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. This
screenshot shows the scheduling of the date command at 22:01 and the
sleep command at 22:03.
root@linux:~# at 22:01
at> date
at> <EOT>
job 1 at Wed Aug 1 22:01:00 2007
root@linux:~# at 22:03
at> sleep 10
at> <EOT>
job 2 at Wed Aug 1 22:03:00 2007
root@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.
root@linux:~# atq
1 Wed Aug 1 22:01:00 2007 a root
2 Wed Aug 1 22:03:00 2007 a root
root@linux:~# at -l
1 Wed Aug 1 22:01:00 2007 a root
2 Wed Aug 1 22:03:00 2007 a root
root@linux:~#
The at command understands English words like tomorrow and teatime to schedule commands the next day and at four in the afternoon.
root@linux:~# at 10:05 tomorrow
at> sleep 100
at> <EOT>
job 5 at Thu Aug 2 10:05:00 2007
root@linux:~# at teatime tomorrow
at> tea
at> <EOT>
job 6 at Thu Aug 2 16:00:00 2007
root@linux:~# atq
6 Thu Aug 2 16:00:00 2007 a root
5 Thu Aug 2 10:05:00 2007 a root
root@linux:~#
atrm
Jobs in the at queue can be removed with atrm.
root@linux:~# atq
6 Thu Aug 2 16:00:00 2007 a root
5 Thu Aug 2 10:05:00 2007 a root
root@linux:~# atrm 5
root@linux:~# atq
6 Thu Aug 2 16:00:00 2007 a root
root@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
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
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
/etc/cron.*
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
/etc/cron.*
Note that Red Hat uses anacron to schedule daily, weekly and monthly
cron jobs.
root@linux:/etc# cat 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
root@linux:/etc#
practice : scheduling
1. Schedule two jobs with at, display the at queue and remove a
job.
2. As normal user, use crontab -e to schedule a script to run every
four minutes.
3. As root, display the crontab file of your normal user.
4. As the normal user again, remove your crontab file.
5. Take a look at the cron files and directories in /etc and
understand them. What is the run-parts command doing ?
solution : scheduling
1. 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
2. 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
3. As root, display the crontab file of your normal user.
root@linux ~# crontab -l -u paul
*/4 * * * * echo `date` >> /home/paul/crontest.txt
4. As the normal user again, remove your crontab file.
student@linux ~$ crontab -r
student@linux ~$ crontab -l
no crontab for paul
5. 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