You are going to learn what systemd is about and how to use systemctl to control systemd units.
The systemd is an init system used by CoreOS for starting, stopping, and managing processes.
Basically, it is a system and service manager for CoreOS. On CoreOS, systemd will be used almost exclusively to manage the life cycle of Docker containers. The systemd records initialization instructions for each process in the unit file, which has many types, but we will mainly be covering the "service" unit file, as covering all of them is beyond the scope for this book.
The systemd records initialization instructions/properties for each process in the "service" unit file we want to run. On CoreOS, unit files installed by the user manually or via cloud-init are placed at /etc/systemd/system, which is a read-write filesystem, as a large part of CoreOS has only read-only access. Units curated by the CoreOS team are placed in /usr/lib64...