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), 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>