Provided by: procmail_3.22-21ubuntu0.2_amd64 
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 (v3.23pre) 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@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>