Ubuntu Manpage: flock - manage locks from shell scripts

Provided by: util-linux_2.40.2-14ubuntu1.1_amd64

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.

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/>.

util-linux 2.40.2                                  2024-07-04                                           FLOCK(1)