How many unit files are required for a systemd

Question

https://fedoramagazine.org/systemd-converting-sysvinit-scripts/ states

Here’s a fun fact: everything handled by that 184 line shell script is now handled by 27 lines of systemd configuration, spread across two unit files.

I can find the first file at /usr/lib/systemd/system/sshd.service:

[Unit]
Description=OpenSSH server daemon
Documentation=man:sshd(8) man:sshd_config(5)
After=network.target sshd-keygen.service
Wants=sshd-keygen.service

[Service]
EnvironmentFile=/etc/sysconfig/sshd
ExecStart=/usr/sbin/sshd -D $OPTIONS
ExecReload=/bin/kill -HUP $MAINPID
KillMode=process
Restart=on-failure
RestartSec=42s

[Install]
WantedBy=multi-user.target

What second file is required and where is it located?

Answer 1

The init script in the article starts sshd-keygen before starting sshd so the second unit file being referred is probably the /usr/lib/systemd/system/sshd-keygen.service file.

Note that, in the systemd case, sshd-keygen is not strictly required by sshd since it configured with Wants=sshd-keygen.service option. Even if sshd-keygen.service fails or is not available, sshd will still be started by systemd. However, in the init script linked in the article, it is a strict requirement since the script will fail if sshd-keygen fails or is not available.

Answer 2

The second file is sshd-keygen.service, which is referenced in your snippet it above.

You can use locate sshd-keygen.service to find the file on your system. It's likely located at /usr/lib/systemd/system/sshd-keygen.service.