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The Shell & Terminal

Every automation task, every server deployment, every data pipeline — it all runs through the shell at some point. Learning to use the shell is not just a skill for sysadmins; it is the foundation of every developer's power-user toolkit.

Learning Objectives

By the end of this section you will be able to:

  • Explain the difference between a shell and a terminal emulator
  • Navigate the filesystem using pwd, cd, and ls
  • Read documentation using man pages
  • Use tab completion and command history to work faster
  • Identify which shell you are running and check its version

What Is a Shell?

A shell is a program that accepts text commands, interprets them, and passes them to the operating system kernel for execution. Think of it as a translator sitting between you and the hardware.

The most common shell on Linux and macOS is bash (Bourne Again SHell). Most scripts in this course target bash specifically.

bash vs zsh

macOS changed its default shell to zsh in Catalina (2019). For scripting, bash and zsh are nearly identical at the level of this course. Scripts start with #!/usr/bin/env bash to be explicit about which interpreter to use.

Shells vs Terminal Emulators

These two terms are often used interchangeably, but they are different programs:

Term What it is Examples
Shell The program that interprets commands bash, zsh, sh, fish, ksh
Terminal emulator The window/app that displays the shell GNOME Terminal, iTerm2, Windows Terminal, Alacritty

The terminal emulator handles the visual side — colors, fonts, scrollback buffer. The shell handles the logic — what happens when you type a command.

Common Shells

Shell Where you will see it Notes
sh POSIX scripts, minimal containers Oldest, most portable
bash Linux servers, most scripts Most common for scripting
zsh macOS default, power users Better interactive experience
fish Developer workstations Friendly, but non-POSIX
dash Ubuntu's /bin/sh Faster than bash, fewer features

Check your shell

echo $SHELL        # your default login shell
echo $0            # the currently running shell process
bash --version     # bash version

These three commands are the ones you will type thousands of times. Learn them now and use them constantly.

pwd — Print Working Directory

pwd

/home/user

pwd tells you where you are. Run it whenever you feel lost.

ls — List Directory Contents

ls

Desktop  Documents  Downloads  Music  Pictures  scripts


ls -la

total 48
drwxr-xr-x  8 user user 4096 Jan 15 09:23 .
drwxr-xr-x  3 root root 4096 Jan 10 08:00 ..
-rw-------  1 user user  220 Jan 10 08:00 .bash_logout
-rw-------  1 user user 3526 Jan 10 08:00 .bashrc
drwxr-xr-x  2 user user 4096 Jan 15 09:23 Desktop
drwxr-xr-x  3 user user 4096 Jan 12 14:05 Documents
drwxr-xr-x  2 user user 4096 Jan 14 11:30 Downloads
drwxr-xr-x  4 user user 4096 Jan 15 09:10 scripts

The -l flag gives long format (permissions, owner, size, date). The -a flag shows hidden files (files whose names start with .). You can combine flags: -la or -l -a.

cd — Change Directory

cd Documents
pwd

/home/user/Documents


cd ..        # go up one level
cd ~         # go to your home directory
cd -         # go back to where you just were
cd /etc      # absolute path — starts from filesystem root

Tab completion

Type the first few characters of a path and press Tab. Bash completes it automatically. If multiple matches exist, press Tab twice to see all options. Use this constantly — it prevents typos and saves time.

Reading Documentation with man

man opens the manual page for any command:

man ls

This opens the manual in a pager (usually less). Navigation:

Key Action
j / k Scroll down / up one line
Space Next page
b Previous page
/pattern Search for text
n Next search match
q Quit

man page sections

Man pages are divided into sections. Section 1 is user commands, section 5 is file formats, section 8 is system administration. If a name exists in multiple sections (e.g., passwd), specify the section: man 5 passwd.

When man is not enough:

ls --help          # short built-in help for most commands
type ls            # tells you whether ls is a builtin, alias, function, or binary
which bash         # shows the full path to the bash executable
command -v bash    # POSIX-portable version of which

Command History

bash saves commands you run in ~/.bash_history. Access your history:

history            # list recent commands with line numbers
history 20         # last 20 commands only
!!                 # re-run the last command
!ls                # re-run the last command that started with ls
Ctrl+R             # reverse search — type to find a previous command

Build the habit now

Tab completion and Ctrl+R are the two single biggest speed improvements for working in the shell. Use them from your very first session. After a week they become automatic.

