Cronjob, every 20 minutes. Starting direct after reboot

running a cronjob like

*/15 * * * * sh /home/me/skript.sh  >> /home/me/out.log 2>&1

will execute the skript.sh every 15 minutes. Starting the computer at 12:10, I have to wait 5 minutes.

Is there a way to get the cronjob starting immediately after reboot and then every 15 minutes?

Thanks, Martin

systemd to the rescue

This is not possible with cron but systemd can do that. You need to create two systemd units, one that starts your script, and one for the timer.

file /etc/systemd/system/my-fifteen-minutes.timer:

[Unit]
Description=15 minute timer

[Timer]
# start this 0 minutes after boot:
OnBootSec=0 min

# ... and then every 15 minutes:
OnActiveSec=15 min

[Install]
WantedBy=multi-user.target

file /etc/systemd/system/my-fifteen-minutes.service (note the different extension):

[Unit]
Description=My script

[Service]
Type=oneshot
ExecStart=/bin/sh -c "/home/me/skript.sh >> /home/me/out.log 2>&1"
User=me

Put these files in the directory /etc/systemd/system and enable the timer with

# make systemd aware of them
sudo systemctl daemon-reload

# make sure the timer is engaged at startup
sudo systemctl enable my-fifteen-minutes.timer

# start the timer "now" (without rebooting):
sudo systemctl start my-fifteen-minutes.timer

# examine the status:
systemctl status my-fifteen-minutes.timer my-fifteen-minutes.service

The command systemctl status my-fifteen-minutes.timer will show something like

● my-fifteen-minutes.timer - 15 minute timer
   Loaded: loaded (/etc/systemd/system/my-fifteen-minutes.timer; enabled; vendor preset: enabled)
   Active: active (waiting) since Sun 2018-07-01 14:42:05 CEST; 1s ago
  Trigger: Sun 2018-07-01 14:57:05 CEST; 14min left

Jul 01 14:42:05 stratum9 systemd[1]: Started 15 minute timer.

This means: The timer was triggered 1s ago and will be triggered again in ~14 minutes (at "Sun 2018-07-01 14:57:05 CEST").

Note that the timer and the service are two different things, and you need to define both. By default, a timer unit starts a service unit with the same name (except for the extension .timer vs. service), i.e. foo.timer would control foo.service (you can override that, though). The timer unit just defines when something happens to the service unit, and the service unit defines the actual action (in your case: start the script /home/me/skript.sh).

Cronjob, every 20 minutes. Starting direct after reboot

systemd to the rescue

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