LOCKFILE(1)LOCKFILE(1)NAMElockfile - conditional semaphore-file creator
SYNOPSISlockfile-sleeptime | -r retries |
-l locktimeout | -s suspend | -! | -ml | -mu | file-
name ...
DESCRIPTIONlockfile 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 pre-
vent the inadvertent immediate removal of any newly cre-
ated 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 sim-
plicity'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' com-
mands.
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 ALSOrm(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 directo-
ry 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 neces-
sarily intuitive or consistent. When testing lockfile's
return value, shell script writers should consider care-
fully whether they want to use the -! flag, simply re-
verse 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 lock-
filename 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-pack-
age (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>
BuGless 2001/06/23 LOCKFILE(1)
