*BASH User Commands Ubuntu 10.04.4 LTS Server coreutils
FLOCK(1)                        H. Peter Anvin                        FLOCK(1)

       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  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.

       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, how-
       ever, 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 com-
              mand 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 can-
              not 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 exe-
              cuting 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 <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-ng package and is available
       from ftp://ftp.kernel.org/pub/linux/utils/util-linux-ng/.

flock utility                     4 Feb 2006                          FLOCK(1)