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       flock - Manage locks from shell scripts


       flock [-sxon] [-w timeout] lockfile [-c] command...

       flock [-sxon] [-w timeout] lockdir [-c] command...

       flock [-sxun] [-w timeout] fd


       This  utility  manages  flock(2) locks from within shell scripts or the
       command line.

       The first and second forms  wraps  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.

       The  third form is convenient inside shell scripts, and is usually used
       the following manner:

         flock -s 200
         # ... commands executed under lock ...
       ) 200>/var/lock/mylockfile

       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.

       By  default,  if  the  lock cannot be immediately acquired, flock waits
       until the lock is available.


       -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 holding the lock.

       -n, --nb, --nonblock
              Fail  (with  an  exit  code  of  1) rather than wait if the lock
              cannot be immediately acquired.

       -w, --wait, --timeout seconds
              Fail (with an exit code of 1) if the  lock  cannot  be  acquired
              within  seconds seconds.  Decimal fractional values are allowed.

       -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 hold ing the lock.

       -c, --command command
              Pass a single command to the shell with -c.

       -h, --help
              Print a help message.


       Written by H. Peter Anvin <>.


       Copyright © 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-ng package and is available