Linux Job Scheduling

The at and batch commands put jobs into the at queue. What is the at queue? Well, there is a directory, /var/spool/at, which is accessible only to the dæmon user and the superuser (everything is available to the superuser). For each job, there is a file in the directory. The file is a shell script that sets up the environment and umask, cd's to the working directory and then runs the programs specified to at/batch in succession.

The commands go into the shell script exactly as they were typed/piped to at. Each is run in turn. If you used &, && or ; to background jobs, or make jobs dependent on one another, these will be observed.

Important note! The shell /bin/sh is used to run these jobs. If you normally use some other shell, such as tcsh, be aware that you can't use the semantics of that shell because /bin/sh will be used instead.

At this point, documenting the dæmon is rather anticlimactic. The atd dæmon examines the /var/spool/at directory. The names of the files actually encode their runtimes, queues and batch vs. at status. These files are shell scripts that set up the environment and run the job as described above. Output from the jobs is temporarily stored in /var/spool/at/spool until the jobs are completed, upon which the output is mailed to the invoking user.

at is less widely known than cron, but is in many ways the more powerful tool.

Potentially every user on the system has a crontab, which is a portmanteau word made from CRON TABle. The command to create, examine and modify crontabs is called crontab.

There are four ways to invoke crontab.

crontab <file>
crontab -l
crontab -r
crontab -e

Generally, crontab works on your own crontab. All four forms accept the -u option followed by a user name. In most cases, you will be able to view and edit other users' crontabs only if you are the superuser. You might want to check your system security if you are able to edit another user's crontabs. You probably have some problems!

The first form stores the named file as the crontab, replacing any current crontab. The second form dumps the current crontab to stdout. The third form removes the current crontab. The fourth form opens the current crontab in the editor specified by the VISUAL or EDITOR environment variable.

If you want to experiment with your crontab, it's a good idea to do a

crontab -l working-crontab

to save your current crontab if any, then use

crontab -e

crontab -r working-crontab

At this point, you may be wondering what a crontab looks like and what it does.

A crontab is a list of program command lines along with a specification of when to run that command line. It is a whitespace-delimited file with a newline between commands. Blank lines and lines beginning with a pound character (#) are ignored.

The fields are:

minute  hour   day of month    month    day of week    command

Any of the time fields may be an asterisk (*), which means “every”. Thus, an entry of:

* * * * * fetchmail

Ranges of numbers are allowed. So:

* 8-17 * * 1-5 fetchmail

will run fetchmail once a minute, between 8 a.m. and 5 p.m., Monday through Friday (0 or 7 represents Sunday).

Lists are allowed. Thus:

0,20,40 * * * 1-5 fetchmail

will run fetchmail at the hour, at 20 past, and again at 40 past the hour every hour of the day, Monday through Friday.

Step values are allowed after asterisks and ranges. They are of the form <range>/<step>. So,

*/5 8-17/2 * * * cp /var/log/* /log/backup

will run that cp command (just in case you had started thinking you could run only fetchmail) every five minutes in the 8 a.m., 10 a.m., noon, 2 p.m. and 4 p.m. hours of every day.

Finally, names may be used for months (jan-dec, case insensitive) and days of the week (sun-sat, case insensitive). The Red Hat man pages claim that you can't use names in ranges, but I gave it a try myself and it appeared to work correctly.