lockfile
can be used to create one or more
semaphorefiles.
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:
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
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.
