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FLOCK(1)				  User Commands 				 FLOCK(1)

NAME
       flock - manage locks from shell scripts

SYNOPSIS
       flock [options] <file|directory> <command> [command args]
       flock [options] <file|directory> -c <command>
       flock [options] <file descriptor number>

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

       The first and second forms wrap the lock around the executing a command, in a manner simi-
       lar 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.  By default, if the lock
       cannot be immediately acquired, flock waits until the lock is available.

       The third form uses open file by file descriptor number.  See examples  how  that  can  be
       used.

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 rather than wait if the lock cannot	be  immediately  acquired.   See  the  -E
	      option for the exit code used.

       -w, --wait, --timeout seconds
	      Fail  if the lock cannot be acquired within seconds.  Decimal fractional values are
	      allowed.	See the -E option for the exit code used.

       -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.

       -E, --conflict-exit-code number
	      The exit code used when the -n option is in use, and the conflicting  lock  exists,
	      or the -w option is in use, and the timeout is reached. The default value is 1.

       -c, --command command
	      Pass a single command, without arguments, to the shell with -c.

       -h, --help
	      Print a help message.

       -V, --version
	      Show version number and exit.

EXAMPLES
       shell1> flock /tmp -c cat
       shell2> flock -w .007 /tmp -c echo; /bin/echo $?
	      Set exclusive lock to directory /tmp and the second command will fail.

       shell1> flock -s /tmp -c cat
       shell2> flock -s -w .007 /tmp -c echo; /bin/echo $?
	      Set  shared  lock  to  directory /tmp and the second command will not fail.  Notice
	      that attempting to get exclusive lock with second command would fail.

       shell> flock -x local-lock-file echo 'a b c'
	      Grab the exclusive lock "local-lock-file" before running echo with 'a b c'.

       (
	 flock -n 9 || exit 1
	 # ... commands executed under lock ...
       ) 9>/var/lock/mylockfile
	      The form is convenient inside shell scripts.   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.

       [ "${FLOCKER}" != "$0" ] && exec env FLOCKER="$0" flock -en "$0" "$0" "$@" || :
	      This  is useful boilerplate code for shell scripts.  Put it at the top of the shell
	      script you want to lock and it'll automatically lock itself on the first	run.   If
	      the env var $FLOCKER is not set to the shell script that is being run, then execute
	      flock and grab an exclusive non-blocking lock (using the script itself as the  lock
	      file)  before re-execing itself with the right arguments.  It also sets the FLOCKER
	      env var to the right value so it doesn't run again.

EXIT STATUS
       The command uses sysexits.h return values for everything else but  an  options  -n  or  -w
       failures which return either the value given by the -E option, or 1 by default.

AUTHOR
       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  Linux  Kernel
       Archive <ftp://ftp.kernel.org/pub/linux/utils/util-linux/>.

util-linux				  September 2011				 FLOCK(1)
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