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# / ___| _   _ ___| |_ ___ _ __ ___ |  _ \
# \___ \| | | / __| __/ _ \ '_ ` _ \| | | |
#  ___) | |_| \__ \ ||  __/ | | | | | |_| |
# |____/ \__, |___/\__\___|_| |_| |_|____/
#        |___/
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SystemD: The most common service manager for Linux Systems

SystemD is the service manager for linux systems that replaces the more traditional SysV init style systems used in the past. Although most commonly implemented, systemd is not universally adopted, and several Linux distrobutions still avoid foregoing it’s implementation. One of the things that separates systemd from it’s predecessor is it’s implementation of built-in services, which traditionally were provided by external third party programs.

Writing SystemD Unit Files

Systemd breaks services down into what it refers to as units. Each service and the dependencies of that service making up a single unit. The accomodates for service dependency resolution, and streamlines the service management process.

It is particularly important to know how to write your own service unit files for systemd in linux system management, because of the fast paced development environment surrounding Linux software. So below we will break down the various parts.

Overview

A minimal service configuration will appear like so:

[Unit]
Description=A Service Description

[Service]
ExecStart=/usr/sbin/foobar

[Install]
WantedBy=Multi-user.target

Notice the three different sections of the systemd service file: Unit, Service, and Install. Each of these will be discussed further below.

For a full list of available attributes please refer to the man pages man systemd.unit and man systemd.service, information on systemd.install is contained within the man systemd.unit man page.

Dependency Management

Systemd defines dependencies on two seperate priority association levels, “Require” and “Want”. “Require” is used to denote dependencies with the highest degree of association. Meaning, that if one of the required dependency fails, then so does the service unit that required it. “Want” is used to denote a lesser degree of association, where if the dependency fails it will have not impact on the unit wanting it.

Unit

As the name indicates, the unit section provides attributes that defines the service unit. Below we list a few of the most common attributes in customary table format.

Service

The service section provides attributes that define and control the particular service the unit is controlling.

Service Types

Below are a list of definitions which describe the available service types which can be used in the Type definition of the service file.

• simple - Starts the service immediately
• forking - Service start succeeds upon process forking.
• oneshot - Requires the process to exit for success.
• dbus - Succeeds when dbus name is acquired.
• notify - Succeeds on reception of signal.
• idle - Execution succeeds only after system is idle.

Install

The install section provides attributes the deal with the enabling and disabling of the service.

References

• SystemD Service File Examples