         'fistar {}.fits -f 100 -F id,x,y,flux -o {}.star' 2>star.log

       4 pexec -r *.png -e IMG -c -o - -- \
         'convert $IMG ${IMG%.png}.jpeg ; "echo $IMG: done"'

       4 ls *.png | parallel 'convert {} {.}.jpeg; echo {}: done'

       5 pexec -r *.png -i %s -o %s.jpg -c 'pngtopnm | pnmtojpeg'

       5 ls *.png | parallel 'pngtopnm < {} | pnmtojpeg > {}.jpg'

       6 for p in *.png ; do echo ${p%.png} ; done | \
         pexec -f - -i %s.png -o %s.jpg -c 'pngtopnm | pnmtojpeg'

       6 ls *.png | parallel 'pngtopnm < {} | pnmtojpeg > {.}.jpg'

       7 LIST=$(for p in *.png ; do echo ${p%.png} ; done)
         pexec -r $LIST -i %s.png -o %s.jpg -c 'pngtopnm | pnmtojpeg'

       7 ls *.png | parallel 'pngtopnm < {} | pnmtojpeg > {.}.jpg'

       8 pexec -n 8 -r *.jpg -y unix -e IMG -c \
         'pexec -j -m blockread -d $IMG | \
         jpegtopnm | pnmscale 0.5 | pnmtojpeg | \
         pexec -j -m blockwrite -s th_$IMG'

       8 Combining GNU parallel and GNU sem.

       8 ls *jpg | parallel -j8 'sem --id blockread cat {} | jpegtopnm |' \
         'pnmscale 0.5 | pnmtojpeg | sem --id blockwrite cat > th_{}'

       8 If reading and writing is done to the same disk, this may be faster as only one process
       will be either reading or writing:

       8 ls *jpg | parallel -j8 'sem --id diskio cat {} | jpegtopnm |' \
         'pnmscale 0.5 | pnmtojpeg | sem --id diskio cat > th_{}'

   DIFFERENCES BETWEEN xjobs AND GNU Parallel
       xjobs is also a tool for running jobs in parallel. It only supports running jobs on your
       local computer.

       xjobs deals badly with special characters just like xargs. See the section DIFFERENCES
       BETWEEN xargs AND GNU Parallel.

       Here are the examples from xjobs's man page with the equivalent using GNU parallel:

       1 ls -1 *.zip | xjobs unzip

       1 ls *.zip | parallel unzip

       2 ls -1 *.zip | xjobs -n unzip

       2 ls *.zip | parallel unzip >/dev/null

       3 find . -name '*.bak' | xjobs gzip

       3 find . -name '*.bak' | parallel gzip

       4 ls -1 *.jar | sed 's/\(.*\)/\1 > \1.idx/' | xjobs jar tf

       4 ls *.jar | parallel jar tf {} '>' {}.idx

       5 xjobs -s script

       5 cat script | parallel

       6 mkfifo /var/run/my_named_pipe; xjobs -s /var/run/my_named_pipe & echo unzip 1.zip >>
       /var/run/my_named_pipe; echo tar cf /backup/myhome.tar /home/me >> /var/run/my_named_pipe

       6 mkfifo /var/run/my_named_pipe; cat /var/run/my_named_pipe | parallel & echo unzip 1.zip
       >> /var/run/my_named_pipe; echo tar cf /backup/myhome.tar /home/me >>
       /var/run/my_named_pipe

   DIFFERENCES BETWEEN prll AND GNU Parallel
       prll is also a tool for running jobs in parallel. It does not support running jobs on
       remote computers.

       prll encourages using BASH aliases and BASH functions instead of scripts. GNU parallel
       supports scripts directly, functions if they are exported using export -f, and aliases if
       using env_parallel.

       prll generates a lot of status information on stderr (standard error) which makes it
       harder to use the stderr (standard error) output of the job directly as input for another
       program.

