cron utility allows you to schedule a program to be run
periodically. Once you’ve defined a ‘cron job’, the operating system will make
sure that job is run at the times you’ve specified.
Jobs are defined in the ‘cron table’, which can be manipulated with the
An example job definition might look like this in the crontab.
The command to be run is
echo foo. The numbers preceding the command specify
when the command is to be run: minute, hour, day of the month, month, and the
day of the week.
This job will then run at 01:01 am on January 1st, and at the same time every
Monday in January (cron will run the job as long as at least one of the two day
fields matches the current time).
There are already plenty of tutorials for crontabs, so I won’t dwell on them to
man crontab and
man 5 crontab are good starting points), but I’ll
mention two other interesting parts of the crontab syntax.
In the first example, we’ve used the asterisk
* syntax to tell cron to match
any value of the field. In this case, cron will run the job at 01:01 am every
day of every month.
In the second example, the slash
/ syntax tells cron to run in steps on that
*/5 means cron will run the command every 5 minutes when the
hour is 01.
Kerberos jobs at CERN
If you want to run a job periodically, you won’t be around to enter your password every time the job runs, so your job won’t be able to see any of your files! The way around this restriction is to use acron, which will automatically provide your job context with the Kerberos token it needs to be able to have the same access rights as you.
It works in a very similar way to regular cron, but now you edit an ‘acron
table’ with the
acrontab -e command, and there’s one additional field you
need to specify for each row.
The extra field is before the command, still
echo foo, where here we’ve
lxplus.cern.ch. This field specifies the machine that acron will run
your job on. In this case, our job will run on an lxplus node, which
is usual cluster of machines that CERN users run interactive jobs on.
Differences to cron
The syntax allowed in an acrontab file is near-identical to that in a regular
crontab, except that step definitions, like
*/5 we saw earlier, aren’t
supported, and neither is a
* value in the minute field. This is to prevent
the acron system being overloaded with many jobs to run. (After all, CERN has
thousands of users, and whereas cron usually runs on your own machine that has
one or two users, acron needs to be able to run jobs for everyone.)
You can define jobs to run periodically on machines in the CERN network with
acron, and those jobs will have Kerberos tokens automatically.
You can manipulate the acron table with the