Ubuntu Manpage: xargs - build and execute command lines from standard input

Provided by: findutils_4.6.0+git+20170828-2_amd64

NAME

       xargs - build and execute command lines from standard input

SYNOPSIS

       xargs [options] [command [initial-arguments]]

DESCRIPTION

This manual page documents the GNU version of xargs. xargs reads items from the standard input,
delimited by blanks (which can be protected with double or single quotes or a backslash) or newlines, and
executes the command (default is /bin/echo) one or more times with any initial-arguments followed by
items read from standard input. Blank lines on the standard input are ignored.

The command line for command is built up until it reaches a system-defined limit (unless the -n and -L
options are used). The specified command will be invoked as many times as necessary to use up the list
of input items. In general, there will be many fewer invocations of command than there were items in the
input. This will normally have significant performance benefits. Some commands can usefully be executed
in parallel too; see the -P option.

Because Unix filenames can contain blanks and newlines, this default behaviour is often problematic;
filenames containing blanks and/or newlines are incorrectly processed by xargs. In these situations it
is better to use the -0 option, which prevents such problems. When using this option you will need to
ensure that the program which produces the input for xargs also uses a null character as a separator. If
that program is GNU find for example, the -print0 option does this for you.

If any invocation of the command exits with a status of 255, xargs will stop immediately without reading
any further input. An error message is issued on stderr when this happens.

OPTIONS

-0, --null
Input items are terminated by a null character instead of by whitespace, and the quotes and
backslash are not special (every character is taken literally). Disables the end of file string,
which is treated like any other argument. Useful when input items might contain white space,
quote marks, or backslashes. The GNU find -print0 option produces input suitable for this mode.

-a file, --arg-file=file
Read items from file instead of standard input. If you use this option, stdin remains unchanged
when commands are run. Otherwise, stdin is redirected from /dev/null.

--delimiter=delim, -d delim
Input items are terminated by the specified character. The specified delimiter may be a single
character, a C-style character escape such as \n, or an octal or hexadecimal escape code. Octal
and hexadecimal escape codes are understood as for the printf command. Multibyte characters are
not supported. When processing the input, quotes and backslash are not special; every character
in the input is taken literally. The -d option disables any end-of-file string, which is treated
like any other argument. You can use this option when the input consists of simply newline-
separated items, although it is almost always better to design your program to use --null where
this is possible.

-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 ignored. If neither -E nor -e is used, no end of file string is used.

-e[eof-str], --eof[=eof-str]
This option is a synonym for the -E option. Use -E instead, because it is POSIX compliant 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.

-I replace-str
Replace occurrences of replace-str in the initial-arguments with names read from standard input.
Also, unquoted blanks do not terminate input items; instead the separator is the newline
character. Implies -x and -L 1.

-i[replace-str], --replace[=replace-str]
This option is a synonym for -Ireplace-str if replace-str is specified. If the replace-str
argument is missing, the effect is the same as -I{}. This option is deprecated; use -I instead.

-L max-lines
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. Implies -x.

-l[max-lines], --max-lines[=max-lines]
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.

-n max-args, --max-args=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 xargs will
exit.

-P max-procs, --max-procs=max-procs
Run up to max-procs processes at a time; the default is 1. If max-procs is 0, xargs will run as
many processes as possible at a time. Use the -n option or the -L option with -P; otherwise
chances are that only one exec will be done. While xargs is running, you can send its process a
SIGUSR1 signal to increase the number of commands to run simultaneously, or a SIGUSR2 to decrease
the number. You cannot increase it above an implementation-defined limit (which is shown with
--show-limits). You cannot decrease it below 1. xargs never terminates its commands; when asked
to decrease, it merely waits for more than one existing command to terminate before starting
another.

Please note that it is up to the called processes to properly manage parallel access to shared
resources. For example, if more than one of them tries to print to stdout, the output will be
produced in an indeterminate order (and very likely mixed up) unless the processes collaborate in
some way to prevent this. Using some kind of locking scheme is one way to prevent such problems.
In general, using a locking scheme will help ensure correct output but reduce performance. If you
don't want to tolerate the performance difference, simply arrange for each process to produce a
separate output file (or otherwise use separate resources).

-o, --open-tty
Reopen stdin as /dev/tty in the child process before executing the command. This is useful if you
want xargs to run an interactive application.

-p, --interactive
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.