       Here is the example from prll's man page with the equivalent using GNU parallel:

         prll -s 'mogrify -flip $1' *.jpg
         parallel mogrify -flip ::: *.jpg

   DIFFERENCES BETWEEN dxargs AND GNU Parallel
       dxargs is also a tool for running jobs in parallel.

       dxargs does not deal well with more simultaneous jobs than SSHD's MaxStartups. dxargs is
       only built for remote run jobs, but does not support transferring of files.

   DIFFERENCES BETWEEN mdm/middleman AND GNU Parallel
       middleman(mdm) is also a tool for running jobs in parallel.

       Here are the shellscripts of http://mdm.berlios.de/usage.html ported to GNU parallel:

         seq 19 | parallel buffon -o - | sort -n > result
         cat files | parallel cmd
         find dir -execdir sem cmd {} \;

   DIFFERENCES BETWEEN xapply AND GNU Parallel
       xapply can run jobs in parallel on the local computer.

       Here are the examples from xapply's man page with the equivalent using GNU parallel:

       1 xapply '(cd %1 && make all)' */

       1 parallel 'cd {} && make all' ::: */

       2 xapply -f 'diff %1 ../version5/%1' manifest | more

       2 parallel diff {} ../version5/{} < manifest | more

       3 xapply -p/dev/null -f 'diff %1 %2' manifest1 checklist1

       3 parallel --link diff {1} {2} :::: manifest1 checklist1

       4 xapply 'indent' *.c

       4 parallel indent ::: *.c

       5 find ~ksb/bin -type f ! -perm -111 -print | xapply -f -v 'chmod a+x' -

       5 find ~ksb/bin -type f ! -perm -111 -print | parallel -v chmod a+x

       6 find */ -... | fmt 960 1024 | xapply -f -i /dev/tty 'vi' -

       6 sh <(find */ -... | parallel -s 1024 echo vi)

       6 find */ -... | parallel -s 1024 -Xuj1 vi

       7 find ... | xapply -f -5 -i /dev/tty 'vi' - - - - -

       7 sh <(find ... |parallel -n5 echo vi)

       7 find ... |parallel -n5 -uj1 vi

       8 xapply -fn "" /etc/passwd

       8 parallel -k echo < /etc/passwd

       9 tr ':' '\012' < /etc/passwd | xapply -7 -nf 'chown %1 %6' - - - - - - -

       9 tr ':' '\012' < /etc/passwd | parallel -N7 chown {1} {6}

       10 xapply '[ -d %1/RCS ] || echo %1' */

       10 parallel '[ -d {}/RCS ] || echo {}' ::: */

       11 xapply -f '[ -f %1 ] && echo %1' List | ...

       11 parallel '[ -f {} ] && echo {}' < List | ...

   DIFFERENCES BETWEEN AIX apply AND GNU Parallel
       apply can build command lines based on a template and arguments - very much like GNU
       parallel. apply does not run jobs in parallel. apply does not use an argument separator
       (like :::); instead the template must be the first argument.

       Here are the examples from
       https://www-01.ibm.com/support/knowledgecenter/ssw_aix_71/com.ibm.aix.cmds1/apply.htm

       1. To obtain results similar to those of the ls command, enter:

         apply echo *
         parallel echo ::: *

       2. To compare the file named a1 to the file named b1, and the file named a2 to the file
       named b2, enter:

         apply -2 cmp a1 b1 a2 b2
         parallel -N2 cmp ::: a1 b1 a2 b2

       3. To run the who command five times, enter:

         apply -0 who 1 2 3 4 5
         parallel -N0 who ::: 1 2 3 4 5

       4. To link all files in the current directory to the directory /usr/joe, enter:

         apply 'ln %1 /usr/joe' *
         parallel ln {} /usr/joe ::: *

   DIFFERENCES BETWEEN paexec AND GNU Parallel
       paexec can run jobs in parallel on both the local and remote computers.

       paexec requires commands to print a blank line as the last output. This means you will
       have to write a wrapper for most programs.

       paexec has a job dependency facility so a job can depend on another job to be executed
       successfully. Sort of a poor-man's make.

