Systemd

Introduction

systemd is a software suite that provides an array of system components for Linux operating systems. Its main aim is to unify service configuration and behavior across Linux distributions; systemd's primary component is a "system and service manager"—an init system used to bootstrap user space and manage user processes.

Documentation

Common Parameters

Unit

Option Description
Description A short description of the unit.
Documentation A list of URIs referencing documentation.
Before, After The order in which units are started.
Requires If this unit gets activated, the units listed here will be activated as well. If one of the other units gets deactivated or fails, this unit will be deactivated.
Wants Configures weaker dependencies than Requires. If any of the listed units does not start successfully, it has no impact on the unit activation. This is the recommended way to establish custom unit dependencies.
Conflicts If a unit has a Conflicts setting on another unit, starting the former will stop the latter and vice versa.

Get a complete list of parameters by running man systemd.unit

Install

Option Description
Alias A space-separated list of additional names for the unit. Most systemctl commands, excluding systemctl enable, can use aliases instead of the actual unit name.
RequiredBy, WantedBy The current service will be started when the listed services are started. See the description of Wants and Requires in the [Unit] section for details.
Also Specifies a list of units to be enabled or disabled along with this unit when a user runs systemctl enable or systemctl disable.

Get a complete list of parameters by running man systemd.unit

Service

Option Description
Type Configures the process start-up type. One of:
  • simple (default) – starts the service immediately. It is expected that the main process of the service is defined in ExecStart.
  • forking – considers the service started up once the process forks and the parent has exited.
  • oneshot – similar to simple, but it is expected that the process has to exit before systemd starts follow-up units (useful for scripts that do a single job and then exit). You may want to set RemainAfterExit=yes as well so that systemd still considers the service as active after the process has exited.
  • dbus – similar to simple, but considers the service started up when the main process gains a D-Bus name.
  • notify – similar to simple, but considers the service started up only after it sends a special signal to systemd.
  • idle – similar to simple, but the actual execution of the service binary is delayed until all jobs are finished.
ExecStart Commands with arguments to execute when the service is started. Type=oneshot enables specifying multiple custom commands that are then executed sequentially. ExecStartPre and ExecStartPost specify custom commands to be executed before and after ExecStart.
ExecStop Commands to execute to stop the service started via ExecStart.
ExecReload Commands to execute to trigger a configuration reload in the service.
Restart With this option enabled, the service shall be restarted when the service process exits, is killed, or a timeout is reached with the exception of a normal stop by the systemctl stop command.
RemainAfterExit If set to True, the service is considered active even when all its processes exited. Useful with Type=oneshot. Default value is False.

Get a complete list of parameters by running man systemd.service

Example

[Unit]
Description=The NGINX HTTP and reverse proxy server
After=syslog.target network.target remote-fs.target nss-lookup.target

[Service]
Type=forking
PIDFile=/run/nginx.pid
ExecStartPre=/usr/sbin/nginx -t
ExecStart=/usr/sbin/nginx
ExecReload=/bin/kill -s HUP $MAINPID
ExecStop=/bin/kill -s QUIT $MAINPID
PrivateTmp=true

[Install]
WantedBy=multi-user.target
[Unit]
Description=The Apache HTTP Server
After=network.target remote-fs.target nss-lookup.target

[Service]
Type=notify
EnvironmentFile=/etc/sysconfig/httpd
ExecStart=/usr/sbin/httpd $OPTIONS -DFOREGROUND
ExecReload=/usr/sbin/httpd $OPTIONS -k graceful
ExecStop=/bin/kill -WINCH ${MAINPID}
KillSignal=SIGCONT
PrivateTmp=true

[Install]
WantedBy=multi-user.target
[Unit]
Description=Redis persistent key-value database
After=network.target
[Service]
ExecStart=/usr/bin/redis-server /etc/redis.conf --daemonize no
ExecStop=/usr/bin/redis-shutdown
User=redis
Group=redis

[Install]
WantedBy=multi-user.target

source: shellhacks.com