FLOCK(1) H. Peter Anvin FLOCK(1)
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 man-
ner 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, how-
ever, 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 com-
mand 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 can-
not 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 exe-
cuting command. This is useful if command spawns a child pro-
cess 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)
flock utility 4 Feb 2006 FLOCK(1)
