flock

FLOCK(1)                         User Commands                        FLOCK(1)



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/⟩.



util-linux                         July 2014                          FLOCK(1)