--process-slot-var=name
Set the environment variable name to a unique value in each running child process. Values are
reused once child processes exit. This can be used in a rudimentary load distribution scheme, for
example.

-r, --no-run-if-empty
If the standard input does not contain any nonblanks, do not run the command. Normally, the
command is run once even if there is no input. This option is a GNU extension.

-s max-chars, --max-chars=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, less 2048 bytes of headroom. If this value is more than 128KiB, 128Kib is used as
the default value; otherwise, the default value is the maximum. 1KiB is 1024 bytes. xargs
automatically adapts to tighter constraints.

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

-t, --verbose
Print the command line on the standard error output before executing it.

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

--help Print a summary of the options to xargs and exit.

--version
Print the version number of xargs and exit.

EXAMPLES

       find /tmp -name core -type f -print | xargs /bin/rm -f

       Find files named core in or below the  directory  /tmp  and  delete  them.   Note  that  this  will  work
       incorrectly if there are any filenames containing newlines or spaces.

       find /tmp -name core -type f -print0 | xargs -0 /bin/rm -f

       Find  files named core in or below the directory /tmp and delete them, processing filenames in such a way
       that file or directory names containing spaces or newlines are correctly handled.

       find /tmp -depth -name core -type f -delete

       Find files named core in or below the directory /tmp and delete them, but more efficiently  than  in  the
       previous example (because we avoid the need to use fork(2) and exec(2) to launch rm and we don't need the
       extra xargs process).

       cut -d: -f1 < /etc/passwd | sort | xargs echo

       Generates a compact listing of all the users on the system.

EXIT STATUS

       xargs exits with the following status:
       0 if it succeeds
       123 if any invocation of the command exited with status 1-125
       124 if the command exited with status 255
       125 if the command is killed by a signal
       126 if the command cannot be run
       127 if the command is not found
       1 if some other error occurred.

       Exit codes greater than 128 are used by the shell to indicate that a program died due to a fatal signal.

STANDARDS CONFORMANCE

       As of GNU xargs version 4.2.9, the default behaviour of xargs  is  not  to  have  a  logical  end-of-file
       marker.  POSIX (IEEE Std 1003.1, 2004 Edition) allows this.

       The  -l  and  -i  options appear in the 1997 version of the POSIX standard, but do not appear in the 2004
       version of the standard.  Therefore you should use -L and -I instead, respectively.

       The -o option is an extension to the POSIX standard for better compatibility with BSD.

       The POSIX standard allows implementations to have a limit on the size of arguments to the exec functions.
       This  limit  could  be  as  low  as  4096 bytes including the size of the environment.  For scripts to be
       portable, they must not rely on a larger value.  However, I know of no implementation whose actual  limit
       is  that  small.   The  --show-limits  option  can  be used to discover the actual limits in force on the
       current system.

BUGS

The -L option is incompatible with the -I option, but perhaps should not be.

It is not possible for xargs to be used securely, since there will always be a time gap between the
production of the list of input files and their use in the commands that xargs issues. If other users
have access to the system, they can manipulate the filesystem during this time window to force the action
of the commands xargs runs to apply to files that you didn't intend. For a more detailed discussion of
this and related problems, please refer to the ``Security Considerations'' chapter in the findutils
Texinfo documentation. The -execdir option of find can often be used as a more secure alternative.

When you use the -I option, each line read from the input is buffered internally. This means that there
is an upper limit on the length of input line that xargs will accept when used with the -I option. To
work around this limitation, you can use the -s option to increase the amount of buffer space that xargs
uses, and you can also use an extra invocation of xargs to ensure that very long lines do not occur. For
example:

somecommand | xargs -s 50000 echo | xargs -I '{}' -s 100000 rm '{}'

Here, the first invocation of xargs has no input line length limit because it doesn't use the -i option.
The second invocation of xargs does have such a limit, but we have ensured that the it never encounters a
line which is longer than it can handle. This is not an ideal solution. Instead, the -i option should
not impose a line length limit, which is why this discussion appears in the BUGS section. The problem
doesn't occur with the output of find(1) because it emits just one filename per line.

The best way to report a bug is to use the form at http://savannah.gnu.org/bugs/?group=findutils. The
reason for this is that you will then be able to track progress in fixing the problem. Other comments
about xargs(1) and about the findutils package in general can be sent to the bug-findutils mailing list.
To join the list, send email to bug-findutils-request@gnu.org.

XARGS(1)