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

systemd Overview

systemd is the primary init system and service manager on modern Linux distributions, replacing the traditional init because it supports parallel start‑up, better dependency handling, and richer unit management.

Key Features

Adopted by major distributions (RHEL 7, CentOS 7, Ubuntu 15…)

Parallel service start‑up improves boot speed

Shutdown only stops running services, unlike legacy init

Units are defined by configuration files instead of shell scripts

Handles lingering child processes that older service commands missed

systemd Command Syntax

<code>systemctl [command] [unit]</code>

Common commands:

start – start a unit (e.g.,

systemctl start nginx

)

stop – stop a unit

restart – restart a unit

reload – reload configuration

enable – enable unit at boot

disable – disable unit at boot

status – show unit status

Unit Configuration Files

Each unit has a

.service

file that tells systemd how to manage it.

Files are stored in

/usr/lib/systemd/system/

with symlinks in

/etc/systemd/system

after enabling.

File name ends with

.service

.

Two directories:

system

for system‑wide services and

user

for per‑user services.

Configuration sections are case‑sensitive and enclosed in brackets.

Practical Example 1 – Managing Nginx with systemd

Install build dependencies and compile Nginx from source:

<code>yum -y install gcc gcc-c++ openssl-devel pcre-devel gd-devel iproute net-tools telnet wget curl
wget http://nginx.org/download/nginx-1.15.5.tar.gz
tar zxf nginx-1.15.5.tar.gz
cd nginx-1.15.5
./configure --prefix=/usr/local/nginx \
    --with-http_ssl_module \
    --with-http_stub_status_module
make -j 4 && make install</code>

Start Nginx manually:

<code>/usr/local/nginx/sbin/nginx          # start
/usr/local/nginx/sbin/nginx -s reload # reload
/usr/local/nginx/sbin/nginx -s quit   # stop</code>

Create a systemd service unit:

<code>[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</code>

Control the service with

systemctl

:

<code>systemctl restart nginx
systemctl enable nginx
systemctl stop nginx</code>

Practical Example 2 – Managing Tomcat with systemd

Install JDK and extract Tomcat:

<code>wget 120.78.77.38/file/jdk-8u231-linux-x64.rpm
rpm -ivh jdk-8u231-linux-x64.rpm
wget 120.78.77.38/file/apache-tomcat-9.0.27.tar.gz
tar -xf apache-tomcat-9.0.27
mv apache-tomcat-9.0.27 /usr/local/tomcat</code>

Start and stop Tomcat manually:

<code>/usr/local/tomcat/bin/startup.sh   # start
/usr/local/tomcat/bin/shutdown.sh  # stop</code>

Create

/usr/lib/systemd/system/tomcat.service

:

<code>[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</code>

Manage Tomcat with systemd:

<code>systemctl restart tomcat
systemctl enable tomcat
systemctl stop tomcat
systemctl status tomcat</code>

Practical Example 3 – Managing a Custom JAR Application

Sample command to run the JAR:

<code>java -jar decode.jar -Dconfig=/usr/local/abc/application.properties</code>

Create a simple start/stop script (

demo.sh

) and a corresponding systemd unit (

abc.service

).

<code># 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
</code>

Systemd unit file:

<code>[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</code>

Control the custom service:

<code>systemctl restart abc
systemctl enable abc
systemctl stop abc
systemctl status abc</code>