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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 there-
fore 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 environ-
ment, 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 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 (com-
pare 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 insuffi-
cient privileges.
BUGS
Definitely less than one.
WARNINGS
The behavior of the -! flag, while useful, is not necessari-
ly intuitive or consistent. When testing lockfile's return
value, shell script writers should consider carefully wheth-
er 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 mail-
folders (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 con-
vention, 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
(v3.15.1) 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:
<[email protected]>
for submitting questions/answers.
<[email protected]>
for subscription requests.
If you would like to stay informed about new versions and
official patches send a subscription request to
[email protected]
(this is a readonly list).
AUTHORS
Stephen R. van den Berg
<[email protected]>
Philip A. Guenther
<[email protected]>
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