Master systemd: From Basics to Real-World Service Management on Linux

systemd Overview

systemd is the default init system on modern Linux distributions such as CentOS 7, RedHat 7, and Ubuntu 15+. It replaces the traditional init, providing parallel service startup, better shutdown handling, and a unified command set (systemctl, hostnamectl, timedatectl, localectl).

Key Features

Adopted by the latest Linux distributions.

Parallel service start improves boot speed.

Only running services are stopped on shutdown.

No more /etc/init.d scripts; services are defined as Units.

Handles shortcomings of legacy service management (e.g., orphaned child processes).

systemd Command Syntax

systemctl [command] [unit]
# commands:
# start   – start a unit, e.g., systemctl start nginx
# stop    – stop a unit, e.g., systemctl stop nginx
# restart – restart a unit, e.g., systemctl restart nginx
# reload  – reload configuration, e.g., systemctl reload nginx
# enable  – enable unit at boot, e.g., systemctl enable nginx
# disable – disable unit at boot
# status  – show unit status, e.g., systemctl status nginx

Unit Configuration Files

Each Unit has a .service file placed in /usr/lib/systemd/system/. After enabling, a symlink is created in /etc/systemd/system/. The file consists of sections [Unit], [Service], and [Install], and is case‑sensitive.

Practical Example 1 – Managing Nginx with systemd

Compile and install Nginx from source, then create /usr/lib/systemd/system/nginx.service:

[Unit]
Description=nginx
After=network.target

[Service]
Type=forking
ExecStart=/usr/local/nginx/sbin/nginx
ExecReload=/usr/local/nginx/sbin/nginx -s reload
ExecStop=/usr/local/nginx/sbin/nginx -s quit
PrivateTmp=true

[Install]
WantedBy=multi-user.target

Control the service with:

systemctl start nginx
systemctl enable nginx
systemctl restart nginx
systemctl status nginx

Practical Example 2 – Managing Tomcat with systemd

Install JDK, set environment variables, extract Tomcat, then create /usr/lib/systemd/system/tomcat.service:

[Unit]
Description=tomcat server
Wants=network-online.target
After=network.target

[Service]
Type=forking
Environment="JAVA_HOME=/usr/java/jdk1.8.0_231-amd64"
Environment="PATH=$JAVA_HOME/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/root/bin"
ExecStart=/usr/local/tomcat/bin/startup.sh
ExecStop=/usr/local/tomcat/bin/shutdown.sh
Restart=on-failure

[Install]
WantedBy=multi-user.target

Manage Tomcat with the usual systemctl commands (start, stop, enable, status).

Practical Example 3 – Managing a Java JAR with systemd

Write a simple Bash wrapper demo.sh that starts and stops the JAR, then create a service file /usr/lib/systemd/system/abc.service:

[Unit]
Description=uams server
Wants=network-online.target
After=network.target

[Service]
Type=forking
WorkingDirectory=/usr/local/abc/
ExecStart=/bin/bash /usr/local/abc/demo.sh start
ExecStop=/bin/bash /usr/local/abc/demo.sh stop
ExecReload=/bin/bash /usr/local/abc/demo.sh restart
Restart=on-failure

[Install]
WantedBy=multi-user.target

Control the JAR service with systemctl restart abc, systemctl enable abc, systemctl stop abc, and systemctl status abc.