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
	   Display 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
       <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/>.

util-linux 2.41.2		  2025-09-22			      FLOCK(1)