scheduling

(Written by Paul Cobbaut, https://github.com/paulcobbaut/)

Linux administrators use the at to schedule one time jobs. Recurring jobs are better scheduled with cron. The next two sections will discuss both tools.

one time jobs with at

at

Simple scheduling can be done with the at command, followed by a time specification. When you press Enter, you will get a prompt (at>) where you can type the commands you want to schedule. To finish the input, press Ctrl-D (that can also be used to exit a shell).

This example shows the scheduling of the date command at 22:01 (that will write its output to a log file) and the sleep command at 22:03.

student@linux:~$ at 22:01
at> date -Is >> /tmp/date.log
at> <EOT>
job 1 at Wed Aug  1 22:01:00 2007
student@linux:~$ at 22:03
at> sleep 10
at> <EOT>
job 2 at Wed Aug  1 22:03:00 2007
student@linux:~$

In real life you will hopefully be scheduling more useful commands ;-)

atq

It is easy to check when jobs are scheduled with the atq or at -l commands.

student@linux:~$ atq
1       Wed Aug  1 22:01:00 2007 a student
2       Wed Aug  1 22:03:00 2007 a student
student@linux:~$ at -l
1       Wed Aug  1 22:01:00 2007 a student
2       Wed Aug  1 22:03:00 2007 a student
student@linux:~$

The at command understands English words like tomorrow and teatime to schedule commands the next day and at four in the afternoon.

student@linux:~$ at 10:05 tomorrow
at> sleep 100
at> <EOT>
job 5 at Thu Aug  2 10:05:00 2007
student@linux:~$ at teatime tomorrow
at> tea
at> <EOT>
job 6 at Thu Aug  2 16:00:00 2007
student@linux:~$ atq
6       Thu Aug  2 16:00:00 2007 a student
5       Thu Aug  2 10:05:00 2007 a student
student@linux:~$

atrm

Jobs in the at queue can be removed with atrm.

student@linux:~$ atq
6       Thu Aug  2 16:00:00 2007 a student
5       Thu Aug  2 10:05:00 2007 a student
student@linux:~$ atrm 5
student@linux:~$ atq
6       Thu Aug  2 16:00:00 2007 a student
student@linux:~$

at.allow and at.deny

You can also use the /etc/at.allow and /etc/at.deny files to manage who can schedule jobs with at.

The /etc/at.allow file can contain a list of users that are allowed to schedule at jobs. When /etc/at.allow does not exist, then everyone can use at unless their username is listed in /etc/at.deny.

If none of these files exist, then everyone can use at.

cron

Where at is used to schedule one time jobs, cron is used to schedule recurring jobs. The cron daemon crond is started at boot time and runs in the background, checking every minute whether there are jobs to be executed.

crontab file

The crontab(1) command can be used to maintain the crontab(5) file. Each user can have their own crontab file to schedule jobs at a specific time. This time can be specified with five fields in this order: minute, hour, day of the month, month and day of the week. If a field contains an asterisk (*), then this means all values of that field.

The following example means : run script42 eight minutes after two, every day of the month, every month and every day of the week.

8 14 * * * script42

Run script8472 every month on the first of the month at 25 past midnight.

25 0 1 * * script8472

Run this script33 every two minutes on Sunday (both 0 and 7 refer to Sunday).

*/2 * * * 0 script33

Instead of these five fields, you can also type one of these: \@reboot, \@yearly or \@annually, \@monthly, \@weekly, \@daily or \@midnight, and \@hourly.

crontab command

Users should not edit the crontab file directly, instead they should type crontab -e which will use the editor defined in the EDITOR or VISUAL environment variable. Users can display their cron table with crontab -l.

cron.allow and cron.deny

The cron daemon crond is reading the cron tables, taking into account the /etc/cron.allow and /etc/cron.deny files.

These files work in the same way as at.allow and at.deny. When the cron.allow file exists, then your username has to be in it, otherwise you cannot use cron. When the cron.allow file does not exists, then your username cannot be in the cron.deny file if you want to use cron.

/etc/crontab

The /etc/crontab file contains entries for when to run hourly/daily/weekly/monthly tasks. It will look similar to this output.

SHELL=/bin/sh
PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin

20 3 * * *        root    run-parts --report /etc/cron.daily
40 3 * * 7        root    run-parts --report /etc/cron.weekly
55 3 1 * *        root    run-parts --report /etc/cron.monthly

the `/etc/cron.*` directories

The directories shown in the next screenshot contain the tasks that are run at the times scheduled in /etc/crontab. The /etc/cron.d directory is for special cases, to schedule jobs that require finer control than hourly/daily/weekly/monthly.

student@linux:~$ ls -ld /etc/cron.*
drwxr-xr-x 2 root root 4096 2008-04-11 09:14 /etc/cron.d
drwxr-xr-x 2 root root 4096 2008-04-19 15:04 /etc/cron.daily
drwxr-xr-x 2 root root 4096 2008-04-11 09:14 /etc/cron.hourly
drwxr-xr-x 2 root root 4096 2008-04-11 09:14 /etc/cron.monthly
drwxr-xr-x 2 root root 4096 2008-04-11 09:14 /etc/cron.weekly

Note that Red Hat uses anacron to schedule daily, weekly and monthly cron jobs.

[student@el:~]$ cat /etc/anacrontab
# /etc/anacrontab: configuration file for anacron

# See anacron(8) and anacrontab(5) for details.

SHELL=/bin/sh
PATH=/sbin:/bin:/usr/sbin:/usr/bin
MAILTO=root
# the maximal random delay added to the base delay of the jobs
RANDOM_DELAY=45
# the jobs will be started during the following hours only
START_HOURS_RANGE=3-22

#period in days   delay in minutes   job-identifier   command
1       5       cron.daily              nice run-parts /etc/cron.daily
7       25      cron.weekly             nice run-parts /etc/cron.weekly
@monthly 45     cron.monthly            nice run-parts /etc/cron.monthly

practice : scheduling

Schedule two jobs with at, display the at queue and remove a job.
As normal user, use crontab -e to schedule a script to run every four minutes.
As root, display the crontab file of your normal user.
As the normal user again, remove your crontab file.
Take a look at the cron files and directories in /etc and understand them. What is the run-parts command doing ?

solution : scheduling

Schedule two jobs with at, display the at queue and remove a job.

root@linux ~# at 9pm today
at> echo go to bed >> /root/todo.txt  
at> <EOT>
job 1 at 2010-11-14 21:00
root@linux ~# at 17h31 today
at> echo go to lunch >> /root/todo.txt
at> <EOT>
job 2 at 2010-11-14 17:31
root@linux ~# atq
2   2010-11-14 17:31 a root
1   2010-11-14 21:00 a root
root@linux ~# atrm 1
root@linux ~# atq
2   2010-11-14 17:31 a root
root@linux ~# date
Sun Nov 14 17:31:01 CET 2010
root@linux ~# cat /root/todo.txt 
go to lunch

As normal user, use crontab -e to schedule a script to run every four minutes.

student@linux ~$ crontab -e
no crontab for paul - using an empty one
crontab: installing new crontab

As root, display the crontab file of your normal user.

root@linux ~# crontab -l -u paul
*/4 * * * * echo `date` >> /home/paul/crontest.txt

As the normal user again, remove your crontab file.

student@linux ~$ crontab -r
student@linux ~$ crontab -l
no crontab for paul

Take a look at the cron files and directories in /etc and understand them. What is the run-parts command doing ?

run-parts runs a script in a directory by executing all the scripts located in the specified directory, one of /etc/cron.daily, /etc/cron.hourly, /etc/cron.weekly and /etc/cron.monthly.