Linux: What Is the Shell?

In one line: the shell is a program that takes what you type and runs it — the terminal is just the window it lives in.

Companion to: William Shotts, The Linux Command Line, chapter 1.

The moving parts

Term	What it is
Shell	The program that reads your commands and runs them. On most Linux systems it’s bash
Terminal emulator	The window that talks to the shell — GNOME Terminal, Konsole, Windows Terminal (via WSL)
Virtual console	Text-only screens behind the desktop, reached with `Ctrl+Alt+F3`–`F6` (varies by distro)
CLI vs GUI	Typing commands vs clicking. The CLI is faster to repeat, script and run remotely

echo $SHELL        # your login shell, e.g. /bin/bash
bash --version     # which bash
cat /etc/shells    # shells installed on this system

andrew@lab:~$

Part	Meaning
`andrew`	Your username
`lab`	The machine’s hostname
`~`	Current directory (`~` = your home directory)
`$`	Normal user
`#`	Root (superuser) — careful, nothing will stop you

If you type a command that doesn’t exist, bash tells you: bash: xyz: command not found.

Key	Does
`↑` / `↓`	Scroll through command history
`←` / `→`	Move the cursor to edit the line
`Enter`	Run the line
`Ctrl+C`	Interrupt the running command — not copy

Method	How
Keyboard	`Ctrl+Shift+C` / `Ctrl+Shift+V` in most terminal emulators
Mouse (X11)	Highlighting copies; middle-click pastes

Muscle memory from Windows is the trap: Ctrl+C in a terminal kills whatever is running.

Command	Does
`date`	Current date and time
`cal`	Calendar for this month
`df`	Free space on each mounted filesystem (`-h` for human-readable sizes)
`free`	Memory and swap usage (`-h` for human-readable)
`clear`	Clear the screen
`exit`	Close the shell (or press `Ctrl+D` on an empty line)

Option	How
Linux VM in your lab	Ubuntu VM — snapshot it first
WSL on Windows	`wsl --install` in an elevated PowerShell, then reboot
SSH into a lab box	`ssh andrew@192.168.1.50`

A # prompt means root. Work as a normal user and use sudo for the one command that needs it — every sudo call is logged.
Pasting commands from websites is risky. A page can hide extra characters or a newline in what you copy, so it runs the moment you paste (“pastejacking”).
- Bash 5.1+ enables bracketed paste by default, which stops pasted text running until you press Enter.
- Still read it before running it.
Bash keeps history in ~/.bash_history. Investigators read it; attackers try to switch it off with tricks like unset HISTFILE. An empty or missing history file on a server is itself a clue.

1. What does root@web01:/var/log# tell you?

You’re root, on host web01, in /var/log.

2. You pressed Ctrl+C to copy some output and the command stopped. Why?

In a terminal Ctrl+C sends an interrupt. Copy with Ctrl+Shift+C or by highlighting.

3. How much disk space is free, in GB rather than blocks?

df -h

4. Which shell are you running?

echo $SHELL (and bash --version for the version)

The shell runs commands; the terminal emulator is just the window.
Read the prompt: who you are, where you are, and $ vs #.
↑ recalls history. Ctrl+C interrupts — it doesn’t copy.
date, cal, df -h, free -h, exit — you can use a Linux box from minute one.