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at does exactly what it sounds like -- it does a job at a set time. For instance, the command who > /tmp/who-list.tmp | at 16:30 September 1 will run who, save the output in a file (/tmp/who-list.tmp), and do it at 16:30 on September 1 of the current year. If you want to run it for a future year, you can specify the year on the command line after the day of the month. When you invoke it, at will assign a job number and will confirm the details to the screen:

job 412345 at 2005-09-01 16:30

You can also schedule work by calling at from the command line, typing in the details of the work to be carried out, and then pressing Ctrl-D to exit:

at 16:30 September 1
who > /tmp/who-list.tmp
Ctrl-D

If you happen to type in a time that has passed (e.g. typing in 16:30 when it is actually 16:35), at will execute the commands immediately.

You can also save your commands to a file and then submit your file to at:
at 16:30 September 1 or cat myfile | at 16:30 September 1

at lets you schedule tasks to run at noon or midnight, today or tomorrow, using those keywords. Best of all, if you want to run your commands at 16:00, use at teatime. How more civilized can you get?

If you don't like using the 24-hour format, at is just as happy using a.m. and p.m.; at 9 AM September 1 is the same as at 09:00 September 1.

Along with absolute dates and times, you can also define relative times by using a plus-sign along with the keyword minutes, hours, days, or weeks, as in at teatime tomorrow + 1 week. You can't use seconds. at is, however, very obliging about whether you use plurals or not; it's fine to say at 09:00 September 2 + 7 hours. You can also use the now statement, as in at now + 3 hours. If you do make a mistake, at will return the error Garbled time.

at decides who can schedule jobs according to the contents (or absence) of two file -- /etc/at.allow and /etc/at.deny. If /etc/at.allow exists then only users listed in it may create new jobs. If the file doesn't exist then at will look for the file /etc/at.deny and will stop any users listed there from creating schedules. If neither file exists then only root may use at. By default most system have an empty /etc/at.deny file, meaning that all users may use at.

Checking what at's up to

So you've been busy setting up your schedule -- but how do you know what's going to run and when? If you use the command at -l or atq and if there are jobs waiting to be run, you will see the command return something like:

4123418      2005-08-10 16:00 a bainm
4123415      2006-06-01 16:30 a bainm

In this directory are a number of at-related files -- one for each job waiting to be processed -- though it can be difficult to work out which file relates to which job.

If you need to remove a job, and you're able to work out which job it is that you need to delete, then use the command at -d or atrm, as in at -d 4123418 or atrm 4123418 to remove job number 4123418.

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