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/.