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FLOCK(1)			 User Commands			      FLOCK(1)

       flock - manage locks from shell scripts

       flock [options] file|directory command [arguments]
       flock [options] file|directory -c command
       flock [options] number

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

       The first and second of the above forms wrap the	lock around the	execu-
       tion  of	 a  command,  in a manner similar to su(1) or newgrp(1).  They
       lock a specified	file or	directory, which is created  (assuming	appro-
       priate  permissions)  if	it does	not already exist.  By default,	if the
       lock cannot be immediately acquired, flock  waits  until	 the  lock  is

       The  third  form	 uses an open file by its file descriptor number.  See
       the examples below for how that can be used.

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

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

       -F, --no-fork
	      Do not fork before executing command.  Upon execution the	 flock
	      process is replaced by command which continues to	hold the lock.
	      This option is incompatible with --close as there	 would	other-
	      wise be nothing left to hold the lock.

       -e, -x, --exclusive
	      Obtain  an  exclusive lock, sometimes called a write lock.  This
	      is the default.

       -n, --nb, --nonblock
	      Fail rather than wait if the  lock  cannot  be  immediately  ac-
	      quired.  See the -E option for the exit code used.

       -o, --close
	      Close  the file descriptor on which the lock is held before exe-
	      cuting command.  This  is	 useful	 if  command  spawns  a	 child
	      process which should not be holding the lock.

       -s, --shared
	      Obtain a shared lock, sometimes called a read lock.

       -u, --unlock
	      Drop  a lock.  This is usually not required, since a lock	is au-
	      tomatically 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.

       -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.  The zero number of seconds is interpreted as	--non-

	      Report how long it took to acquire the lock,  or	why  the  lock
	      could not	be obtained.

       -V, --version
	      Display version information and exit.

       -h, --help
	      Display help text	and exit.

       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

       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 automati-
	      cally 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.

       The  command  uses sysexits.h return values for everything, except when
       using either of the options -n or -w which report a failure to  acquire
       the lock	with a return value given by the -E option, or 1 by default.

       When  using  the	 command variant, and executing	the child worked, then
       the exit	status is that of the child command.

       H. Peter	Anvin <>

       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


       The flock command is part of the	util-linux package  and	 is  available
       from Linux Kernel Archive <

util-linux			   July	2014			      FLOCK(1)


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