flock

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.

       --verbose
           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
       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
       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 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 (C) 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/karelzak/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 2.37.2                 2021-06-02                          FLOCK(1)