       Here are the examples from paexec's example catalog with the equivalent using GNU
       parallel:

       1_div_X_run:
          ../../paexec -s -l -c "`pwd`/1_div_X_cmd" -n +1 <<EOF [...]
          parallel echo {} '|' `pwd`/1_div_X_cmd <<EOF [...]

       all_substr_run:
          ../../paexec -lp -c "`pwd`/all_substr_cmd" -n +3 <<EOF [...]
          parallel echo {} '|' `pwd`/all_substr_cmd <<EOF [...]

       cc_wrapper_run:
          ../../paexec -c "env CC=gcc CFLAGS=-O2 `pwd`/cc_wrapper_cmd" \
                     -n 'host1 host2' \
                     -t '/usr/bin/ssh -x' <<EOF [...]
          parallel echo {} '|' "env CC=gcc CFLAGS=-O2 `pwd`/cc_wrapper_cmd" \
                     -S host1,host2 <<EOF [...]
          # This is not exactly the same, but avoids the wrapper
          parallel gcc -O2 -c -o {.}.o {} \
                     -S host1,host2 <<EOF [...]

       toupper_run:
          ../../paexec -lp -c "`pwd`/toupper_cmd" -n +10 <<EOF [...]
          parallel echo {} '|' ./toupper_cmd <<EOF [...]
          # Without the wrapper:
          parallel echo {} '| awk {print\ toupper\(\$0\)}' <<EOF [...]

   DIFFERENCES BETWEEN map AND GNU Parallel
       map sees it as a feature to have less features and in doing so it also handles corner
       cases incorrectly. A lot of GNU parallel's code is to handle corner cases correctly on
       every platform, so you will not get a nasty surprise if a user for example saves a file
       called: My brother's 12" records.txt

       map's example showing how to deal with special characters fails on special characters:

         echo "The Cure" > My\ brother\'s\ 12\"\ records

         ls | \
           map 'echo -n `gzip < "%" | wc -c`; echo -n '*100/'; wc -c < "%"' | bc

       It works with GNU parallel:

         ls | \
           parallel 'echo -n `gzip < {} | wc -c`; echo -n '*100/'; wc -c < {}' | bc

       And you can even get the file name prepended:

         ls | \
           parallel --tag '(echo -n `gzip < {} | wc -c`'*100/'; wc -c < {}) | bc'

       map has no support for grouping. So this gives the wrong results without any warnings:

         parallel perl -e '\$a=\"1{}\"x10000000\;print\ \$a,\"\\n\"' '>' {} \
           ::: a b c d e f
         ls -l a b c d e f
         parallel -kP4 -n1 grep 1 > out.par ::: a b c d e f
         map -p 4 'grep 1' a b c d e f > out.map-unbuf
         map -p 4 'grep --line-buffered 1' a b c d e f > out.map-linebuf
         map -p 1 'grep --line-buffered 1' a b c d e f > out.map-serial
         ls -l out*
         md5sum out*

       The documentation shows a workaround, but not only does that mix stdout (standard output)
       with stderr (standard error) it also fails completely for certain jobs (and may even be
       considered less readable):

         parallel echo -n {} ::: 1 2 3

         map -p 4 'echo -n % 2>&1 | sed -e "s/^/$$:/"' 1 2 3 | sort | cut -f2- -d:

       maps replacement strings (% %D %B %E) can be simulated in GNU parallel by putting this in
       ~/.parallel/config:

         --rpl '%'
         --rpl '%D $_=::shell_quote(::dirname($_));'
         --rpl '%B s:.*/::;s:\.[^/.]+$::;'
         --rpl '%E s:.*\.::'

       map cannot handle bundled options: map -vp 0 echo this fails

       map does not have an argument separator on the command line, but uses the first argument
       as command. This makes quoting harder which again may affect readability. Compare:

         map -p 2 perl\\\ -ne\\\ \\\'/^\\\\S+\\\\s+\\\\S+\\\$/\\\ and\\\ print\\\ \\\$ARGV,\\\"\\\\n\\\"\\\' *

         parallel -q perl -ne '/^\S+\s+\S+$/ and print $ARGV,"\n"' ::: *

       map can do multiple arguments with context replace, but not without context replace:

         parallel --xargs echo 'BEGIN{'{}'}END' ::: 1 2 3

       map does not set exit value according to whether one of the jobs failed:

         parallel false ::: 1 || echo Job failed

         map false 1 || echo Never run

       map requires Perl v5.10.0 making it harder to use on old systems.

       map has no way of using % in the command (GNU Parallel has -I to specify another
       replacement string than {}).

