How to use systemctl and systemd in linux

With the systemd subsystem came the systemctl command. It allows you to manage the main Linux processes. It is cheat sheet of the most used commands.

General Syntax

Without options, systemctl lists running services, mount points, devices, and other units.

systemctl

Cheat sheet on frequently used systemctl commands

How to view status service:

systemctl status httpd

Start the service:

systemctl start mysql

Stop the service:

systemctl stop nginx

Restart the service:

systemctl restart apache2

Disable autorun of the service:

systemctl disable firewalld

Run a command on a remote system:

systemctl --host root@192.168.0.15 stop cron

Restart the server:

systemctl reboot

Checking the operation of the service.

systemctl is-active docker

Status of service which you can see:

1. If the service is running, we will see:

active

1. If not running:

failed

…or:

inactive

1. If there is no such service in the system:

unknown

…or:

inactive

If the service does not work or it is not in the system, the command will return an error code, thus the construction:

systemctl is-active docker && docker run hello-world

will result in the execution of the docker run hello-world command only if the docker service is running.

Autorun

The systemd subsystem can also be used to autorun services or scripts. To do this, in the /usr/lib/systemd/system directory, create a unit (file) with the .service extension. Let’s take a closer look at the example of the bind service:

nano /usr/lib/systemd/system/named.service

The content may be as follows:

[Unit]

Description=Berkeley Internet Name Domain (DNS)

Wants=nss-lookup.target

Wants=named-setup-rndc.service

Before=nss-lookup.target

After=network.target

After=named-setup-rndc.service

[Service]

Type=forking

Environment=NAMEDCONF=/etc/named.conf

EnvironmentFile=-/etc/sysconfig/named

Environment=KRB5_KTNAME=/etc/named.keytab

PIDFile=/run/named/named.pid

ExecStartPre=/bin/bash -c 'if [ ! "$DISABLE_ZONE_CHECKING" == "yes" ]; then /usr/sbin/named-checkconf -z "$NAMEDCONF"; else echo "Checking of zone files is disabled"; fi'

ExecStart=/usr/sbin/named -u named -c ${NAMEDCONF} $OPTIONS

ExecReload=/bin/sh -c '/usr/sbin/rndc reload > /dev/null 2>&1 || /bin/kill -HUP $MAINPID'

ExecStop=/bin/sh -c '/usr/sbin/rndc stop > /dev/null 2>&1 || /bin/kill -TERM $MAINPID'

PrivateTmp=true

[Install]

WantedBy=multi-user.target

Basically, the file is divided into 3 parts:

Unit — allows you to define metadata for a unit. Service — the section for the main configuration of the unit. Install — defining the behavior for a unit when it is enabled or disabled.

After making changes and saving the file, you need to re-read the changes with the command:

systemctl daemon-reload

Now you can allow autorun:

systemctl enable named

Editing services

If we want to make changes to the service unit file that was installed with the last one, we need to use a drop-in file or a settings override file. Otherwise, after updating the program, our changes may be deleted.

And so, for example, we took a unit for bind. To create a drop-in file for it, enter:

systemctl edit named

And we make, for example, such changes:

[Service]
Restart=on-failure

the /etc/systemd/system/name.service.d/override.conf file will be created, which will override the settings of the main unit file. In this example, we indicate the need to restart the service in case of a failure.

To make sure that the Drop-In file is used, look at the service status:

systemctl status named

We need to see something like:

Drop-In: /etc/systemd/system/named.service.d
           — override.conf

Timers

Above, we considered the possibility of automatically starting services using systemd. We can also configure the periodic launch of these units by timer. To do this, we need to create a unit with the same name, but there must be a suffix.timer at the end.

Suppose we want to create a run on the named timer, for which we created a service unit in the example above. Then create a file:

nano /usr/lib/systemd/system/named.timer

[Unit]
Description=Run named every 10 min

[Timer]
OnBootSec=5min
OnUnitActiveSec=10min

[Install]
WantedBy=timers.target

in this example, our timer will be created for the named unit; it will start 5 minutes after the start of the service and will start it every 10 minutes.

Reread the changes in systemd:

systemctl daemon-reload

We allow our timer:

systemctl enable named.timer

Let’s launch it:

systemctl start named.timer

You can output a list of timers with the command:

systemctl list-timers