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RedHat 9 (Linux i386) - man page for lockfile (redhat section 1)

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 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)  lock-
       file 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 infinite loops, failures for any rea-
       son 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	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 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 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  deter-
			      mined 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 reverse the test, or do a switch on the  ex-
       act  exitcode.	In  general, the -!  flag should only be used when lockfile is the condi-
       tional 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 ev-
       ery 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 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  convention, 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  (v3.22)  available 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-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  sub-
       scription 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)


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