flock

       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 execu-
       tion  of  a  command,  in a manner similar to su(1) or newgrp(1).  They
       lock a specified file or directory, which is created  (assuming  appro-
       priate  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  con-
              flicting  lock exists, or the -w option is in use, and the time-
              out is reached.  The default value is 1.

       -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  exe-
              cuting  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  com-
              mand group may have forked a background process which should not
              be holding 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  --non-

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,  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  PARTICULAR
       PURPOSE.
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