flock man page on aLinux

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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  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.

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 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.

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-ng package and is available
       from ftp://ftp.kernel.org/pub/linux/utils/util-linux-ng/.

flock utility			  4 Feb 2006			      FLOCK(1)
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