Cron Jobs

Cron Jobs essentially consist in running some command from the terminal at a predefined time, or every given interval.
They are great and useful since they can be used to automate any type of task.

Configuration

Short answer

Run crontab -e

Long answers

• How do I set up a Cron Job? on AskUbuntu
• Creating a custom Cron Job, in DreamHost Knowledge Base
• More detailed information in Ubuntu Community Help Wiky → CronHowto
• Crontab Guru, simple and insightful tool to configure and check crontabs.

Enable logging

Cron Jobs do not log activity by default, to activate logging for an easier debugging, go to /etc/rsyslog.conf or /etc/rsyslog.d/50-default.conf and uncomment the following line:

cron.*					/var/log/cron.log

then run

sudo service rsyslog restart && sudo service cron restart

Cron Jobs logs will appear in /var/log/cron.log

Troubleshooting

• Solve (CRON) info (No MTA installed, discarding output)

My Cron Jobs

rtcwake

This Cron Job schedules Linuxplosion boot, and it switches it off until the following week.

35 18 * * 1 sudo udisksctl mount -b /dev/sda2 && echo "`date`: Linuxplosion is up and running!" >> ~/rtcwake-log.txt
0 21 * * 1 echo "`date`: Linuxplosion is going back to sleep until next monday at 7PM." >> ~/rtcwake-log.txt && sudo rtcwake -m off -t "$(date -d 'next Monday 18:30' '+%s')"

wayback_archiver

This Cron Job saves a list of predefined pages to The Wayback Machine using wayback_archiver

0 1 * * 1 /usr/local/bin/wayback_archiver https://tommi.space/pages-to-archive --crawl --limit=100 --verbose --log=$HOME/wayback_archiver.log && echo "\n$(date) wayback_archiver success!" >> $HOME/wayback_archiver.log