Startup and init systems (systemd) in Linux CLI - Step-by-Step Execution

Concept Flow - Startup and init systems (systemd)

Power On

↓

Firmware (BIOS/UEFI)

↓

Load bootloader (GRUB)

↓

Start kernel

↓

systemd starts

↓

Read unit files

↓

Start target units (like multi-user.target)

↓

Start services in order

↓

System ready for use

This flow shows how systemd starts after the kernel loads, reads unit files, and starts services to make the system ready.

Execution Sample

Linux CLI

systemctl start sshd.service
systemctl status sshd.service
systemctl enable sshd.service

Starts the SSH service, checks its status, and enables it to start on boot.

Execution Table

Step	Command	Action	Result	Output
1	systemctl start sshd.service	Start sshd service now	Success	● sshd.service - OpenSSH server daemon Loaded: loaded (/usr/lib/systemd/system/sshd.service; disabled) Active: active (running) since ...
2	systemctl status sshd.service	Check sshd service status	Active	● sshd.service - OpenSSH server daemon Active: active (running) since ...
3	systemctl enable sshd.service	Enable sshd to start at boot	Success	Created symlink /etc/systemd/system/multi-user.target.wants/sshd.service → /usr/lib/systemd/system/sshd.service
4	systemctl is-enabled sshd.service	Verify enable status	enabled	enabled
5	systemctl stop sshd.service	Stop sshd service	Success
6	systemctl status sshd.service	Check sshd service status	Inactive	● sshd.service - OpenSSH server daemon Active: inactive (dead) since ...

💡 Commands complete; sshd service started, enabled, then stopped and status checked.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 5	Final
sshd.service status	inactive	active (running)	active (running)	active (running)	inactive (dead)	inactive (dead)
sshd.service enabled	disabled	disabled	disabled	enabled	enabled	enabled

Key Moments - 3 Insights

Why does 'systemctl start' not enable the service to start on boot?

'systemctl start' only starts the service now but does not change boot settings. Enabling requires 'systemctl enable' as shown in step 3.

What does 'enabled' mean in systemd context?

'Enabled' means systemd will start the service automatically at boot. This is confirmed in step 4 with 'systemctl is-enabled'.

Why is the service status 'inactive' after stopping it?

Stopping the service with 'systemctl stop' (step 5) halts it immediately, so status changes to 'inactive' as seen in step 6.

Visual Quiz - 3 Questions

Look at the execution table, what is the status of sshd.service after step 1?

A failed

B inactive (dead)

C active (running)

D enabled

💡 Hint

Check the 'Result' and 'Output' columns in row for step 1.

At which step is sshd.service enabled to start at boot?

A Step 3

B Step 1

C Step 5

D Step 6

💡 Hint

Look for the command 'systemctl enable sshd.service' in the execution table.

If you skip 'systemctl enable', what happens on next boot?

A sshd.service starts automatically

B sshd.service does not start automatically

C sshd.service fails to start

D sshd.service is removed

💡 Hint

Refer to the meaning of 'enabled' in the key moments section.

Concept Snapshot

systemd manages startup by reading unit files and starting services.
Use 'systemctl start <service>' to run now.
Use 'systemctl enable <service>' to start on boot.
Check status with 'systemctl status <service>'.
Stopping a service changes status to inactive.

Full Transcript

When a Linux system boots, systemd starts after the kernel loads. It reads unit files that describe services and targets. Using systemctl commands, you can start services immediately, check their status, and enable them to start automatically on boot. For example, 'systemctl start sshd.service' runs the SSH server now, but does not enable it. 'systemctl enable sshd.service' sets it to start on future boots. Stopping a service with 'systemctl stop' makes it inactive. This flow helps manage system services clearly and reliably.