       By design map is option incompatible with xargs, it does not have remote job execution, a
       structured way of saving results, multiple input sources, progress indicator, configurable
       record delimiter (only field delimiter), logging of jobs run with possibility to resume,
       keeping the output in the same order as input, --pipe processing, and dynamically
       timeouts.

   DIFFERENCES BETWEEN ladon AND GNU Parallel
       ladon can run multiple jobs on files in parallel.

       ladon only works on files and the only way to specify files is using a quoted glob string
       (such as \*.jpg). It is not possible to list the files manually.

       As replacement strings it uses FULLPATH DIRNAME BASENAME EXT RELDIR RELPATH

       These can be simulated using GNU parallel by putting this in ~/.parallel/config:

           --rpl 'FULLPATH $_=::shell_quote($_);chomp($_=qx{readlink -f $_});'
           --rpl 'DIRNAME $_=::shell_quote(::dirname($_));chomp($_=qx{readlink -f $_});'
           --rpl 'BASENAME s:.*/::;s:\.[^/.]+$::;'
           --rpl 'EXT s:.*\.::'
           --rpl 'RELDIR $_=::shell_quote($_);chomp(($_,$c)=qx{readlink -f $_;pwd});s:\Q$c/\E::;$_=::dirname($_);'
           --rpl 'RELPATH $_=::shell_quote($_);chomp(($_,$c)=qx{readlink -f $_;pwd});s:\Q$c/\E::;'

       ladon deals badly with filenames containing " and newline, and it fails for output larger
       than 200k:

           ladon '*' -- seq 36000 | wc

       EXAMPLES FROM ladon MANUAL

       It is assumed that the '--rpl's above are put in ~/.parallel/config and that it is run
       under a shell that supports '**' globbing (such as zsh):

       1 ladon "**/*.txt" -- echo RELPATH

       1 parallel echo RELPATH ::: **/*.txt

       2 ladon "~/Documents/**/*.pdf" -- shasum FULLPATH >hashes.txt

       2 parallel shasum FULLPATH ::: ~/Documents/**/*.pdf >hashes.txt

       3 ladon -m thumbs/RELDIR "**/*.jpg" -- convert FULLPATH -thumbnail 100x100^ -gravity
       center -extent 100x100 thumbs/RELPATH

       3 parallel mkdir -p thumbs/RELDIR\; convert FULLPATH -thumbnail 100x100^ -gravity center
       -extent 100x100 thumbs/RELPATH ::: **/*.jpg

       4 ladon "~/Music/*.wav" -- lame -V 2 FULLPATH DIRNAME/BASENAME.mp3

       4 parallel lame -V 2 FULLPATH DIRNAME/BASENAME.mp3 ::: ~/Music/*.wav

   DIFFERENCES BETWEEN jobflow AND GNU Parallel
       jobflow can run multiple jobs in parallel.

       Just like xargs output from jobflow jobs running in parallel mix together by default.
       jobflow can buffer into files (placed in /run/shm), but these are not cleaned up - not
       even if jobflow dies unexpectently. If the total output is big (in the order of RAM+swap)
       it can cause the system to run out of memory.

       jobflow gives no error if the command is unknown, and like xargs redirection requires
       wrapping with bash -c.

       jobflow makes it possible to set ressource limits on the running jobs. This can be
       emulated by GNU parallel using bash's ulimit:

         jobflow -limits=mem=100M,cpu=3,fsize=20M,nofiles=300 myjob

         parallel 'ulimit -v 102400 -t 3 -f 204800 -n 300 myjob'

