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       flock [options] <file|directory> <command> [command args]
       flock [options] <file|directory> -c <command>
       flock [options] <file descriptor number>


       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.


       -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.

       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 else but an
       options -n or -w failures which return either the value given by the -E
       option, or 1 by default.


       H. Peter Anvin <hpa@zytor.com>


       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  <ftp://ftp.kernel.org/pub/linux/utils/util-
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