Basics of systemd

What is systemd?

systemd is the init system and service manager used by most modern Linux distributions (Ubuntu, Fedora, Debian, RHEL/CentOS, Arch, etc.). It’s the first process (PID 1) that starts at boot and manages all other processes, services, and system state.

Core Concepts

Units

Everything in systemd is a unit — a resource that systemd knows how to manage. Common unit types:

Type	Extension	Purpose
Service	`.service`	Manages a daemon/process
Socket	`.socket`	Network/IPC socket activation
Timer	`.timer`	Scheduled tasks (like cron)
Mount	`.mount`	Filesystem mount points
Target	`.target`	Groups of units (like runlevels)
Path	`.path`	Path-based activation

Unit files live in:

/etc/systemd/system/ — local/custom overrides (highest priority)
/run/systemd/system/ — runtime units
/lib/systemd/system/ or /usr/lib/systemd/system/ — package-installed defaults

Essential Commands

Managing services

systemctl start nginx          # Start a service
systemctl stop nginx           # Stop a service
systemctl restart nginx        # Restart a service
systemctl reload nginx         # Reload config without restart
systemctl status nginx         # Check status
systemctl enable nginx         # Start automatically at boot
systemctl disable nginx        # Don't start at boot
systemctl enable --now nginx   # Enable + start in one command

Checking system state

systemctl list-units                  # List active units
systemctl list-units --type=service   # List active services
systemctl list-unit-files             # List all installed units + enabled state
systemctl is-active nginx             # Just check if running
systemctl is-enabled nginx            # Just check if enabled at boot
systemctl daemon-reload               # Reload unit files after editing

Logs (journald)

journalctl -u nginx           # Logs for a specific unit
journalctl -u nginx -f        # Follow logs live (like tail -f)
journalctl -b                 # Logs since last boot
journalctl -p err             # Only error-level logs
journalctl --since "1 hour ago"

Power/session control

systemctl reboot
systemctl poweroff
systemctl suspend

Anatomy of a Service Unit File

[Unit]
Description=My Custom App
After=network.target

[Service]
ExecStart=/usr/bin/myapp --config /etc/myapp.conf
Restart=on-failure
User=myuser

[Install]
WantedBy=multi-user.target

[Unit] — metadata and dependency ordering (After=, Requires=, Wants=)
[Service] — how to run it (ExecStart, Type, Restart, User)
[Install] — how it hooks into boot targets when enabled

Custom unit files go in /etc/systemd/system/myapp.service, then:

systemctl daemon-reload
systemctl enable --now myapp

Targets (replace old “runlevels”)

Target	Roughly equivalent to
`poweroff.target`	Runlevel 0
`rescue.target`	Runlevel 1 (single-user)
`multi-user.target`	Runlevel 3 (no GUI)
`graphical.target`	Runlevel 5 (GUI)
`reboot.target`	Runlevel 6

Check/set default target:

systemctl get-default
systemctl set-default multi-user.target

Timers (cron alternative)

A .timer unit pairs with a .service unit of the same name:

# mybackup.timer
[Timer]
OnCalendar=daily
Persistent=true

[Install]
WantedBy=timers.target

systemctl enable --now mybackup.timer
systemctl list-timers

Quick Troubleshooting Flow

systemctl status <service> — see if it’s running and recent log lines
journalctl -u <service> -e — jump to end of that unit’s logs
systemctl cat <service> — view the actual unit file being used
systemctl daemon-reload — after any manual edit to a unit file