Which command enables a service to run at startup on systemd-based systems?

• A. update-rc.d ssh enable
• B. service ssh enable
• C. systemctl start ssh
• D. systemctl enable ssh

Answer: (D)

On systemd-based systems, making a service run automatically at startup is about enabling its unit so systemd knows to start it during boot. The right command enables the ssh service, which creates the necessary boot-time symlinks so ssh is started when the default target boots. It doesn’t start the service immediately by itself, but it ensures it will launch on the next boot. If you also want to start it right away, you can run a separate start command, but enabling is specifically about boot-time behavior.

The other options don’t fit the boot-start goal: updating SysV-style runlevel links isn’t how systemd handles autostart, and the wrapper for SysV scripts doesn’t provide a persistent boot-start setting. Starting the service with a start command runs it now but won’t persist across reboots. Therefore, the correct approach is to enable the service with systemctl enable ssh.