CentOS / RHEL 7 : Beginners guide to systemd

systemd is the new system and service manager in RHEL 7. It is backward compatible with SysV init scripts used by previous versions of Oracle Linux including RHEL 6. systemd is the first process that starts after the system boots, and is the final process that is running when the system shuts down.

Features

– Speeds up booting by loading services concurrently.
– Allows you to manage various types of units on a system, example:

• services (name.service)
• targets (name.target)
• devices (name.device)
• file system mount points (name.mount)
• sockets (name.socket)

– Snapshotting of the system state and restoration of the system state from a snapshot is supported.
– Mount points can be configured as systemd targets.

systemd unit types

Below is a most commonly used list of systemd unit types.

unit	function	extention
service unit	Start and control daemons and the processes they consist of.	.service
target unit	replaces sysV init run levels.	.target
mount unit	Control mount points in the file system.	.mount
device unit	Expose kernel devices in systemd	.device
Snapshot unit	Can be used to temporarily save the state of the set of systemd units, which can later be restored by activating the saved snapshot unit.	.snapshot
swap unit	Encapsulate memory swap partitions or swap files.	.swap

systemd configuration files

systemd units are defined by unit configuration files located in the following directories :
/usr/lib/systemd/system
systemd units distributed with installed RPM packages.
/run/systemd/system
systemd units created at runtime. This directory takes precedence over the directory with installed service units.
/etc/systemd/system
systemd units created and managed by the system administrator. This directory takes precedence over the directory with runtime units.