lockfile - conditional semaphore-file creator
| -r retries
| -s suspend
can be used to create one or more semaphore files
If lockfile can't create all the specified files (in the specified order), it
(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.,
) 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
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
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
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
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:
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.
- used as a hint to determine the invoker's loginname
- to verify and/or correct the invoker's loginname (and to find out his HOME
directory, if needed)
- lockfile for the system mailbox, the environment variables present in here
will not be taken from the environment, but will be determined by looking
- 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.
Definitely less than one.
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 -!
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.
Lockfile is NFS-resistant and eight-bit clean.
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.
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
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.
This program is part of the procmail mail-processing-package
available at http://www.procmail.org/ or ftp.procmail.org in
There exists a mailinglist for questions relating to any program in the procmail
for submitting questions/answers.
for subscription requests.
If you would like to stay informed about new versions and official patches send
a subscription request to
(this is a readonly list).
Stephen R. van den Berg
Philip A. Guenther