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 -c.

       -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.  The
              number has to be in the range of 0 to 255.

       -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 --nonblock.

              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 an exit status given by the -E
       option, or 1 by default.  The exit status given by -E has to be
       in the range of 0 to 255.

       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

       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 descriptor.

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

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
       2021-03-21.  (At that time, the date of the most recent commit
       that was found in the repository was 2021-03-19.)  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 man-pages@man7.org

util-linux                      July 2014                       FLOCK(1)

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