SYSTEMD

Systemd is a suite that manages to do many tasks in Debian/ Ubuntu OS. In this lesson, I only focus on its service management aspect.

The command used to manage systemd is systemctl.

1. Systemd units

A unit in systemd can be services (.service), mount points (.mount), devices (.device) or sockets (.socket).

To specify a unit, you have to use its complete name, including its suffix. For example mpd.socket or mpd.service. If you omit the suffix, systemctl will assume that you are addressing a .service.

To list all current loaded units:

systemctl list-units

Service units only:

systemctl list-units | grep .service

To list failed units:

systemctl --failed

2. Check services status

If a systemctl command is just for checking information, it does not need root power (sudo).

Check the status of a service:

systemctl status mpd.service

● mpd.service - Music Player Daemon
   Loaded: loaded (/lib/systemd/system/mpd.service; enabled; vendor preset: enab
   Active: active (running) since Wed 2020-03-11 13:07:20 +07; 1h 9min ago
     Docs: man:mpd(1)
           man:mpd.conf(5)
           file:///usr/share/doc/mpd/user-manual.html
 Main PID: 890 (mpd)
    Tasks: 3 (limit: 4915)
   Memory: 51.6M
   CGroup: /system.slice/mpd.service
           └─890 /usr/bin/mpd --no-daemon

There is several info listed here but please notice the Loaded and Active ones.

  • Loaded: the loaded status above is enabled that means it will auto start if there is a reboot. If it is disable, it will not autostart at the next reboot.
  • Active: active means it is running. Otherwises (inactive/ failed…), you have to look further into it.

2. Enable and disable a service

If a systemctl command changes service state, it needs root power (sudo).

This status is shown in the Loaded line as above.

Enable a service:

sudo systemctl enable mpd.service

Disable a service

sudo systemctl disable mpd.service

3. Stop, start and restart a service

This status is shown in the Active line as above.

sudo systemctl start mpd.service # or stop/ restart

4. reboot/ poweroff and suspend

polkit is necessary for power management as an unprivileged user. If you are in a local systemd-logind user session and no other session is active, the following commands will work without root privileges. If not (for example, because another user is logged into a tty), systemd will automatically ask you for the root password. From Arch wiki

This means if only you are using the desktop, you can use these commands without sudo.

systemctl reboot # or power off/ suspend

5. Create your own systemd service

The reason why I create a systemd service is I need a way for a script to survive a reboot (autostart when boot).

I have used this service for my python-telegram bot script running on my Raspberry Pi.

nano /etc/systemd/system/python-telegram.service

[Unit]
Description=My telegram bot
After=syslog.target
[Service]
Type=simple
User=root
Group=root
WorkingDirectory=/home/pi/telegrambot/
ExecStart=/home/pi/telegrambot/myPiBot.py
StandardOutput=syslog
StandardError=syslog
[Install]
WantedBy=multi-user.target

After you add a service file, you need to start it:

sudo systemctl start python-telegram.service

Explanation:

A. Where to put the service file:

Unit files are loaded from multiple locations (to see the full list, run systemctl show --property=UnitPath), but the main ones are (listed from lowest to highest precedence):

/usr/lib/systemd/system/: units provided by installed packages
/etc/systemd/system/: units installed by the system administrator

B.

The most typical case is that unit A requires the unit B to be running before A is started. In that case add Requires=B and After=B to the [Unit] section of A. If the dependency is optional, add Wants=B and After=B instead. Note that Wants= and Requires= do not imply After=, meaning that if After= is not specified, the two units will be started in parallel.

You can specify the directives User= and Group= in the [Service] section of the unit file.

Type=simple : systemd considers the service to be started up immediately. The process must not fork. Do not use this type if other services need to be ordered on this service, unless it is socket activated.

WorkingDirectory: defines which directory the service will be launched, same as when you use cd to change a directory when you’re working in the shell.

[Install] WantedBy=multi-user.target

normally defines a system state where all network services are started up and the system will accept logins, but a local GUI is not started.

if you omit the WantedBy=multi-user.target line and no other enabled service includes a Requires=your.service or Wants=your.service in its service definition, your service will not be started automatically

systemd works on dependencies, and at boot time, if nothing Requires or Wants your service, it won’t be started even if the service is enabled.

sudo journalctl -u [unit] to view the log of a unit