plan9 man page for flock

FLOCK(1)							   User Commands							  FLOCK(1)

NAME
       flock - manage locks from shell scripts

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

DESCRIPTION
       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 execution of a command, in a manner similar to su(1) or newgrp(1).  They
       lock 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 an open file by its file descriptor number.	See the examples below for how that can be used.

OPTIONS
       -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 conflicting lock exists, or the -w option is in use, and the timeout 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 otherwise 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 acquired.  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.

       -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 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.

       -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 --nonblock.

       --verbose
	      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.

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, 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.

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 <https://www.kernel.org/pub/linux/utils
       /util-linux/>.

util-linux							     July 2014								  FLOCK(1)