       EXAMPLES FROM jobflow README

       1 cat things.list | jobflow -threads=8 -exec ./mytask {}

       1 cat things.list | parallel -j8 ./mytask {}

       2 seq 100 | jobflow -threads=100 -exec echo {}

       2 seq 100 | parallel -j100 echo {}

       3 cat urls.txt | jobflow -threads=32 -exec wget {}

       3 cat urls.txt | parallel -j32 wget {}

       4 find . -name '*.bmp' | jobflow -threads=8 -exec bmp2jpeg {.}.bmp {.}.jpg

       4 find . -name '*.bmp' | parallel -j8 bmp2jpeg {.}.bmp {.}.jpg

   DIFFERENCES BETWEEN gargs AND GNU Parallel
       gargs can run multiple jobs in parallel.

       It caches output in memory. This causes it to be extremely slow when the output is larger
       than the physical RAM, and can cause the system to run out of memory.

       See more details on this in man parallel_design.

       Output to stderr (standard error) is changed if the command fails.

       Here are the two examples from gargs website.

       1 seq 12 -1 1 | gargs -p 4 -n 3 "sleep {0}; echo {1} {2}"

       1 seq 12 -1 1 | parallel -P 4 -n 3 "sleep {1}; echo {2} {3}"

       2 cat t.txt | gargs --sep "\s+" -p 2 "echo '{0}:{1}-{2}' full-line: \'{}\'"

       2 cat t.txt | parallel --colsep "\\s+" -P 2 "echo '{1}:{2}-{3}' full-line: \'{}\'"

   DIFFERENCES BETWEEN orgalorg AND GNU Parallel
       orgalorg can run the same job on multiple machines. This is related to --onall and
       --nonall.

       orgalorg supports entering the SSH password - provided it is the same for all servers. GNU
       parallel advocates using ssh-agent instead, but it is possible to emulate orgalorg's
       behavior by setting SSHPASS and by using --ssh "sshpass ssh".

       To make the emulation easier, make a simple alias:

         alias par_emul="parallel -j0 --ssh 'sshpass ssh' --nonall --tag --linebuffer"

       If you want to supply a password run:

         SSHPASS=`ssh-askpass`

       or set the password directly:

         SSHPASS=P4$$w0rd!

       If the above is set up you can then do:

         orgalorg -o frontend1 -o frontend2 -p -C uptime
         par_emul -S frontend1 -S frontend2 uptime

         orgalorg -o frontend1 -o frontend2 -p -C top -bid 1
         par_emul -S frontend1 -S frontend2 top -bid 1

         orgalorg -o frontend1 -o frontend2 -p -er /tmp -n 'md5sum /tmp/bigfile' -S bigfile
         par_emul -S frontend1 -S frontend2 --basefile bigfile --workdir /tmp  md5sum /tmp/bigfile

       orgalorg has a progress indicator for the transferring of a file. GNU parallel does not.

   DIFFERENCES BETWEEN Rust parallel AND GNU Parallel
       Rust parallel implements a few features from GNU parallel, but lacks many functions. All
       of these fail:

         # -q to protect quoted $ and space
         parallel -q perl -e '$a=shift; print "$a"x10000000' ::: a b c
         # Generation of combination of inputs
         parallel echo {1} {2} ::: red green blue ::: S M L XL XXL
         # Show what would be executed
         parallel --dry-run echo ::: a
         # Run different shell dialects
         zsh -c 'parallel echo \={} ::: zsh && true'
         csh -c 'parallel echo \$\{\} ::: shell && true'
         bash -c 'parallel echo \$\({}\) ::: pwd && true'

       Rust parallel lacks ::::, --pipe, and has no remote facilities.

       Rust parallel buffers in RAM like gargs. This can cause death-by-swapping. See man
       parallel_design.

   DIFFERENCES BETWEEN ClusterSSH AND GNU Parallel
       ClusterSSH solves a different problem than GNU parallel.

       ClusterSSH opens a terminal window for each computer and using a master window you can run
       the same command on all the computers. This is typically used for administrating several
       computers that are almost identical.

