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

       -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)