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 || exit 1
         # ... 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 © 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/.