Mastering systemd: From Basics to Managing Nginx, Tomcat, and Java Services

1. Introduction to systemd

systemd is the primary system and service manager on modern Linux distributions, replacing the traditional init system because init handles processes serially and only runs startup scripts, leading to blocking and limited service control. systemd manages resources called Units and provides commands such as systemctl, hostnamectl, timedatectl, localctl.

2. Features of systemd

Adopted by the latest distributions (RedHat 7, CentOS 7, Ubuntu 15, …).

Supports parallel service start on boot, significantly improving boot speed.

Shuts down only running services on power‑off, unlike CentOS 6 which stops all.

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

Fixes defects of the old model, e.g., proper termination of child processes.

3. systemd command syntax

systemctl [command] [unit]
# command options
# start   – start a unit
# stop    – stop a unit
# restart – restart a unit
# reload  – reload a unit
# enable  – enable unit at boot
# disable – disable unit at boot
# status  – show unit status

4. systemd unit file structure

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

Configuration files are stored in /usr/lib/systemd/system/; enabling a service creates a symlink in /etc/systemd/system/.

Unit files use the .service suffix.

Two directories exist: system (for services that start without a logged‑in user) and user.

Files are divided into sections using square brackets and are case‑sensitive.

5. Comparison of systemd‑related paths

Service start script path: /etc/init.d (CentOS 6) → /usr/lib/systemd/system (CentOS 7)

Boot‑auto start directory: /etc/rcN.d (CentOS 6) → /etc/systemd/system/multi-user.target.wants/ (CentOS 7)

Default run‑level configuration file: /etc/inittab (CentOS 6) → /etc/systemd/system/default.target (CentOS 7)

Practical Example 1 – Compile and install Nginx with systemd control

Install build dependencies:

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 -j4 && make install

Start Nginx manually:

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

Create a systemd service file:

vim /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:

systemctl restart nginx
systemctl enable nginx
systemctl stop nginx
systemctl status nginx

Practical Example 2 – Install Tomcat and manage it with systemd

Install Java runtime (RPM) and download Tomcat archive:

wget 120.78.77.38/file/jdk-8u231-linux-x64.rpm
wget 120.78.77.38/file/apache-tomcat-9.0.27.tar.gz
rpm -ivh jdk-8u231-linux-x64.rpm

Extract and move Tomcat:

tar -xf apache-tomcat-9.0.27.tar.gz
mv apache-tomcat-9.0.27 /usr/local/tomcat

Start and stop Tomcat manually:

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

Create a systemd service file for Tomcat:

vim /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

Control Tomcat with systemctl:

systemctl restart tomcat
systemctl enable tomcat
systemctl stop tomcat
systemctl status tomcat

Practical Example 3 – Manage a custom Java JAR with systemd

Typical launch command:

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

Create a wrapper script (demo.sh) that supports start, stop, restart:

#!/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

Corresponding systemd unit file:

vim /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

Control the custom service:

systemctl restart abc
systemctl enable abc
systemctl stop abc
systemctl status abc