NAME | SYNOPSIS | DESCRIPTION | OPTIONS | EXAMPLES | EXIT STATUS | AUTHOR | COPYRIGHT | SEE ALSO | AVAILABILITY | COLOPHON

FLOCK(1)                        User Commands                       FLOCK(1)

NAME         top

       flock - manage locks from shell scripts

SYNOPSIS         top

       flock [options] <file|directory> <command> [command args]
       flock [options] <file|directory> -c <command>
       flock [options] <file descriptor number>

DESCRIPTION         top

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

       The first and second forms wrap the lock around the executing a
       command, in a manner similar to su(1) or newgrp(1).  It locks 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 open file by file descriptor number.  See
       examples how that can be used.

OPTIONS         top

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

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

       -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.

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

       -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.

       -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.

       -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.

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

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

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

EXAMPLES         top

       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         top

       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         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
       PARTICULAR PURPOSE.

SEE ALSO         top

       flock(2)

AVAILABILITY         top

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

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
       2014-07-09.  If you discover any rendering problems in this HTML ver‐
       sion 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 man‐
       ual page), send a mail to man-pages@man7.org

util-linux                     September 2011                       FLOCK(1)