systemd

Managing services, units, and logs with systemctl and journalctl on modern Linux systems.

General Basics

Systemd controls what is running on modern Linux systems.

Service Management

Command Action
systemctl start myservice.service Start a service
systemctl stop myservice.service Stop a service
systemctl restart myservice.service Restart a service
systemctl status myservice.service Show service status
systemctl enable myservice.service Enable at boot
systemctl disable myservice.service Disable at boot

Listing Units

Command Action
systemctl list-units --type=service List active services
systemctl --all list-unit-files --type=service List all services (including inactive)
systemctl list-sockets List sockets

Reloading Config

After modifying a unit file, reload the daemon:

sudo systemctl daemon-reload

Example: Tomcat Service Unit File

View the full Tomcat unit file 
[Service]
Type=forking
Environment=JAVA_HOME=/usr/lib/jvm/java
Environment=CATALINA_PID=/usr/local/TomcatRootDir/temp/tomcat.pid
Environment=CATALINA_HOME=/usr/local/TomcatRootDir
Environment=CATALINA_BASE=/usr/local/TomcatRootDir

WorkingDirectory=/usr/local/TomcatRootDir
ExecStart=/usr/local/TomcatRootDir/bin/startup.sh
ExecStop=/bin/kill -15 $MAINPID
User=tomcat
Group=tomcat

[Install]
WantedBy=multi-user.target

Power / Reboot

Command Action
sudo systemctl poweroff Shut down the system
sudo systemctl reboot Reboot the system

Logs (journalctl)

Command Action
journalctl -xe Show recent logs with context
journalctl --follow Tail logs live
journalctl -u myservice.service Logs for a specific service
journalctl -u myservice.service --since today Logs since midnight
journalctl -u myservice.service -n 100 Last 100 lines
journalctl --disk-usage Show how much space logs are using
journalctl --vacuum-time=7d Delete logs older than 7 days

Unit File Anatomy

A full service unit has three sections:

View the full unit file 
[Unit]
Description=My Service
After=network.target

[Service]
Type=simple
ExecStart=/usr/bin/myapp
Restart=on-failure
RestartSec=5s
User=myuser

[Install]
WantedBy=multi-user.target
Type= value Meaning
simple Process started by ExecStart is the main process
forking Process forks; parent exits (e.g. traditional daemons)
oneshot Short-lived task; systemd waits for it to exit
notify Service signals readiness via sd_notify()

Timers (cron replacement)

Create a .timer unit alongside a .service unit of the same name:

View the timer example 
# myapp.timer
[Unit]
Description=Run myapp every hour

[Timer]
OnCalendar=hourly
Persistent=true

[Install]
WantedBy=timers.target
Command Action
systemctl list-timers List all active timers
systemctl enable --now myapp.timer Enable and start a timer

Targets (runlevels)

Target Equivalent
multi-user.target Runlevel 3 — multi-user, no GUI
graphical.target Runlevel 5 — multi-user with GUI
rescue.target Runlevel 1 — single user
systemctl get-default Show current default target
systemctl set-default graphical.target Change default target