man > LOCKFILE(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 re‐
       tries 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  encoun‐
       tered.  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  sec‐
       onds have passed since the lockfile was last modified/created (most likely by some other pro‐
       gram  that unexpectedly died a long time ago, and hence could not clean up any leftover lock‐
       files).  Lockfile is clock skew immune.  After a lockfile has been removed by force,  a  sus‐
       pension  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  SUS‐
       PEND 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), 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  ex‐
       tension  `.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  (v3.24)  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 AT procmail.org>
                     for submitting questions/answers.
              <procmail-users-request AT procmail.org>
                     for subscription requests.

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

AUTHORS
       Stephen R. van den Berg
              <srb AT cuci.nl>

                                               BuGless                                   LOCKFILE(1)