flock - manage locks from shell scripts
flock [options] file|directory command
[arguments]
flock [options] file|directory -c
command
flock [options] number
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.
-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.
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.
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.
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.
H. Peter Anvin <hpa@zytor.com>
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.
For bug reports, use the issue tracker at
<https://github.com/util-linux/util-linux/issues>.
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/>.