       GNU parallel runs the same (or different) commands with different arguments in parallel
       possibly using remote computers to help computing. If more than one computer is listed in
       -S GNU parallel may only use one of these (e.g. if there are 8 jobs to be run and one
       computer has 8 cores).

       GNU parallel can be used as a poor-man's version of ClusterSSH:

       parallel --nonall -S server-a,server-b do_stuff foo bar

BUGS

   Quoting of newline
       Because of the way newline is quoted this will not work:

         echo 1,2,3 | parallel -vkd, "echo 'a{}b'"

       However, these will all work:

         echo 1,2,3 | parallel -vkd, echo a{}b
         echo 1,2,3 | parallel -vkd, "echo 'a'{}'b'"
         echo 1,2,3 | parallel -vkd, "echo 'a'"{}"'b'"

   Speed
       Startup

       GNU parallel is slow at starting up - around 250 ms the first time and 150 ms after that.

       Job startup

       Starting a job on the local machine takes around 10 ms. This can be a big overhead if the
       job takes very few ms to run. Often you can group small jobs together using -X which will
       make the overhead less significant. Or you can run multiple GNU parallels as described in
       EXAMPLE: Speeding up fast jobs.

       SSH

       When using multiple computers GNU parallel opens ssh connections to them to figure out how
       many connections can be used reliably simultaneously (Namely SSHD's MaxStartups). This
       test is done for each host in serial, so if your --sshloginfile contains many hosts it may
       be slow.

       If your jobs are short you may see that there are fewer jobs running on the remove systems
       than expected. This is due to time spent logging in and out. -M may help here.

       Disk access

       A single disk can normally read data faster if it reads one file at a time instead of
       reading a lot of files in parallel, as this will avoid disk seeks. However, newer disk
       systems with multiple drives can read faster if reading from multiple files in parallel.

       If the jobs are of the form read-all-compute-all-write-all, so everything is read before
       anything is written, it may be faster to force only one disk access at the time:

         sem --id diskio cat file | compute | sem --id diskio cat > file

       If the jobs are of the form read-compute-write, so writing starts before all reading is
       done, it may be faster to force only one reader and writer at the time:

         sem --id read cat file | compute | sem --id write cat > file

       If the jobs are of the form read-compute-read-compute, it may be faster to run more jobs
       in parallel than the system has CPUs, as some of the jobs will be stuck waiting for disk
       access.

   --nice limits command length
       The current implementation of --nice is too pessimistic in the max allowed command length.
       It only uses a little more than half of what it could. This affects -X and -m. If this
       becomes a real problem for you file a bug-report.

   Aliases and functions do not work
       If you get:

         Can't exec "command": No such file or directory

       or:

         open3: exec of by command failed

       it may be because command is not known, but it could also be because command is an alias
       or a function. If it is a function you need to export -f the function first. An alias will
       only work if you use env_parallel.

REPORTING BUGS

       Report bugs to <bug-parallel@gnu.org> or
       https://savannah.gnu.org/bugs/?func=additem&group=parallel

       See a perfect bug report on
       https://lists.gnu.org/archive/html/bug-parallel/2015-01/msg00000.html

       Your bug report should always include:

       · The error message you get (if any).

       · The complete output of parallel --version. If you are not running the latest released
         version (see http://ftp.gnu.org/gnu/parallel/) you should specify why you believe the
         problem is not fixed in that version.

       · A minimal, complete, and verifiable example (See description on
         http://stackoverflow.com/help/mcve).

         It should be a complete example that others can run that shows the problem including all
         files needed to run the example. This should preferably be small and simple, so try to
         remove as many options as possible. A combination of yes, seq, cat, echo, and sleep can
         reproduce most errors. If your example requires large files, see if you can make them by
         something like seq 1000000 > file or yes | head -n 10000000 > file.

         If your example requires remote execution, see if you can use localhost - maybe using
         another login.

       · The output of your example. If your problem is not easily reproduced by others, the
         output might help them figure out the problem.

