Section: H. Peter Anvin (1)
Updated: 4 Feb 2006
Return to Main Contents
flock - Manage locks from shell scripts
flock [-sxon] [-w timeout] lockfile [-c] command...
flock [-sxun] [-w timeout] fd
This utility manages
locks from within shell scripts or the command line.
The first form wraps the lock around the executing a command, in a manner similar to
It locks a specified file, which is created (assuming appropriate
permissions), if it does not already exist.
The second form is conveninent inside shell scripts, and is usually
used the following manner:
flock -s 200
# ... commands executed under lock ...
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
By default, if the lock cannot be immediately acquired, flock
waits until the lock is available.
- -s, --shared
Obtain a shared lock, sometimes called a read lock.
- -x, -e, --exclusive
Obtain an exclusive lock, sometimes called a write lock. This is the
- -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
- -n, --nb, --nonblock
Fail (with an exit code of 1) rather than wait if the lock cannot be
- -w, --wait, --timeout seconds
Fail (with an exit code of 1) if the lock cannot be acquired within
seconds seconds. Decimal fractional values are allowed.
- -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 hold ing the lock.
- -c, --command command
Pass a single command to the shell with -c.
- -h, --help
Print a help message.
Written by H. Peter Anvin <email@example.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.
- SEE ALSO
linux.jgfs.net manual pages