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sysdemd

systemd is the init system used by most, but not all, Linux-based operating systems.

Summary

systemd does a lot of things, and a short synopsis of its features is difficult. It handles boot initialization, processes, and various other system configuration items. Processes are managed in groups called units.

Application management

Application/process management

Mostly we use the systemctl command to control units or the system itself. This is especially useful for managing important background processes like server daemons, including ssh.

Command	Explanation
systemctl list-units	List units on the system
systemctl status sshd	Get information about the sshd unit
systemctl restart sshd	Restart sshd unit (stop and immediately start again)
systemctl stop sshd	Stop the sshd unit (careful here if you only have remote access via ssh)
systemctl disable sshd	Prevent the sshd unit from starting when the system boots
systemctl enable sshd	Enable the sshd service so it can be run on every boot

See below for troubleshooting a unit.

Troubleshooting

The journalctl command lets you access log information.

Command	Explanation
journalctl -f	Kind of similar to a command like tail -f /var/log/example.log. the -f means follow
journalctl -b	Retrieves logs from current boot
journalctl -b -1	Using a boot offset, -1 would show you the prior butt and -3 would show you -3 boots ago (default is boot 0, the current boot)
journalctl --list-boots	Get a list of prior boots
journalctl --since "1 hour ago"	Get log data from the past hour.
journalctl --since "2025-06-26 22:00:15" --until "2025-06-28 23:59:59"	Log data over a time range, format YYYY-MM-DD HH:00:SS, but other formats are possible
journalctl -n 25 --since "1 hour ago"	Show the last 25 log invents in the past hour
journalctl -r -n 25 --since "1 hour ago"	Show the last 25 log invents in the past hour, but reverse the order (-r flag)

If you want to troubleshoot a unit, you can use more specific commands similar to the above. Using the NetworkManager.service unit as an example:

Command	Explanation
journalctl -u NetworkManager.service	Look at NetworkManager logs
journalctl -u NetworkManager.service -f	Tail the log for NetworkManager in your console
journalctl -u NetworkManager.service -u sshd.service	Look at logs for two units, this this case both NetworkManager and the SSH server

You can adapt a lot of the parameters in the first section to the per-unit commands.