Creating the systemd unit files

The unit files on our system define how systemd starts and runs services.

Each unit file represents a single component as a simple text file that describes its behavior, what needs to run before or afterward, and more.

Unit files are stored in a few different places on a system and systemd looks for them in this order:

1. /etc/systemd/system
2. /run/systemd/system
3. /usr/lib/systemd/system

Unit files that are in the earlier directories override the later ones. This lets us change what we need in the /etc directory, where configuration is expected, leaving the default configuration files in /usr, for example.

But what does a unit file look like? Let's find out.

First, we can get the location of a default unit file by asking systemd about it:

# systemctl status sshd
○ sshd.service - OpenSSH server daemon
     Loaded: loaded (/usr/lib/systemd/system/sshd.service; disabled; vendor preset: disabled...