systemd

Managing services

$ systemctl status sshd            # state, recent logs, main PID
$ systemctl start sshd             # also stop / restart
$ systemctl reload sshd            # re-read config without a full restart
$ systemctl enable --now sshd      # start now AND on every boot
$ systemctl disable --now sshd
$ systemctl is-active sshd; systemctl is-enabled sshd
$ systemctl mask sshd              # fully block it (symlink to /dev/null)

Listing and inspecting units

$ systemctl list-units --type=service        # running units
$ systemctl list-units --state=failed        # what is broken
$ systemctl list-unit-files --state=enabled  # enabled on boot
$ systemctl cat sshd                         # show the unit file(s)
$ systemctl show sshd -p MainPID,ActiveState # raw properties
$ systemctl daemon-reload                    # after editing a unit by hand

Overriding a unit (drop-ins)

Don’t edit vendor units in /usr/lib; use a drop-in so changes survive package updates:

$ systemctl edit sshd    # creates /etc/systemd/system/sshd.service.d/override.conf

[Service]
Restart=always
Environment=FOO=bar

Writing a unit

/etc/systemd/system/myapp.service:

[Unit]
Description=My app
After=network-online.target
Wants=network-online.target

[Service]
ExecStart=/usr/local/bin/myapp
Restart=on-failure
User=myapp

[Install]
WantedBy=multi-user.target

journalctl

$ journalctl -u sshd               # logs for one unit
$ journalctl -u sshd -f            # follow, like tail -f
$ journalctl -b                    # this boot;  -b -1 = previous boot
$ journalctl -p err                # priority err and worse
$ journalctl --since "10 min ago" --until now
$ journalctl -k                    # kernel messages (dmesg)
$ journalctl --disk-usage; journalctl --vacuum-time=7d

Timers (a cron replacement)

A .timer triggers the .service of the same name. backup.timer:

[Unit]
Description=Nightly backup

[Timer]
OnCalendar=*-*-* 02:00:00     # daily at 02:00
Persistent=true               # run on next boot if a scheduled run was missed

[Install]
WantedBy=timers.target

$ systemctl enable --now backup.timer
$ systemctl list-timers            # next/last run of every timer

Targets (runlevels)

$ systemctl get-default
$ systemctl set-default multi-user.target   # boot without a GUI
$ systemctl isolate multi-user.target        # switch now, no reboot

Boot analysis

$ systemd-analyze                  # total boot time
$ systemd-analyze blame            # slowest units
$ systemd-analyze critical-chain   # the critical boot path

User units

Run units as your own user (no root) - this is how rootless container quadlets run:

$ systemctl --user enable --now myapp.service
$ loginctl enable-linger $USER     # let user units run without an active login