systemd — Complete, Concise Notes

What is systemd?

systemd is the init system and service manager for modern Linux distributions.

It is responsible for:

• Booting the system
• Starting & supervising services
• Managing processes, logs, devices, mounts, users, timers, and networking

It replaced SysVinit & Upstart in most distros (Ubuntu, RHEL, CentOS, Debian, Arch).

Why systemd exists (Problems it solved)

Old init systems:

• Sequential startup (slow)
• Poor dependency handling
• No supervision after start
• Scattered logging

systemd provides:

• Parallel startup → faster boot
• Dependency-aware service management
• Automatic restarts & monitoring
• Unified logging & control interface

Core Concepts

1. Units

Everything in systemd is a unit.

A unit is a configuration file describing something systemd manages.

Common unit types:

Example unit name:

nginx.service

2. Unit File Locations

Priority order:

/etc/systemd/system/     # Admin overrides (highest priority) 

/run/systemd/system/     # Runtime units /lib/systemd/system/     

# Distribution default units

How systemd Boots (Boot Flow)

1. BIOS/UEFI
2. Bootloader (GRUB)
3. Kernel loads
4. PID 1 = systemd
5. systemd:
   • Mounts filesystems
   • Starts default target
   • Launches services in parallel
   • Applies dependencies & ordering

Check PID 1:

ps -p 1 -o comm=

Targets (Runlevels Replacement)

Targets replace SysV runlevels.

Default target:

systemctl get-default

Set default:

systemctl set-default multi-user.target

Services

Service lifecycle

systemctl start nginx 

systemctl stop nginx 

systemctl restart nginx 

systemctl reload nginx 

systemctl status nginx

Enable at boot:

systemctl enable nginx

Disable:

systemctl disable nginx

Anatomy of a Service File

Example:

[Unit] Description=Nginx Web Server After=network.target 

[Service] ExecStart=/usr/sbin/nginx -g 'daemon off;' Restart=always User=www-data

[Install] WantedBy=multi-user.target

Sections explained

• [Unit] → description & dependencies
• [Service] → how process runs
• [Install] → when to start (target linkage)

Dependencies & Ordering

Dependency types

• Requires= → hard dependency
• Wants= → soft dependency
• After= → start order
• Before= → inverse order

Example:

Requires=network.target After=network.target

Socket Activation (Key systemd Feature)

systemd:

• Opens socket before service starts
• Starts service only when traffic arrives

Benefits:

• Faster boot
• Lower memory usage

Example:

sshd.socket → sshd.service

Timers (Cron Replacement)

Timer unit + service unit.

Example:

systemctl list-timers

Timer benefits over cron:

• Missed jobs run after reboot
• Dependency-aware
• Unified logging

Logging — journald

systemd logging is handled by journald.

View logs:

journalctl journalctl -u nginx journalctl -b journalctl -f

Persistent logs:

/var/log/journal/

Process & Resource Management (cgroups)

systemd uses cgroups to:

• Track all processes of a service
• Enforce limits

Examples:

MemoryLimit=500M CPUQuota=50%

Check cgroups:

systemd-cgls

systemd vs traditional init

Common Debugging Commands

systemctl status <service> journalctl -xe systemctl list-units systemctl list-unit-files systemctl show <service>

Key systemd Components

Why interviewers care

systemd shows:

• Linux internals knowledge
• Service reliability understanding
• Debugging ability
• Production readiness

One-line summary (remember this)

systemd is a dependency-aware, parallel init and service management system that controls how Linux boots, runs, logs, and manages resources.