Exercise: Schedule a job

Schedule a job

• Create a personal crontab entry and read the cron log.
• Diagnose and fix the percent-sign problem.
• Create a system job in /etc/cron.d.

Start the exercise

On workstation, start the exercise. It removes cron jobs and output files of an earlier run from servera and makes sure crond runs.

[student@workstation ~]$ lab start sa-cron

Later tasks assume that you are logged in to the server they name: ssh student@servera from workstation, then sudo -i for a root shell.

Look at what is scheduled

On servera as root (sudo -i), list the system cron files, show 0hourly, and check whether student has a crontab.

[student@workstation ~]$ ssh student@servera
[student@servera ~]$ sudo -i
[root@servera ~]# ls /etc/cron.d
0hourly  dailyjobs
[root@servera ~]# cat /etc/cron.d/0hourly
# Run the hourly jobs
SHELL=/bin/bash
PATH=/sbin:/bin:/usr/sbin:/usr/bin
MAILTO=root
01 * * * * root run-parts /etc/cron.hourly
[root@servera ~]# crontab -u student -l
no crontab for student

A job that looks right

Install a crontab for student that every minute appends cron ran at HH:MM:SS (use $(date +%T)) to /home/student/cron.out. Wait two minutes. Is there any output? Find out why in /var/log/cron.

[root@servera ~]# echo '* * * * * echo "cron ran at $(date +%T)" >> /home/student/cron.out' > /tmp/ct
[root@servera ~]# crontab -u student /tmp/ct
[root@servera ~]# crontab -u student -l
* * * * * echo "cron ran at $(date +%T)" >> /home/student/cron.out
[root@servera ~]# sleep 90; ls /home/student/cron.out
ls: cannot access '/home/student/cron.out': No such file or directory
[root@servera ~]# grep CROND /var/log/cron | tail -2 | cut -c1-140
Oct  3 17:50:01 servera CROND[732]: (student) CMDOUT (/bin/sh: -c: line 1: unexpected EOF while looking for matching `)')
Oct  3 17:50:01 servera CROND[732]: (student) CMDOUT (/bin/sh: -c: line 2: syntax error: unexpected end of file)

No file, and the log gives the reason: a syntax error. cron treated the % in +%T as a line break.

Fix it

Escape the percent sign, install the crontab again, wait for the next minute, and check the output and the log.

[root@servera ~]# echo '* * * * * echo "cron ran at $(date +\%T)" >> /home/student/cron.out' > /tmp/ct
[root@servera ~]# crontab -u student /tmp/ct
[root@servera ~]# sleep 62; cat /home/student/cron.out
cron ran at 17:52:01
[root@servera ~]# grep CROND /var/log/cron | tail -1 | cut -c1-150
Oct  3 17:52:01 servera CROND[845]: (student) CMDEND (echo "cron ran at $(date +%T)" >> /home/student/cron.out)

A system job

Create /etc/cron.d/demo that runs /usr/bin/logger -t cron-demo hello as root every 2 minutes. Check the file permissions and wait for the first run. Where does the output appear?

[root@servera ~]# echo '*/2 * * * * root /usr/bin/logger -t cron-demo hello' > /etc/cron.d/demo
[root@servera ~]# ls -l /etc/cron.d/demo
-rw-r--r--. 1 root root 52 Oct  3 17:52 /etc/cron.d/demo
[root@servera ~]# sleep 120; journalctl -t cron-demo --no-pager | tail -1 | cut -c1-90
Oct 03 17:54:01 servera.lab.example.com cron-demo[1011]: hello
[root@servera ~]# grep 'demo' /var/log/cron | tail -1 | cut -c1-110
Oct  3 17:54:01 servera CROND[1010]: (root) CMD (/usr/bin/logger -t cron-demo hello)

The message appears in the journal (logger's job), and cron logs only that it started the command.

Grade and finish

Check your work with lab grade sa-cron, then put the exercise away with lab finish sa-cron.