lockfile



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) 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
       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
       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/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 reverse 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
       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 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)

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