Services with systemd

systemd is the program that starts and looks after services on most Linux systems. A service is a program that runs in the background, such as a web server. You control services with systemctl: start, stop, restart, enable (start at boot) and status.

Each service is described by a small text file called a unit file. It says which command to run, which user to run it as and what to do if it crashes, for example Restart=on-failure. Writing your own unit file turns a script or app into something that survives crashes and reboots.

Docker restart policies and Kubernetes Deployments do the same job later, at larger scale. When a service fails, systemctl status shows its last log lines and its exit code.