Cron expression to run every 30 minutes

Field breakdown

*/30 expands to minutes 0 and 30. All other fields are wildcards, so the job fires at 00:00, 00:30, 01:00, 01:30 and so on through the day. 0,30 * * * * is exactly equivalent and arguably clearer, since there are only two values to list.

Half-hourly is a good default for renewals

Certificate clients are the canonical example. Certbot's own guidance is to run the renewal check twice a day at a random minute, but many teams run it more frequently and rely on the client itself to no-op until renewal is actually due. A half-hourly check costs almost nothing and shortens the window in which an expiring certificate goes unnoticed.

The same reasoning applies to token refreshes, DNS updates and health reconciliation loops: the job is cheap when there is nothing to do, and the cost of being late is high.

Using it

# crontab
0,30 * * * * /usr/bin/certbot renew --quiet

# with a random offset so hosts do not stampede
$((RANDOM \% 30)),$((RANDOM \% 30 + 30)) * * * * /usr/bin/certbot renew --quiet

Crontabs do not expand shell arithmetic, so the second example only works if you generate the line from a configuration management tool. Note also the escaped percent sign: an unescaped % in a crontab is converted to a newline.

Offsetting to avoid the top of the hour

The top of the hour is the busiest minute on most systems — log rotation, metric flushes and every other half-hourly job all land there. Shifting a few minutes off the boundary is free:

7,37 * * * *    # :07 and :37 instead of :00 and :30

Variations