lockfile



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 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
       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 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)

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