flock - Unix, Linux Command

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 conveninent 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

Tag	Description
-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 <[email protected]>.