flock(1) — Linux manual page


FLOCK(1)                        User Commands                       FLOCK(1)

NAME         top

       flock - manage locks from shell scripts

SYNOPSIS         top

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

DESCRIPTION         top

       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         top

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

       -E, --conflict-exit-code number
              The exit status 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 status 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
              status used. The zero number of seconds is interpreted as

              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.

EXIT STATUS         top

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

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

EXAMPLES         top

       Note that "shell> " in examples is a command line prompt.

       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.

       shell> exec 4<>/var/lock/mylockfile
       shell> flock -n 4
              This form is convenient for locking a file without spawning a
              subprocess.  The shell opens the lock file for reading and
              writing as file descriptor 4, then flock is used to lock the

AUTHORS         top

       H. Peter Anvin ⟨hpa@zytor.com⟩

COPYRIGHT         top

       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

SEE ALSO         top


AVAILABILITY         top

       The flock command is part of the util-linux package and is available
       from Linux Kernel Archive 

COLOPHON         top

       This page is part of the util-linux (a random collection of Linux
       utilities) project.  Information about the project can be found at 
       ⟨https://www.kernel.org/pub/linux/utils/util-linux/⟩.  If you have a
       bug report for this manual page, send it to
       util-linux@vger.kernel.org.  This page was obtained from the
       project's upstream Git repository
       ⟨git://git.kernel.org/pub/scm/utils/util-linux/util-linux.git⟩ on
       2020-08-13.  (At that time, the date of the most recent commit that
       was found in the repository was 2020-08-12.)  If you discover any
       rendering problems in this HTML version of the page, or you believe
       there is a better or more up-to-date source for the page, or you have
       corrections or improvements to the information in this COLOPHON
       (which is not part of the original manual page), send a mail to

util-linux                        July 2014                         FLOCK(1)

Pages that refer to this page: flock(2)lslocks(8)