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flock

NAME

flock - Manage locks from shell scripts

SYNOPSIS

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

flock [-sxun] [-w timeout] fd

DESCRIPTION

This utility manages flock(2) 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 su(1) or newgrp(1). It locks a specified file, which is created (assuming appropriate permissions), if it does not already exist.

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

(

  flock -s 200

  # ... commands executed under lock ...
) 200>/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.

OPTIONS

-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 default.

-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.

-n, --nb, --nonblock

Fail (with an exit code of 1) rather than wait if the lock cannot be immediately acquired.

-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.

AUTHOR

Written by H. Peter Anvin <hpa@zytor.com>.

COPYRIGHT

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)

AVAILABILITY

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