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FLOCK(1)                        H. Peter Anvin                        FLOCK(1)



NAME
       flock - Manage locks from shell scripts

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

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

       flock [-sxun] [-w timeout] fd

DESCRIPTION
       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 new-
       grp(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.

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 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.

AUTHOR
       Written by H. Peter Anvin <hpaATzytor.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 MERCHANTABIL-
       ITY or FITNESS FOR A PARTICULAR PURPOSE.

SEE ALSO
       flock(2)

AVAILABILITY
       The  flock  command  is  part  of  the   util-linux-ng   package   and   is   available   from   ftp://ftp.ker-
       nel.org/pub/linux/utils/util-linux-ng/.



flock utility                     4 Feb 2006                          FLOCK(1)