flock(1) — Linux manual page

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 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.

       --fcntl
           Instead of flock(2), apply an fcntl(2) open file description
           lock (that is, using the F_OFD_SETLK (non-blocking) or
           F_OFD_SETLKW (blocking) commands). These locks are
           independent of those applied via flock(2), but, unlike
           traditional POSIX fcntl() locks (F_SETLK, F_SETLKW), have
           semantics matching those of flock(2).

       This is only available on kernel versions >= 3.15.

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

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

       -V, --version
           Print version and exit.

EXIT STATUS

       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.

NOTES

       flock does not detect deadlock. See flock(2) for details.

       Some file systems (e. g. NFS and CIFS) have a limited
       implementation of flock(2) and flock may always fail. For details
       see flock(2), nfs(5) and mount.cifs(8). Depending on mount
       options, flock can always fail there.

EXAMPLES

       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
           environment variable $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 environment variable 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
           descriptor.

AUTHORS

       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), fcntl(2)

REPORTING BUGS

       For bug reports, use the issue tracker at
       https://github.com/util-linux/util-linux/issues.

AVAILABILITY

       The flock command is part of the util-linux package which can be
       downloaded from Linux Kernel Archive
       <https://www.kernel.org/pub/linux/utils/util-linux/>. 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
       2024-06-14. (At that time, the date of the most recent commit
       that was found in the repository was 2024-06-10.) 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 2.41.devel-537-e... 2024-06-13                       FLOCK(1)

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

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