SSH config file in Linux CLI - Step-by-Step Execution

Concept Flow - SSH config file

Start

↓

Read ~/.ssh/config

↓

Parse Host entries

↓

Match Host alias with user input

↓

Apply settings for matched Host

↓

Connect using applied settings

↓

End

The SSH client reads the config file, finds the matching Host entry, applies its settings, and then connects.

Execution Sample

Linux CLI

Host myserver
  HostName 192.168.1.10
  User alice
  Port 2222

Defines a shortcut 'myserver' to connect to 192.168.1.10 as user alice on port 2222.

Execution Table

Step	Action	Input/Line	Result/State
1	Read config file	Host myserver	Found Host entry 'myserver'
2	Parse HostName	HostName 192.168.1.10	Set HostName = 192.168.1.10
3	Parse User	User alice	Set User = alice
4	Parse Port	Port 2222	Set Port = 2222
5	Match user input	ssh myserver	Matched Host 'myserver'
6	Apply settings	Use HostName, User, Port	Connection will use 192.168.1.10, alice, port 2222
7	Connect	ssh myserver	SSH connects to 192.168.1.10:2222 as alice

💡 All settings applied and connection established using config file.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
HostName	undefined	192.168.1.10	192.168.1.10	192.168.1.10	192.168.1.10
User	undefined	undefined	alice	alice	alice
Port	undefined	undefined	undefined	2222	2222
Host	undefined	myserver	myserver	myserver	myserver

Key Moments - 3 Insights

Why does typing 'ssh myserver' connect to 192.168.1.10 and not the literal 'myserver'?

Because the SSH client reads the config file (see Step 5) and matches 'myserver' to the Host entry, then uses the HostName setting (Step 2) to connect.

What happens if the config file does not specify a Port?

SSH uses the default port 22. In the execution table, if Step 4 (Port) was missing, Port would remain undefined and default to 22.

Can multiple Host entries exist and how does SSH choose?

Yes, SSH checks each Host entry in order and uses the first that matches the input (Step 5). If none match, it uses default settings.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of User after Step 3?

A myserver

B undefined

C alice

D 2222

💡 Hint

Check the 'User' variable in variable_tracker after Step 3.

At which step does SSH match the input 'ssh myserver' to the Host entry?

A Step 2

B Step 5

C Step 7

D Step 1

💡 Hint

Look at the 'Action' column in execution_table where matching happens.

If the Port line was removed from the config, what would be the final Port used?

A 22

B 2222

C undefined

D myserver

💡 Hint

Recall default SSH port when Port is not set in config.

Concept Snapshot

SSH config file lets you create shortcuts for SSH connections.
Use 'Host' to name the shortcut.
Set 'HostName', 'User', and 'Port' under it.
SSH reads config, matches your shortcut, and applies settings.
This saves typing full commands every time.

Full Transcript

The SSH config file is read by the SSH client to simplify connections. It contains Host entries with settings like HostName, User, and Port. When you type 'ssh' with a shortcut name, SSH finds the matching Host entry, applies its settings, and connects accordingly. This process involves reading the file, parsing each line, matching the input, and then connecting with the specified parameters. If some settings like Port are missing, SSH uses defaults such as port 22. Multiple Host entries can exist, and SSH picks the first matching one. This makes connecting to servers easier and faster.