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systemd — got to learn that one now

Submitted by on Thu, 06/02/2016 - 11:11

The systemd environment and commands replacing the system V start scripts.

systemd has come as the default boot loader since Ubuntu 15.04. The old version, upstart, is now considered deprecated. Systemd has an interface pretty similar to the upstart service command line.

Command Description Support Persist 
systemctl start SERVICE
 Start SERVICE Always No 
systemctl stop SERVICE
 Stop SERVICE Always No 
systemctl restart SERVICE
 Restart SERVICE Always No 
systemctl reload SERVICE
 Reload SERVICE Optional Yes 
systemctl status SERVICE
 Show SERVICE status Always n.a. 
systemctl enable SERVICE
 Enable SERVICE Always Yes 
systemctl disable SERVICE
 Disable SERVICE Always Yes 
systemctl is-enabled SERVICE
 Check whether SERVICE is enabled Always n.a. 
systemctl is-active SERVICE
 Check whether SERVICE is currently active Always n.a. 
systemctl show SERVICE
 Show all of SERVICE's information Always n.a.

Also, when a daemon has problems starting, it may print information about it in the console. However, systemd hides all console output from you.

All the options have an equivalent for the user that make use of the --user command line option. By default, user services get started at the time the user logs in. You can specify required targets to have services only start after certain other events such as the X11 display availability.

IMPORTANT: The fact is logging in, even with ssh, has the effect of starting that user's service, if the service is enabled at that point. Just disabling the service will not stop the currently running instance if it fails over and over again. That special case requires you to (1) disable and (2) log out from all the sessions you currently have opened and that time it will stop the infinite loop trying to restart your system. Now you can re-log back in your system and the service will be stopped and it won't be restarted.

To see the latest output of a process, you'll have to use the journalctl command. In most cases, something like this will work as expected:

journalctl -a -u SERVICE

Note: the -u option means Unit (not user).

It automatically uses less to display the data.