Cron

Cron#

Cron is a utility that allows you to schedule commands in a Linux system. You can manage cron jobs using crontab, a special utility that handles the file defining jobs for cron.

The following cell runs Alpine Linux, which includes cron by default, providing a playground to experiment with it.

docker run --rm -itd --name crontab_test alpine

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Note don’t forget to stop the container after all.

docker stop crontab_test

crontab_test

Cron files#

As usually cron configurations are just a specific files in the filesystem. To my mind editing this file is the best option to update cron job.

Typical location is /var/spool/cron/crontabs, there is separate file for each user.

The following cell shows the contents of the /var/spool/cron/crontabs folder for the container we’re using as an example.

docker exec crontab_test ls -l /var/spool/cron/crontabs

lrwxrwxrwx    1 root     root            21 Dec  5 12:17 /var/spool/cron/crontabs -> ../../../etc/crontabs

There is only one file that describes crontabs for the root user. The next cell lists the contents of this file:

docker exec crontab_test cat /var/spool/cron/crontabs/root

# do daily/weekly/monthly maintenance
# min	hour	day	month	weekday	command
*/15	*	*	*	*	run-parts /etc/periodic/15min
0	*	*	*	*	run-parts /etc/periodic/hourly
0	2	*	*	*	run-parts /etc/periodic/daily
0	3	*	*	6	run-parts /etc/periodic/weekly
0	5	1	*	*	run-parts /etc/periodic/monthly

Typical syntax for linux cron jobs.

From stdin#

You can set your cron schedule using the syntax crontab -, which means that crontab will take the schedule from standard input.

The following cell shows how you can set the crontab through the echo command.

docker exec crontab_test sh -c "
echo \"10 * * * * from_echo\" | crontab -
"

Let’s check gotten cron schedule.

docker exec crontab_test crontab -l

10 * * * * from_echo

Even better, you can use heredoc syntax to set a multiline cron schedule.

docker exec crontab_test sh -c "
cat <<EOF | crontab -
10 * * * * echo line1
* 10 * * * echo line2
EOF"

Let’s check if the schedule from the previous cell was actually added to the cron.

docker exec crontab_test crontab -l

10 * * * * echo line1
* 10 * * * echo line2

Specify period#

To specify the period when each cron job will be executed, you have to use the syntax */<amount> in the corresponding column. For example, */2 in the minutes column means that the job will be executed every 2 minutes.

Note: */1 is equivalent to just *, because * means “every.”

As an example, consider the following Docker container. It runs the crond daemon during startup, so the cron jobs will work.

docker run --rm --name run_cron_example -itd alpine \
    crond -fl 2

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The following cell defines two jobs:

  • The first job executes every minute.

  • The second job executes every two minutes.

Both jobs print their signatures to the /test file, allowing us to check when they have been executed.

docker exec run_cron_example sh -c "
cat << 'EOF' | crontab -
* * * * * echo \"\$(date +%H:%M:%S) every minute\" >> /test
*/2 * * * * echo \"\$(date +%H:%M:%S) every two minutes\" >> /test
EOF"

After a while we can check what we get from our cron jobs:

docker exec run_cron_example cat /test

16:46:00 every minute
16:46:00 every two minutes
16:47:00 every minute
16:48:00 every minute
16:48:00 every two minutes
16:49:00 every minute
16:50:00 every minute
16:50:00 every two minutes

Jobs are executed at the beginning of each minute and every two minutes.

docker stop run_cron_example

run_cron_example