cron
is the daemon responsible for executing scheduled and recurring commands (every hour, every day, every week, etc.). atd
deals with commands to be executed a single time, but at a specific moment in the future.
locate
program;
crontab -e
(its content is stored in the /var/spool/cron/crontabs/user
file).
/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.
crontab
entry describes a scheduled command with the six (or seven) following fields:
Sun
, Mon
, etc.);
/etc/crontab
file and in the fragments located in /etc/cron.d/
, but not in the users' own crontab files);
a-b
describes the interval of all the values between a
and b
. The syntax a-b/c
describes the interval with an increment of c
(example: 0-10/2
means 0,2,4,6,8,10
). An asterisk *
is a wildcard, representing all possible values.
Example 9.2. Sample user crontab
file
#Format #min hour day mon dow command # Download data every night at 7:25 pm 25 19 * * * $HOME/bin/get.pl # 8:00 am, on weekdays (Monday through Friday) 00 08 * * 1-5 $HOME/bin/dosomething # every two hours * */2 * * * $HOME/bin/dosomethingelse # Restart the IRC proxy after each reboot @reboot /usr/bin/dircproxy
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
(27.07.22
thus representing 27 July 2022), YYYY-MM-DD
(this same date being expressed as 2022-07-27
), MM/DD/[CC]YY
(i.e., 12/25/22
or 12/25/2022
will be December 25, 2022), or simple MMDD[CC]YY
(so that 122522
or 12252022
will, likewise, represent December 25, 2022). 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 27.07.22 <<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 1 at Wed Jul 27 09:00:00 2022
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.
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.