Your First Session

A typical navigation session from scratch:

# Where am I?
pwd

/home/user


# What is here?
ls -la

total 48
drwxr-xr-x  8 user user 4096 Jan 15 09:23 .
drwxr-xr-x  3 root root 4096 Jan 10 08:00 ..
-rw-------  1 user user 3526 Jan 10 08:00 .bashrc
drwxr-xr-x  2 user user 4096 Jan 15 09:23 Desktop
drwxr-xr-x  3 user user 4096 Jan 12 14:05 Documents
drwxr-xr-x  4 user user 4096 Jan 15 09:10 scripts


# Move into a directory and back
cd Documents
pwd

/home/user/Documents


cd -
pwd

/home/user


# Create a practice directory
mkdir -p ~/scripts/practice
cd ~/scripts/practice
pwd

/home/user/scripts/practice

Common Mistakes

Case sensitivity

Linux filesystems are case-sensitive. Documents, documents, and DOCUMENTS are three different directories. Windows and macOS are case-insensitive by default, so this surprises people coming from those platforms.

Spaces in paths

A space in a filename breaks naive commands. Always quote paths that might contain spaces: cd "My Documents", not cd My Documents. The second version tells bash to cd to My — which probably does not exist.

Confusing shells and terminals

If someone says "open a terminal," they mean open a terminal emulator. If someone asks "which shell are you using?", they mean bash/zsh/fish. The terminal is just a window; the shell is the program interpreting your commands.

Practice Exercises

Warm-Up (run and observe)

  1. Open a terminal. Run pwd. Note your home directory path.
  2. Run ls -la ~. Identify three hidden files (names starting with .).
  3. Navigate to /etc using cd /etc. Run ls | head -20. Navigate back with cd ~.
  4. Run man ls. Find the flag that sorts files by modification time. Press q to exit.
  5. Run type cd and type ls. Why does one say "shell builtin" and the other a file path?

Main (write a short script)

Create ~/scripts/practice/explore.sh:

#!/usr/bin/env bash
set -euo pipefail

echo "=== Current directory ==="
pwd

echo ""
echo "=== Contents ==="
ls -la

echo ""
echo "=== Your shell ==="
echo "$SHELL"

echo ""
echo "=== System info ==="
uname -a

Run it:

chmod +x ~/scripts/practice/explore.sh
bash ~/scripts/practice/explore.sh

Stretch

  1. Use man bash to find what the HISTSIZE and HISTFILESIZE variables control. What are the defaults on your system? (Hint: check ~/.bashrc.)
  2. Press Ctrl+R and search for a command you ran earlier in this session. How do you execute it? How do you cancel the search without running anything?
  3. Run echo $PATH. What is $PATH? Why is your current directory usually not in it?

Interview Questions

  1. What is the difference between a shell and a terminal emulator?
Show answer

A terminal emulator is the GUI application that provides a text window (examples: GNOME Terminal, iTerm2, Windows Terminal). A shell is the program running inside that window that interprets your commands (examples: bash, zsh, fish). When you close the terminal window, both programs exit — but they are distinct programs with distinct responsibilities.

  1. How do you find out which shell you are currently running?
Show answer

Run echo $SHELL to see your default login shell (what opens when you start a terminal). Run echo $0 to see the name of the currently active shell process. These can differ if you launched a different shell explicitly (e.g., typed zsh inside a bash session).

  1. What does cd - do?
Show answer

It changes to the previous working directory — whatever $OLDPWD contained before your last cd command. It is useful for toggling between two directories without typing their full paths. bash prints the directory it switched to.

  1. Why would you use man 5 passwd instead of man passwd?
Show answer

man passwd (section 1) shows the man page for the passwd command used to change passwords. man 5 passwd shows the man page for the /etc/passwd file format. When a name has pages in multiple sections, you must specify the section number to get the right one.

  1. What does type ls tell you that which ls does not?
Show answer

type ls tells you whether ls is a shell builtin, an alias, a function, or an external binary — and shows the full definition if it is an alias or function. which ls only searches $PATH for external binaries; it cannot reveal aliases or shell builtins.

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