NAME
flock - manage locks from shell scripts
SYNOPSIS
flock [options] file|directorycommand [arguments]
flock [options] file|directory-ccommand
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, --commandcommand
-E, --conflict-exit-codenumber
-F, --no-fork
-e, -x, --exclusive
-n, --nb, --nonblock
-o, --close
-s, --shared
-u, --unlock
-w, --wait, --timeoutseconds
--fcntl
This is only available on kernel versions >= 3.15.
--verbose
-h, --help
-V, --version
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 $?
shell1> flock -s /tmp -c cat; shell2> flock -s -w .007 /tmp -c echo; /bin/echo $?
shell> flock -x local-lock-file echo 'a b c'
(; flock -n 9 || exit 1; # ... commands executed under lock ...; ) 9>/var/lock/mylockfile
[ "${FLOCKER}" != "$0" ] && exec env FLOCKER="$0" flock -en "$0" "$0" "$@" || :
shell> exec 4<>/var/lock/mylockfile; shell> flock -n 4
AUTHORS
H. Peter Anvin <[email protected]>
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/>.