lockfile(1) - phpMan

LOCKFILE(1) General Commands Manual LOCKFILE(1)

NAME
lockfile - conditional semaphore-file creator

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 en-
countered. 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 deter-
mined 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 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 (v3.23pre) 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 sub-
scription request to
procmail-announce-request AT procmail.org
(this is a readonly list).

AUTHORS
Stephen R. van den Berg
<srb AT cuci.nl>
Philip A. Guenther
<guenther AT sendmail.com>

BuGless 2001/06/23 LOCKFILE(1)

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