NAME

       flock - manage locks from shell scripts

SYNOPSIS

       flock [options] file|directory command [arguments]
       flock [options] file|directory -c command
       flock [options] number

DESCRIPTION

       This utility manages flock(2) locks from within shell scripts or from the command line.

       The  first  and second of the above forms wrap the lock around the execution of a command,
       in a manner similar to su(1) or newgrp(1).  They lock a specified file or directory, which
       is  created  (assuming appropriate permissions) if it does not already exist.  By default,
       if the lock cannot be immediately acquired, flock waits until the lock is available.

       The third form uses an open file by its file descriptor number.  See  the  examples  below
       for how that can be used.

OPTIONS

       -c, --command command
              Pass a single command, without arguments, to the shell with -c.

       -E, --conflict-exit-code number
              The  exit  code used when the -n option is in use, and the conflicting lock exists,
              or the -w option is in use, and the timeout is reached.  The default value is 1.

       -F, --no-fork
              Do not fork before executing command.  Upon execution the flock process is replaced
              by  command  which  continues  to  hold  the lock. This option is incompatible with
              --close as there would otherwise be nothing left to hold the lock.

       -e, -x, --exclusive
              Obtain an exclusive lock, sometimes called a write lock.  This is the default.

       -n, --nb, --nonblock
              Fail rather than wait if the lock cannot  be  immediately  acquired.   See  the  -E
              option for the exit code used.

       -o, --close
              Close the file descriptor on which the lock is held before executing command.  This
              is useful if command spawns a child process which should not be holding the lock.

       -s, --shared
              Obtain a shared lock, sometimes called a read lock.

       -u, --unlock
              Drop a lock.  This is usually not required, since a lock is  automatically  dropped
              when the file is closed.  However, it may be required in special cases, for example
              if the enclosed command group may have forked a background process which should not
              be holding the lock.

       -w, --wait, --timeout seconds
              Fail  if the lock cannot be acquired within seconds.  Decimal fractional values are
              allowed.  See the -E option for the exit code used. The zero number of  seconds  is
              interpreted as --nonblock.

       --verbose
              Report how long it took to acquire the lock, or why the lock could not be obtained.

       -V, --version
              Display version information and exit.

       -h, --help
              Display help text and exit.

EXAMPLES

       shell1> flock /tmp -c cat
       shell2> flock -w .007 /tmp -c echo; /bin/echo $?
              Set exclusive lock to directory /tmp and the second command will fail.

       shell1> flock -s /tmp -c cat
       shell2> flock -s -w .007 /tmp -c echo; /bin/echo $?
              Set  shared  lock  to  directory /tmp and the second command will not fail.  Notice
              that attempting to get exclusive lock with second command would fail.

       shell> flock -x local-lock-file echo 'a b c'
              Grab the exclusive lock "local-lock-file" before running echo with 'a b c'.

       (
         flock -n 9 || exit 1
         # ... commands executed under lock ...
       ) 9>/var/lock/mylockfile
              The form is convenient inside shell scripts.   The  mode  used  to  open  the  file
              doesn't matter to flock; using > or >> allows the lockfile to be created if it does
              not already exist, however, write permission is required.  Using  <  requires  that
              the file already exists but only read permission is required.

       [ "${FLOCKER}" != "$0" ] && exec env FLOCKER="$0" flock -en "$0" "$0" "$@" || :
              This  is useful boilerplate code for shell scripts.  Put it at the top of the shell
              script you want to lock and it'll automatically lock itself on the first  run.   If
              the env var $FLOCKER is not set to the shell script that is being run, then execute
              flock and grab an exclusive non-blocking lock (using the script itself as the  lock
              file)  before re-execing itself with the right arguments.  It also sets the FLOCKER
              env var to the right value so it doesn't run again.

EXIT STATUS

       The command uses sysexits.h return values for everything, except when using either of  the
       options  -n  or -w which report a failure to acquire the lock with a return value given by
       the -E option, or 1 by default.

       When using the command variant, and executing the child worked, then the  exit  status  is
       that of the child command.

AUTHOR

       H. Peter Anvin ⟨hpa@zytor.com⟩

COPYRIGHT

       Copyright © 2003-2006 H. Peter Anvin.
       This  is  free software; see the source for copying conditions.  There is NO warranty; not
       even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

SEE ALSO

       flock(2)

AVAILABILITY

       The flock command is part of the util-linux package and is  available  from  Linux  Kernel
       Archive ⟨https://www.kernel.org/pub/linux/utils/util-linux/⟩.