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 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
       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  hold-
	      ing 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 lock-
	      file 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 PARTICU-
       LAR 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)