Systemd

• systemd is goal-oriented. These goals are defined as ‘units’

• Units are systemd objects used for organizing boot and maintenance tasks. Units consist of mounts, services, sockets, devices, and timers, etc.

• There are 11 unit types

  • Services units tells the init system what it needs to know about the life cycle of an application
  • systemd is the init system typically
  • Use systemctl cat sshd.service to view the unit file for a service
  • Use systemctl edit --full sshd.service to edit a unit file
  • use systemctl revert sshd.service to revert the unit file to the default

• To prevent a service from being started, you can mask it

  • systemctl mask sshd.service

• Targets are simply logical collections of units.

  • Target files end in the .target extension.

  • Systemd includes several predefined targets:

    • halt: shuts down and halts the system
    • poweroff: shuts down and powers off the system
    • shutdown: shuts down the system
    • rescue: boots into single user mode for recovery. All local file systems are mounted. Networking is disabled. Some essential services are started
    • emergency: Runs an emergency shell. The root file system is mounted in read-only mode, other file systems are not mounted. Network and other services are disabled
    • multi-user: full network support, but without a GUI
    • graphical: full network support with a GUI
    • reboot: shuts down and reboots the system
    • default: a special soft link that points to the default system boot target (multi-user or graphical)
    • hibernate: Puts the system into hibernation

  • systemctl get-default will show you the default target

  • use systemctl set-default multi-user.target to set the default operating mode, then reboot

  • Useful targets:

  • emergency.target = root file system is read-only. Minimal amount of programs loaded

  • rescue.target = a few services are loaded and you are dropped into a root shell

    • You must have a password set for the root user to use either of these operating modes

  • You can switch to a target without booting by typing systemctl isolate graphical.target, but this does not change the default boot target

• Each unit has its own config file

• When you boot a system, you’re activating a default unit, usually a target unit called default.target that groups together a number of service and mount units as dependencies.

• There are two main directories that store systemd unit files:

  • /lib/systemd/system or /usr/lib/system/system - system unit directory (avoid making changes. The operating system will maintain these files for you.)
  • /etc/systemd/system - system configuration directory (make changes here)
  • You can check the current systemd configuration search path with this command: systemctl -p UnitPath show

• You can interact with systemd using the systemctl command

• One of systemds features is the ability to delay a daemon startup until it is absolutely needed

• While upgrading software, if systemd’s components are upgraded, you will typically need to reboot

Systemd example

Let’s create a simple echo service

First, define a socket (create a file named echo.socket in /etc/systemd/system)::

[Unit]
Description=my echo socket

[Socket]
ListenStream=8081
Accept=true

Next, define a service for echoing a response (create a file named echo@.service in /etc/systemd/system):

[Unit]
Description=my echo service

[Service]
ExecStart=/bin/cat
StandardInput=socket

Now, we need to start the socket we created in step 1.

systemctl start echo.socket

We can get the status of our socket:

ryan:system/ $ sudo systemctl status echo.socket
● echo.socket - my echo socket
     Loaded: loaded (/etc/systemd/system/echo.socket; static)
     Active: active (listening) since Tue 2023-01-17 06:02:51 EST; 7s ago
     Listen: [::]:8081 (Stream)
   Accepted: 0; Connected: 0;
      Tasks: 0 (limit: 38033)
     Memory: 8.0K
        CPU: 911us
     CGroup: /system.slice/echo.socket

Jan 17 06:02:51 xerxes systemd[1]: Listening on my echo socket.

Now you can connect to the socket and see it repeat whatever you say!

ryan:system/ $ nc localhost 8081
hello
hello
nice day, isn't it?
nice day, isn't it?