       · Whether you have watched the intro videos
         (http://www.youtube.com/playlist?list=PL284C9FF2488BC6D1), walked through the tutorial
         (man parallel_tutorial), and read the EXAMPLE section in the man page (man parallel -
         search for EXAMPLE:).

       If you suspect the error is dependent on your environment or distribution, please see if
       you can reproduce the error on one of these VirtualBox images:
       http://sourceforge.net/projects/virtualboximage/files/
       http://www.osboxes.org/virtualbox-images/

       Specifying the name of your distribution is not enough as you may have installed software
       that is not in the VirtualBox images.

       If you cannot reproduce the error on any of the VirtualBox images above, see if you can
       build a VirtualBox image on which you can reproduce the error. If not you should assume
       the debugging will be done through you. That will put more burden on you and it is extra
       important you give any information that help. In general the problem will be fixed faster
       and with less work for you if you can reproduce the error on a VirtualBox.

AUTHOR

       When using GNU parallel for a publication please cite:

       O. Tange (2011): GNU Parallel - The Command-Line Power Tool, ;login: The USENIX Magazine,
       February 2011:42-47.

       This helps funding further development; and it won't cost you a cent.  If you pay 10000
       EUR you should feel free to use GNU Parallel without citing.

       Copyright (C) 2007-10-18 Ole Tange, http://ole.tange.dk

       Copyright (C) 2008,2009,2010 Ole Tange, http://ole.tange.dk

       Copyright (C) 2010,2011,2012,2013,2014,2015,2016 Ole Tange, http://ole.tange.dk and Free
       Software Foundation, Inc.

       Parts of the manual concerning xargs compatibility is inspired by the manual of xargs from
       GNU findutils 4.4.2.

LICENSE

       Copyright (C) 2007,2008,2009,2010,2011,2012,2013,2014,2015,2016 Free Software Foundation,
       Inc.

       This program is free software; you can redistribute it and/or modify it under the terms of
       the GNU General Public License as published by the Free Software Foundation; either
       version 3 of the License, or at your option any later version.

       This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY;
       without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
       See the GNU General Public License for more details.

       You should have received a copy of the GNU General Public License along with this program.
       If not, see <http://www.gnu.org/licenses/>.

   Documentation license I
       Permission is granted to copy, distribute and/or modify this documentation under the terms
       of the GNU Free Documentation License, Version 1.3 or any later version published by the
       Free Software Foundation; with no Invariant Sections, with no Front-Cover Texts, and with
       no Back-Cover Texts.  A copy of the license is included in the file fdl.txt.

   Documentation license II
       You are free:

       to Share to copy, distribute and transmit the work

       to Remix to adapt the work

       Under the following conditions:

       Attribution
                You must attribute the work in the manner specified by the author or licensor
                (but not in any way that suggests that they endorse you or your use of the work).

       Share Alike
                If you alter, transform, or build upon this work, you may distribute the
                resulting work only under the same, similar or a compatible license.

       With the understanding that:

       Waiver   Any of the above conditions can be waived if you get permission from the
                copyright holder.

       Public Domain
                Where the work or any of its elements is in the public domain under applicable
                law, that status is in no way affected by the license.

       Other Rights
                In no way are any of the following rights affected by the license:

                · Your fair dealing or fair use rights, or other applicable copyright exceptions
                  and limitations;

                · The author's moral rights;

                · Rights other persons may have either in the work itself or in how the work is
                  used, such as publicity or privacy rights.

       Notice   For any reuse or distribution, you must make clear to others the license terms of
                this work.

       A copy of the full license is included in the file as cc-by-sa.txt.

DEPENDENCIES

       GNU parallel uses Perl, and the Perl modules Getopt::Long, IPC::Open3, Symbol, IO::File,
       POSIX, and File::Temp. For remote usage it also uses rsync with ssh.

SEE ALSO

       ssh(1), ssh-agent(1), sshpass(1), ssh-copy-id(1), rsync(1), find(1), xargs(1), dirname(1),
       make(1), pexec(1), ppss(1), xjobs(1), prll(1), dxargs(1), mdm(1)