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

systemd Introduction

systemd is the main init system on modern Linux, replacing the traditional init because init runs sequentially and only executes startup scripts. Since CentOS 7, systemd is the default process manager.

All resources managed by systemd are called Units. systemd provides commands such as systemctl, hostnamectl, timedatectl, and localectl, which replace older tools like chkconfig and service.

systemd Features

Latest distributions (RedHat 7, CentOS 7, Ubuntu 15…) use systemd.

CentOS 7 supports parallel service start, significantly speeding up boot.

CentOS 7 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 previous issues, e.g., ensuring child processes are terminated.

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 File Description

Each Unit requires a configuration file.

Files reside in /usr/lib/systemd/system/ and are linked to /etc/systemd/system/ when enabled.

Unit files usually have a .service suffix.

System units are placed under /usr/lib/systemd/system/; user units have a separate directory.

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

Practical Example 1 – Manage Nginx with systemd

Install build dependencies and compile Nginx from source:

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

Start Nginx directly:

/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 for Nginx:

[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 – Manage Tomcat with systemd

Download and install JDK and Tomcat, set environment variables, and verify Java installation.

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
tar -xf apache-tomcat-9.0.27
mv apache-tomcat-9.0.27 /usr/local/tomcat
sh /usr/local/tomcat/bin/startup.sh   # start
sh /usr/local/tomcat/bin/shutdown.sh  # stop

Systemd unit for Tomcat:

[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 – Manage a Java JAR Service with systemd

Example start command for a JAR application:

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

Sample control 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

Corresponding systemd unit (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

Original link: https://blog.csdn.net/weixin_43546282