systemd Basics

This article is mainly the notes from “systemd” Tutorials

Why “systemd”?

What is systemd?

A new kind of init system.
Replaces SysV Init system
Created at Redhat
Chief Architect: Lennart Poettering
Manages system services, targets, and resources.

Why systemd?

Faster boot-up: Daemons start in parallel, reducing boot time.
Simplified dependencies: No need to manually order daemon startup (no more numbered “S” and “K” links).
Simpler config files: Easier to manage.
Auto-restart: Automatically restarts crashed services.
Better security: Services run in isolated cgroups; some services (e.g., auditd) cannot be stopped by root.
Resource management: Tracks resource usage per process.
Consistent administration: Unified commands across Linux distributions.

Shundown and Reboot Commands

systemd replaces traditional commands like shutdown, service, and chkconfig on Red Hat-type systems.

# Shutdown
sudo systemctl poweroff

# Reboot
sudo systemctl reboot

# Traditional commands still work but are deprecated
shutdown
reboot

For more details: man systemctl

Daemon Management

# Enable/disable services
systemctl enable name.service
systemctl disable name.service
systemctl is-enabled name.service

# Start/stop services
systemctl start name.service
systemctl stop name.service

# Check status
systemctl status name.service
systemctl status -l name.service  # More detailed

# List all services
systemctl list-unit-files --type service
systemctl list-unit-files

Tragets vs. Runlevels

Targets are systemd’s equivalent of SysV runlevels.

# List all targets
sudo systemctl list-units --type target --all

# Set default target (runlevel)
sudo systemctl set-default multi-user.target  # Text mode
sudo systemctl set-default graphical.target   # Graphical mode

# Check current default
sudo systemctl get-default

# Switch to a target immediately
sudo systemctl isolate graphical.target  # To graphical
sudo systemctl isolate multi-user.target # To text mode

Automatically Restart Crashed Services

To enable auto-restart, edit the service file:

sudo systemctl edit name.service

Add under [Service]:

Restart=always

Then reload and restart:

sudo systemctl daemon-reload
sudo systemctl restart name.service

Edit systemd Service Files Correctly

Use systemctl edit to override service files without modifying originals:

sudo systemctl edit nginx.service

This creates drop-in files in /etc/systemd/system/nginx.service.d/.

Controlling the auditd Service

auditd is a security auditing service.

sudo systemctl start auditd
sudo systemctl stop auditd
sudo systemctl status auditd

Note: auditd cannot be stopped or restarted by any user, including root, for security reasons.

Setting the hostname with systemd

# Check current hostname
hostname

# Set new hostname
sudo hostnamectl set-hostname newhostname

Setting the timezone with timedatectl

# Check current time and timezone
timedatectl

# Set timezone (e.g., to UTC)
sudo timedatectl set-timezone UTC

# Set time manually
sudo timedatectl set-time "2023-01-01 12:00:00"

Systemd Service Files

Service files are in /etc/systemd/system/ or /usr/lib/systemd/system/.

Example service file:

[Unit]
Description=ABC service
Documentation=https://example.com/abc.html
After=network.target

[Service]
Environment=VAR1=abc VAR2=abc VAR3=abc
ExecStart=/usr/local/bin/abc
Restart=on-failure
RestartSec=5
User=ubuntu
Group=ubuntu
Type=simple
WorkingDirectory=/data/abc

[Install]
WantedBy=multi-user.target

Another example:

[Unit]
Description=ABC service
After=network.target

[Service]
User=ubuntu
Group=ubuntu
Type=simple
Restart=always
RestartSec=5s
WorkingDirectory=/data/abc
ExecStart=/data/abc/abc

[Install]
WantedBy=multi-user.target

[Unit]: Dependencies and description.
[Service]: How to run the service.
[Install]: When to start the service.

manual

man systemd

Additional Important Points

Units: systemd manages units (services, targets, sockets, timers, etc.). Services are just one type.

Logs: Use journalctl to view system logs.

journalctl -u name.service  # Logs for a service
journalctl -f              # Follow logs in real-time

Timers: systemd timers replace cron jobs.
```
systemctl list-timers
```
Network Management: systemd-networkd for network configuration.
Resource Control: Use systemctl show to see cgroup info.
Masking Services: Prevent a service from starting: sudo systemctl mask name.service
Reloading Configuration: After editing files: sudo systemctl daemon-reload