flock — manage locks from shell scripts
flock [options] file |
directory command [arguments]
flock [options] file |
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
directory, which is
created (assuming appropriate permissions) if it does not
already exist. By default, if the lock cannot be immediately
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.
Pass a single
arguments, to the shell with
The exit code 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
−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 code
Close the file descriptor on which the lock is held
command. This is useful
spawns a child process which should not be holding the
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.
−w, −−wait, −−timeout
Fail if the lock cannot be acquired within
Decimal fractional values are allowed. See the
−E option for the
exit code used. The zero number of seconds is
Report how long it took to acquire the lock, or why the lock could not be obtained.
Display version information and exit.
Display help text and exit.
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.
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
allows the lockfile to be created if it does not
already exist, however, write permission is required.
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 env var $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 env var to the right value so it doesn't run again.
The command uses
return values for everything, except when using either of the
−w which report a failure to acquire
the lock with a return value given by the
−E option, or 1 by default.
When using the
command variant, and executing
the child worked, then the exit status is that of the child
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.
The flock command is part of the util-linux package and is available from Linux Kernel Archive
Copyright 2003-2006 H. Peter Anvin - All Rights Reserved
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