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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 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 -n 9
         # ... commands executed under lock ...
       ) 9>/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 holding 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 <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.

SEE ALSO
       flock(2)

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

flock utility                     4 Feb 2006                          FLOCK(1)