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flock

manage locks from shell scripts

flock [-sxon] [-w timeout] lockfile [-c] command...

flock [-sxon] [-w timeout] lockdir [-c] command...

flock [-sxun] [-w timeout] fd

This utility manages flock(2) locks from within shell scripts or the command line.

The first and second forms wraps the lock around the executing a command, in a manner similar to su(1) or newgrp(1). It locks a specified file or directory, which is created (assuming appropriate permissions), if it does not already exist.

The third form is convenient inside shell scripts, and is usually used the following manner:

(
flock -n 9 || exit 1
# ... commands executed under lock ...
) 9>/var/lock/mylockfile

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.

By default, if the lock cannot be immediately acquired, flock waits until the lock is available.

Obtain a shared lock, sometimes called a read lock.
Obtain an exclusive lock, sometimes called a write lock. This is the default.
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 (with an exit code of 1) rather than wait if the lock cannot be immediately acquired.
Fail (with an exit code of 1) if the lock cannot be acquired within seconds seconds. Decimal fractional values are allowed.
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.
Pass a single command to the shell with -c.
Print a help message.

Written by 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.

flock(2)

The flock command is part of the util-linux package and is available from ftp://ftp.kernel.org/pub/linux/utils/util-linux/.