Cron and crontab

cron runs commands on chronological periods.

cron expressions

A cron expression has five parts....

*     *     *   *    *        command to be executed
-     -     -   -    -
|     |     |   |    |
|     |     |   |    +----- day of week (0 - 6) (Sunday=0)
|     |     |   +------- month (1 - 12)
|     |     +--------- day of        month (1 - 31)
|     +----------- hour (0 - 23)
+------------- min (0 - 59)

MINUTES

* * * * * ls         # Every  minute
*/2 * * * * ls       # Even minutes
1-59/2 * * * * ls    # Odd minutes
*/5 * * * * ls       # Every 5 minutes
*/15 * * * * ls      # Every 15 minutes
22 * * * * ls        # 22 minutes past the hour every hour...

0 * * * * ls         # Every hour On the hour
0 */2 * * * ls       # Even hours, on the hour
0 1-23/2 * * * ls    # Odd hours on the hour
0 */6 * * * ls       # Every 6 hours
0 3 * * * ls         # At 3 AM (every day)...
0 0 * * * ls         # Every day at midnight
0 0 */2 * * ls       # EVEN days at midnight
0 0 */5 * * ls       # Every 5 days

crontab

Your crontab file stores all of the jobs you run periodically. You can edit this file with:

sudo crontab -e

If you've never used crontab before, then the first time you run it it will ask which editor to use:

$ sudo crontab -e
no crontab for root - using an empty one

Select an editor.  To change later, run 'select-editor'.
  1. /bin/ed
  2. /bin/nano        <---- easiest
  3. /usr/bin/vim.basic
  4. /usr/bin/vim.tiny

Choose 1-4 [2]: 2

If you're a BASIC like me then choose nano

Next it will open nano with your empty crontab file (empty apart from explanatory comments)...

Add your cronexpressions at the bottom of the file.

e.g.

15 3 * * * /usr/bin/certbot renew --quiet
@reboot /usr/bin/supervisord -n -c /etc/supervisor/supervisord.conf 2>&1

check cron logs

sudo grep CRON /var/log/syslog

If you see this in your cron logs:

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

It's because one of the cron commands is producing output, and cron thinks it should email you, but it can't find a Mail-Transfer-Agent.

You can just pipe the output to nowhere by adding this to the command:

>/dev/null 2>&1