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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  com-
       mand,  in a manner similar 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 imme-
       diately 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  com-
	      mand 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 exe-
	      cuting 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 con-
	      flicting lock exists, or the -w option is in use, and the	 time-
	      out 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 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.

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)