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LOCKFILE(1)							  LOCKFILE(1)



NAME
  lockfile - conditional semaphore-file	creator

SYNOPSIS
  lockfile -sleeptime |	-r retries |
       -l locktimeout |	-s suspend | -!	| -ml |	-mu | filename ...

DESCRIPTION
  lockfile can be used to create one or	more semaphore	files.	 If  lockfile
  can't	 create	 all  the  specified files (in the specified order), it	waits
  sleeptime (defaults to 8) seconds and	retries	the  last  file	 that  didn't
  succeed.   You  can  specify	the  number of retries to do until failure is
  returned.  If	the number of retries is -1 (default,  i.e.   -r-1)  lockfile
  will retry forever.

  If the number	of retries expires before all files have been created,	lock-
  file	returns	 failure  and  removes	all the	files it created up till that
  point.

  Using	lockfile as the	condition of a loop in a shell	script	can  be	 done
  easily by using the -! flag to invert	the exit status.  To prevent infinite
  loops, failures for any reason other than the	lockfile already existing are
  not inverted to success but rather are still returned	as failures.

  All flags can	be specified anywhere on the command line, they	will be	 pro-
  cessed  when	encountered.   The command line	is simply parsed from left to
  right.

  All files created by lockfile	will be	read-only, and therefore will have to
  be removed with rm -f.

  If you specify a locktimeout then a lockfile will be removed by force	after
  locktimeout	seconds	  have	 passed	  since	  the	lockfile   was	 last
  modified/created (most likely	by some	other program that unexpectedly	 died
  a  long  time	 ago,  and  hence could	not clean up any leftover lockfiles).
  Lockfile is clock skew immune.  After	a lockfile has been removed by force,
  a  suspension	of suspend seconds (defaults to	16) is taken into account, in
  order	to prevent the inadvertent immediate removal  of  any  newly  created
  lockfile by another program (compare SUSPEND in procmail(1)).

  Mailbox locks

  If the permissions on	the system mail	spool directory	allow it, or if	lock-
  file	is  suitably  setgid,  it will be able to lock and unlock your system
  mailbox by using the options -ml and -mu respectively.









EXAMPLES
  Suppose you want to make sure	that access to the file	"important" is	seri-
  alised,  i.e.	no more	than one program or shell script should	be allowed to
  access it.  For simplicity's sake, let's suppose that	it is a	shell script.
  In this case you could solve it like this:
       ...
       lockfile	important.lock
       ...
       access_"important"_to_your_hearts_content
       ...
       rm -f important.lock
       ...
  Now if all the scripts that access "important" follow	this  guideline,  you
  will	be  assured  that  at  most  one script	will be	executing between the
  `lockfile' and the `rm' commands.

ENVIRONMENT

  LOGNAME		 used as a hint	to determine the invoker's loginname

FILES

  /etc/passwd		 to verify and/or  correct  the	 invoker's  loginname
			 (and to find out his HOME directory, if needed)

  /var/spool/mail/$LOGNAME.lock
			 lockfile for the  system  mailbox,  the  environment
			 variables present in here will	not be taken from the
			 environment, but will be determined  by  looking  in
			 /etc/passwd

SEE ALSO
  rm(1), mail(1), binmail(1), sendmail(8), procmail(1)

DIAGNOSTICS

  Filename too long, ... Use shorter filenames.

  Forced unlock	denied on "x"
			 No write permission in	the directory where  lockfile
			 "x"  resides,	or  more  than one lockfile trying to
			 force a lock at exactly the same time.

  Forcing lock on "x"	 Lockfile "x" is going to be removed by	force because
			 of a timeout (compare LOCKTIMEOUT in procmail(1)).

  Out of memory, ...	 The system is out of swap space.

  Signal received, ...	 Lockfile will remove anything it  created  till  now
			 and terminate.

  Sorry, ...		 The retries limit has been reached.

  Truncating "x" and retrying lock
			 "x" does not seem to be a valid filename.

  Try praying, ...	 Missing subdirectories	or insufficient	privileges.









BUGS
  Definitely less than one.




WARNINGS
  The behavior of the -! flag, while useful, is	not necessarily	intuitive  or
  consistent.	When  testing  lockfile's  return value, shell script writers
  should consider carefully whether they want to use the -! flag, simply  re-
  verse	 the  test, or do a switch on the exact	exitcode.  In general, the -!
  flag should only be used when	lockfile is the	conditional of a loop.

MISCELLANEOUS
  Lockfile is NFS-resistant and	eight-bit clean.

NOTES
  Calling up lockfile with the -h or -?	options	will cause it  to  display  a
  command-line	help page.  Calling it up with the -v option will cause	it to
  display its version information.

  Multiple -! flags will toggle	the return status.

  Since	flags can occur	anywhere on the	command	line, any  filename  starting
  with a '-' has to be preceded	by './'.

  The number of	retries	will not be reset when any following  file  is	being
  created  (i.e.  they	are  simply  used  up).	 It can, however, be reset by
  specifying -rnewretries after	every file on the command line.

  Although files with any name can be used as lockfiles, it is	common	prac-
  tice	to  use	 the extension `.lock' to lock mailfolders (it is appended to
  the mailfolder name).	 In case one does not want to have to worry about too
  long	filenames and does not have to conform to any other lockfilename con-
  vention, then	an excellent way to generate a lockfilename corresponding  to
  some	already	 existing  file	is by taking the prefix	`lock.'	and appending
  the i-node number of the file	which is to be locked.

SOURCE
  This program is part of the procmail mail-processing-package (v3.14) avail-
  able at http://www.procmail.org/ or ftp.procmail.org in pub/procmail/.

MAILINGLIST
  There	exists a mailinglist for questions relating to	any  program  in  the
  procmail package:
       <procmail-usersATprocmail.org>
	 for submitting	questions/answers.
       <procmail-users-requestATprocmail.org>
	 for subscription requests.

  If you would like to stay informed about new versions	and official  patches
  send a subscription request to
       procmail-announce-requestATprocmail.org
  (this	is a readonly list).

AUTHOR
  Stephen R. van den Berg
       <srbATcuci.nl>