9.7. Scheduling Tasks with cron
and atd
cron
is the daemon responsible for executing scheduled and recurring commands (every day, every week, etc.); atd
is that which deals with commands to be executed a single time, but at a specific moment in the future.
In a Unix system, many tasks are scheduled for regular execution:
By default, all users can schedule the execution of tasks. Each user has thus their own crontab in which they can record scheduled commands. It can be edited by running crontab -e
(its content is stored in the /var/spool/cron/crontabs/user
file).
The root user has their own crontab, but can also use the /etc/crontab
file, or write additional crontab files in the /etc/cron.d
directory. These last two solutions have the advantage of being able to specify the user identity to use when executing the command.
The cron package includes by default some scheduled commands that execute:
programs in the /etc/cron.hourly/
directory once per hour;
programs in /etc/cron.daily/
once per day;
programs in /etc/cron.weekly/
once per week;
programs in /etc/cron.monthly/
once per month.
Many Debian packages rely on this service: by putting maintenance scripts in these directories, they ensure optimal operation of their services.
9.7.2. Using the at
Command
The at
executes a command at a specified moment in the future. It takes the desired time and date as command-line parameters, and the command to be executed in its standard input. The command will be executed as if it had been entered in the current shell. at
even takes care to retain the current environment, in order to reproduce the same conditions when it executes the command. The time is indicated by following the usual conventions: 16:12
or 4:12pm
represents 4:12 pm. The date can be specified in several European and Western formats, including DD.MM.YY
(27.07.12
thus representing 27 July 2012), YYYY-MM-DD
(this same date being expressed as 2012-07-27
), MM/DD/[CC]YY
(ie., 12/25/12
or 12/25/2012
will be December 25, 2012), or simple MMDD[CC]YY
(so that 122512
or 12252012
will, likewise, represent December 25, 2012). Without it, the command will be executed as soon as the clock reaches the time indicated (the same day, or tomorrow if that time has already passed on the same day). You can also simply write “today” or “tomorrow”, which is self-explanatory.
$
at 09:00 27.07.14 <<END
>
echo "Don't forget to wish a Happy Birthday to Raphaël!" \
>
| mail lolando@debian.org
>
END
warning: commands will be executed using /bin/sh
job 31 at Fri Jul 27 09:00:00 2012
An alternative syntax postpones the execution for a given duration: at now + number
period
. The period
can be minutes
, hours
, days
, or weeks
. The number
simply indicates the number of said units that must elapse before execution of the command.
To cancel a task scheduled by cron
, simply run crontab -e
and delete the corresponding line in the crontab file. For at
tasks, it is almost as easy: run atrm task-number
. The task number is indicated by the at
command when you scheduled it, but you can find it again with the atq
command, which gives the current list of scheduled tasks.