Mastering systemd: From Basics to Real-World Service Management

Introduction to systemd

systemd is the primary system daemon manager on modern Linux, replacing init because init handles processes serially and only runs startup scripts, leading to blocking and limited service management. Since CentOS 7, systemd is the default.

Features of systemd

All recent major distributions (RedHat 7, CentOS 7, Ubuntu 15, etc.) use systemd.

CentOS 7 supports parallel service start‑up, significantly improving boot speed.

Shutdown only stops running services, unlike CentOS 6 which stops everything.

Service start/stop no longer rely on scripts under /etc/init.d.

systemd resolves defects of the old model, such as services not terminating child processes.

systemd command syntax

systemctl [command] [unit]
# command options:
# start   : systemctl start nginx
# stop    : systemctl stop nginx
# restart : systemctl restart nginx
# reload  : systemctl reload nginx
# enable  : systemctl enable nginx   (auto‑start on boot)
# disable : systemctl disable nginx  (prevent auto‑start)
# status  : systemctl status nginx

Configuration file overview

Each Unit requires a configuration file to tell systemd how to manage the service.

Files are stored in /usr/lib/systemd/system/ and linked from /etc/systemd/system/ after enabling.

Unit files use the .service suffix.

The directory contains separate system and user sub‑directories.

Configuration sections are case‑sensitive and enclosed in square brackets.

Practical Example 1 – Managing Nginx with systemd

Install build dependencies, compile Nginx, then create /usr/lib/systemd/system/nginx.service with appropriate sections and control it via systemctl commands.

# /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

Practical Example 2 – Managing Tomcat with systemd

Install JDK, extract Tomcat, then create /usr/lib/systemd/system/tomcat.service with environment variables and start/stop commands.

# /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=$JAVA_HOME/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/root/bin"
Environment="CLASSPATH=.:$JAVA_HOME/lib/dt.jar:$JAVA_HOME/lib/tools.jar"
ExecStart=/usr/local/tomcat/bin/startup.sh
ExecStop=/usr/local/tomcat/bin/shutdown.sh
Restart=on-failure

[Install]
WantedBy=multi-user.target

Practical Example 3 – Managing a Java JAR service

Create a simple start/stop script ( demo.sh) and a systemd unit file ( /usr/lib/systemd/system/abc.service) that invokes the script.

# demo.sh
#!/bin/bash
source /etc/profile
jarName="abc-web.jar"
workDir="/usr/local/abc"

start() {
  cd ${workDir} && java -jar ${jarName} --spring.profiles.active=prod --server.port=9630 >uams.log 2>&1 &
}
stop() {
  ps -ef | grep -qP "(?<=-jar)\s+${jarName}" && kill $(ps -ef | grep -P "(?<=-jar)\s+${jarName}" | awk '{print $2}')
}
case $1 in
  start)   start   ;;
  stop)    stop    ;;
  restart) stop; start ;;
esac

# /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 demo.sh start
ExecStop=/bin/bash demo.sh stop
ExecReload=/bin/bash demo.sh restart
Restart=on-failure

[Install]
WantedBy=multi-user.target

Controlling the services

Use systemctl restart, enable, stop, and status to manage the created services.