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LOCKFILE(1)		    General Commands Manual		   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 lock-
       file 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,
       lockfile	 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 in-
       finite 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
       processed  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 modi-
       fied/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	 proc-
       mail(1)).

   Mailbox locks
       If  the	permissions on the system mail spool directory allow it, or if
       lockfile	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
       serialised, 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	login-
			      name

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

       /var/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 look-
			      ing 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 lock-
			      file "x" resides,	or more	than one lockfile try-
			      ing to force a lock at exactly the same time.

       Forcing lock on "x"    Lockfile "x" is going to be removed by force be-
			      cause of a timeout (compare LOCKTIMEOUT in proc-
			      mail(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	privi-
			      leges.

BUGS
       Definitely less than one.

WARNINGS
       The  behavior  of  the -!  flag,	while useful, is not necessarily intu-
       itive or	consistent.   When  testing  lockfile's	 return	 value,	 shell
       script  writers	should consider	carefully whether they want to use the
       -!  flag, simply	reverse	the test, or do	a switch on  the  exact	 exit-
       code.   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	start-
       ing with	a '-' has to be	preceded by './'.

       The  number of retries will not be reset	when any following file	is be-
       ing 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
       practice	to use the extension `.lock' to	lock mailfolders  (it  is  ap-
       pended  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 oth-
       er  lockfilename	 convention, then an excellent way to generate a lock-
       filename	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  (v3.22)
       available  at http://www.procmail.org/ or ftp.procmail.org in pub/proc-
       mail/.

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

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

AUTHORS
       Stephen R. van den Berg
	      <srb@cuci.nl>
       Philip A. Guenther
	      <guenther@sendmail.com>

BuGless				  2001/06/23			   LOCKFILE(1)

NAME | SYNOPSIS | DESCRIPTION | EXAMPLES | ENVIRONMENT | FILES | SEE ALSO | DIAGNOSTICS | BUGS | WARNINGS | MISCELLANEOUS | NOTES | SOURCE | MAILINGLIST | AUTHORS

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