NAME
flock - manage locks from shell scriptsSYNOPSIS
flock [options] file|directory command [arguments]DESCRIPTION
This utility manages flock(2) locks from within shell scripts or from the command line.OPTIONS
-c, --command commandPass a single command, without
arguments, to the shell with -c.
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.
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.
Obtain an exclusive lock, sometimes called a
write lock. This is the default.
Fail rather than wait if the lock cannot be
immediately acquired. See the -E option for the exit status used.
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.
Obtain a shared lock, sometimes called a read
lock.
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.
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.
Report how long it took to acquire the lock,
or why the lock could not be obtained.
Display help text and exit.
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.NOTES
flock does not detect deadlock. See flock(2) for details.EXAMPLES
Note that "shell> " in examples is a command line prompt.Set exclusive lock to directory /tmp
and the second command will fail.
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.
Grab the exclusive lock
"local-lock-file" before running echo with 'a b c'.
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.
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.
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. <[email protected]>PeterCOPYRIGHT
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/>.2022-05-11 | util-linux 2.38.1 |