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.

       --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)

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/>.