systemd

systemd is a service manager for Linux computers. Its job is to manage all of the other services running on the system at a given time. It’s the first service that runs when the computer is booted, and it initializes (and manages) all of the other components of the system while the computer is running.

When we want to manage services from the command line, we’ll typically use the systemctl tool, which provides us with a way to start, stop, or check the status of a service, e.g.:

sudo systemctl start <service_name> sudo systemctl stop <service_name> systemctl status <service_name>

We can also view logs using the journalctl tool, e.g.:

journalctl -u <service_name>

Configuring Services

We can configure how services run on a machine by creating and editing .service files. Imagine we have a service named daren and we want to configure it running with systemd. We could do this by editing a service file, e.g.:

sudo nvim /etc/systemd/system/daren.service

We might include the following in this service file:

[Unit]
Description=Daren Go Web Server
After=network.target
 
[Service]
Type=simple
User=USERNAME
WorkingDirectory=/home/USERNAME/daren
ExecStart=/home/USERNAME/daren/daren
Restart=always
RestartSec=5
 
[Install]
WantedBy=multi-user.target

The [Unit] section includes metadata, including a description of the service and an instruction not to start the service until after the network is up and running (since this is a web app, it needs a network connection).

The [Service] section defines the data that systemd needs to run the service:

• I’m not 100% sure what Type = simple means, but I think it’s that the application started by ExecStart is basically the whole application?
• User = USERNAME is a security feature that runs the application with user permissions rather than with root permissions.
• WorkingDirectory and ExecStart specify the working directory and the executable to be run, respectively.
• Restart and RestartSec tells systemd to restart the app if it crashes, but to wait 5 seconds before doing so.

Finally, the [Install] section describes how the service should be activated. Again, I’m not 100% sure about what’s happening here, but my understanding is that WantedBy=multi-user.target basically means to run the app when the computer boots up normally.