On Tue, 4 Jan 2005 22:48:22 -0500, Paul W. Abrahams wrote:

Is there lurking somewhere in bash or as a utility program something I can use on a command line to construct a semaphore? The idea is that I have some resource and a couple of processes that want that resource. I want to make sure that if one gets the resource, the other is locked out until the first one is finished.

For what it's worth, here's the specific case I'm interested in. I want to back up my home directory once a day. Easy enough, it seems -- create a cron job that does the deed at noon each day. But what if my machine isn't turned on at noon (since I don't leave it on all the time)? Again -- easy enough: do the backup when the machine is turned on, and set a signal indicating that it's been done. If the signal is set at noon, then don't do the backup.

But things get interesting if I turn the machine on very slightly before noon. Then it's possible that the job will either get done twice or not at all if I don't handle the signal correctly. That's the problem that semaphores are supposed to solve.

Paul

I don't know of one, but you can write your own in c code pretty easily. #include main() { fd = open("/tmp/my_semaphore", O_CREAT | O_EXCL); if (fd >= 0) exit(0); // return success exit (-1) // return failure } That is how semaphores were written in UNIX 20 years ago, and it should still work. I may have the return values wrong for how the shell treats success / failure, but I think the above is right. Then in your shell script, you "rm /tmp/my_semapore" to release the semaphore. See, there is value in having us old farts hanging around. HTH Greg -- Greg Freemyer

Paul, On Tuesday 04 January 2005 19:48, Paul W. Abrahams wrote:

Is there lurking somewhere in bash or as a utility program something I can use on a command line to construct a semaphore?

I don't know about lurking, though "apropos lock" produces 234 hits on my system. Narrowing it down to commands (i.e., excluding library functions) by filtering the apropos output through "grep '(1'" cuts that down to a mere 30, which makes it easy to pick out "lockfile": % man lockfile LOCKFILE(1) LOCKFILE(1) NAME lockfile - conditional semaphore-file creator SYNOPSIS lockfile -sleeptime | -r retries | -l locktimeout | -s suspend | -! | -ml | -mu | file­ name ... 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. ... I believe this will do what you want.

...

Paul

And by the way, I did not know of this command before you asked. Randall Schulz