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Linux 2.6 - man page for flock (linux section 1)

FLOCK(1)				  H. Peter Anvin				 FLOCK(1)

NAME
       flock - Manage locks from shell scripts

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

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

       flock [-sxun] [-w timeout] fd

DESCRIPTION
       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 sim-
       ilar 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 man-
       ner:

       (
	 flock -n 9
	 # ... 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.

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 sec-
	      onds.  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 holding 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 (C) 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 package and is available  from  ftp://ftp.ker-
       nel.org/pub/linux/utils/util-linux/.

flock utility				    4 Feb 2006					 FLOCK(1)


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