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.
-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/>.
util-linux 2.38.1 2022-05-11 FLOCK(1)
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