What Is `systemd`? (Need Revision from linkedIn series PTC)

systemd is a system and service manager for Linux operating systems. It initializes the system at boot, manages background services (daemons), and handles logging via journald.

A service in systemd is a unit of work that typically runs in the background. These are defined by .service files located in:

/etc/systemd/system/ — Custom or user-defined services
/lib/systemd/system/ — Installed by packages
/run/systemd/system/ — Runtime-generated services

Here’s a minimal example of a systemd service:


[Unit]
Description=My Sample Web Server
After=network.target
 
[Service]
ExecStart=/usr/bin/python3 -m http.server 8080
Restart=always
User=www-data
 
[Install]
WantedBy=multi-user.target

Key Sections

Section	Purpose
`[Unit]`	Metadata and dependencies
`[Service]`	How the service starts/stops
`[Install]`	When/where to start the service

Task	Command
Start a service	`sudo systemctl start nginx`
Stop a service	`sudo systemctl stop nginx`
Restart a service	`sudo systemctl restart nginx`
Reload without restarting	`sudo systemctl reload nginx`
Enable (auto-start at boot)	`sudo systemctl enable nginx`
Disable	`sudo systemctl disable nginx`
Check if running	`systemctl status nginx`
View all active services	`systemctl list-units --type=service`
View all installed service files	`systemctl list-unit-files`
Show details	`systemctl show nginx`
Analyze boot time of services	`systemd-analyze blame`


$ systemctl status ssh

Sample output:


● ssh.service - OpenBSD Secure Shell server
   Loaded: loaded (/lib/systemd/system/ssh.service; enabled)
   Active: active (running) since Mon 2025-07-21 10:00:00 IST; 3h 22min ago
 Main PID: 1234 (sshd)
    Tasks: 1
   Memory: 2.5M

Field	Meaning
`Loaded`	Service file is recognized and configured
`Active`	Current state (`active`, `inactive`, `failed`, etc.)
`Main PID`	The main process ID of the running service


# Create a custom service file
sudo nano /etc/systemd/system/hello.service

Content:


[Unit]
Description=Hello Service
 
[Service]
ExecStart=/bin/echo "Hello, world!"
 
[Install]
WantedBy=multi-user.target

Then:


sudo systemctl daemon-reexec    # Reload systemd itself (rarely needed)
sudo systemctl daemon-reload    # Reload unit files
sudo systemctl start hello
sudo systemctl enable hello
systemctl status hello

Reload after editing services: sudo systemctl daemon-reload
Check logs: journalctl -u servicename
Debug a failing service: Add StandardOutput=journal+console to your [Service] block.

Flag	Description
`--type=service`	Filter for services only
`--state=failed`	Show failed services
`--now`	Used with enable/disable to act immediately
`--user`	For user-level services

What Is systemd? (Need Revision from linkedIn series PTC)

Key Sections

What Is `systemd`? (Need Revision from linkedIn series PTC)