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

systemd Introduction

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.

systemd Features

Adopted by latest systems (RedHat7, CentOS7, Ubuntu15…)

Parallel service start on boot improves boot speed

Shutdown only stops running services, unlike CentOS6 which stops all

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

Resolves issues such as lingering child processes after service stop

systemd Syntax

systemctl [command] [unit]

Common commands:

start      systemctl start nginx
stop       systemctl stop nginx
restart    systemctl restart nginx
reload     systemctl reload nginx
enable     systemctl enable nginx
disable    systemctl disable nginx
status     systemctl status nginx

systemd Unit Files

Each unit requires a configuration file.

Files are stored in /usr/lib/systemd/system/ with symlinks in /etc/systemd/system/ after enabling.

Service files use the .service suffix.

System units reside in /usr/lib/systemd/system/, user units in .../user/.

Configuration sections are case‑sensitive and enclosed in brackets.

Practical Example 1: Manage Nginx with systemd

Compile and install nginx, 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 with systemctl start|stop|restart|enable nginx.

Practical Example 2: Manage Tomcat with systemd

Install JDK, 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=$JAVA_HOME/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/root/bin"
Environment="CLASSPATH=.:$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

Use systemctl start|stop|restart|enable tomcat.

Practical Example 3: Manage a Java JAR Service

Create a bash script (demo.sh) that starts and stops the JAR, then define a unit 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 uams.sh start
ExecStop=/bin/bash uams.sh stop
ExecReload=/bin/bash uams.sh restart
Restart=on-failure

[Install]
WantedBy=multi-user.target

Control with systemctl restart abc, systemctl enable abc, systemctl stop abc, and systemctl status abc.