Comments on: Scheduling a 1-time future job in OSX?

Question: Scheduling a 1-time future job in OSX?

aberrant — Sun, 11 May 2008 04:17:24 -0800

Mac OS X (Leopard): at(1) replacement?

I need to run a shell script, once, at a specified time in the future (ideally, I'd like to be able to specify "X minutes from now, do this", equivalent to "at now+X minutes").

However, at(1) is broken on OS X (and it's not just because atrun is disabled: at's got some serious time conversion errors) and I can't figure out how to do this via launchd.

Running 10.5.2 if it makes a difference.

Short of using sleep, is there an easy way to queue jobs up for future processing but make it so that the job is only run once, then dequeued/deleted?

By: sophist

sophist — Sun, 11 May 2008 04:38:45 -0800

I think you want to do this with cron. Googling cron and osx should lead you down the correct path, but here are a few top hits anyway [1, 2, 3] and here is a small utility GUI which promises to work although I have no firsthand knowledge of said promise!

By: dseaton

dseaton — Sun, 11 May 2008 05:12:37 -0800

Agree, cron is the way to do this, and CronniX is the best tool for the job.

By: aberrant

aberrant — Sun, 11 May 2008 08:19:48 -0800

Sorry if I wasn't clearer: cron won't do what I want because it's difficult to set it up to schedule a one-time job (how do you automatically remove a cron entry easily)? and the GUI frontends are not useful because I'm scripting the whole thing. I need to programatically submit jobs to run once in the future.

By: ijoshua

ijoshua — Sun, 11 May 2008 08:47:04 -0800

iCal has the option to run a script on an event's alarm. I don't know how it will handle an event that would have occurred while the computer was asleep.

By: ijoshua

ijoshua — Sun, 11 May 2008 08:48:18 -0800

Oh, I missed your reply that your entire workflow is scripted. Perhaps you could use AppleScript to create iCal events with alarms?

By: dreadpiratesully

dreadpiratesully — Sun, 11 May 2008 08:53:33 -0800

If you use launchd instead of cron, I don't think you can set up a one-time job but it's easy to programatically remove the job - 'launchctl remove jobname'. You could use a script that does whatever you need it to and then removes itself from launchd, or at least disables itself.

By: namewithoutwords

namewithoutwords — Sun, 11 May 2008 09:08:38 -0800

cron has enough granularity that you can specify the job only run once, ever....

as far as removing entries programatically, why not simply overwrite the crontab for the user in question with a new one every time you need to run a new job? have the script dump the cron entry out to a (temp) file, and then call crontab to read that file in?

By: Mikey-San

Mikey-San — Sun, 11 May 2008 09:18:13 -0800

However, at(1) is broken on OS X

Define "broken"?

By: aberrant

aberrant — Sun, 11 May 2008 09:37:57 -0800

From the bug report I submitted:

* SUMMARY
at(1) is queueing jobs with the same jobid (688128) and wildly incorrect times.

sh-3.2# ls -l /usr/bin/at; md5 /usr/bin/at
-r-sr-xr-x 4 root wheel 69552 Dec 10 14:40 /usr/bin/at
MD5 (/usr/bin/at) = 9960f880ca5fe80853ec9adee0494479

* STEPS TO REPRODUCE
One example:
echo "ls" | at now+5 minutes

* RESULTS
sh-3.2# date; echo "ls" | at now+5 minutes
Sun May 11 09:11:05 PDT 2008
job 11 at Sun May 11 09:16:00 2008
sh-3.2# at -l
688128 Tue Sep 24 21:01:12 1912

expected:

sh-3.2# at -l
11 Sun 11 May 09:16:00 2008

* REGRESSION
Problem occurs consistently, but the queue time differs.

* NOTES
May be related to the bug in leave(1) (bug ID 5835373).

By: sergent

sergent — Sun, 11 May 2008 09:39:19 -0800

I think that at works, but it's the BSD at, not the GNU one, so it may not accept all the command line flags that you think it does. Have you read the man page?

By: sergent

sergent — Sun, 11 May 2008 09:40:52 -0800

Hm, I'm probably wrong.

By: aberrant

aberrant — Sun, 11 May 2008 09:43:21 -0800

Yeah, read the manpage, this should work. Nothing submitted to at(1) is queued at the proper time.

By: sergent

sergent — Sun, 11 May 2008 09:43:28 -0800

Anyway, you could still write your own cheap version of at in a shell script and run it as a cron job.

By: sergent

sergent — Sun, 11 May 2008 09:57:14 -0800

Well, I found the bug. The source code is here. The stupid makefile sets -DDEFAULT_AT_QUEUE to \"a\" instead of \'a\', which breaks everything. You can work around this by adding the '-q a' flag, or by fixing the makefile.

By: aberrant

aberrant — Sun, 11 May 2008 10:04:12 -0800

FANTASTIC. Just updated the bug report. Thanks, sergent!

By: ctmf

ctmf — Sun, 11 May 2008 11:59:55 -0800

Wow. AskMe above and beyond.

By: sbutler

sbutler — Sun, 11 May 2008 12:03:02 -0800

Ooooh... good catch. DEFAULT_BATCH_QUEUE also needs to be changed then.

By: aberrant

aberrant — Sun, 11 May 2008 13:07:23 -0800

Yup. Already added to the bug report. Hopefully this can be fixed.

leave(1) is suffering from possibly similar timing issues, but to the best of my knowledge, it's not using at(1).

By: sergent

sergent — Sun, 11 May 2008 22:17:09 -0800

Just in case it wasn't clear from my earlier comment, you can use at -q a instead of at to work around the problem. The bug is just in the setting of default for the "-q" flag - the Makefile is setting the default to a string instead of a character, and C just omits a compiler warning for this rather than failing to compile, and Apple evidently doesn't use -Werror.

I'm pretty sure that leave is having a different problem, since it takes no command line flags.