Shell Basics¶

Understanding what bash is and how it fits into the larger picture of terminals, shells, and command-line interfaces.

What is a Shell?¶

A shell is a program that interprets commands and communicates with the operating system. It's the layer between you and the kernel:

┌─────────────────────────────────────┐
│           User (you)                │
├─────────────────────────────────────┤
│     Terminal Emulator (UI)          │
├─────────────────────────────────────┤
│     Shell (bash, zsh, fish)         │
├─────────────────────────────────────┤
│     Operating System Kernel         │
├─────────────────────────────────────┤
│           Hardware                  │
└─────────────────────────────────────┘

The shell:

Reads commands you type
Interprets and expands them (variables, globs, etc.)
Executes programs
Returns output to you

Terminal vs Shell¶

These terms are often confused:

Terminal (or terminal emulator)

The application window you type in
Examples: Terminal.app, iTerm2, Alacritty, Windows Terminal
Handles display, fonts, colors, keyboard input

Shell

The program running inside the terminal
Interprets your commands
Examples: bash, zsh, fish, sh

You can run different shells in the same terminal, and the same shell in different terminals.

What is Bash?¶

Bash (Bourne Again SHell) is:

The default shell on most Linux distributions
Available on macOS (though zsh is now default)
A superset of the original Bourne shell (sh)
Both an interactive shell and a scripting language

Check your current shell:

echo $SHELL

/bin/bash

Or see what shell you're currently in:

echo $0

-bash

Bash Versions¶

Version matters for scripting:

bash --version

GNU bash, version 5.2.15(1)-release (aarch64-apple-darwin23.0.0)

Version	Notable Features
3.2	macOS default, last GPLv2 version
4.0	Associative arrays, `mapfile`, `&>>`
4.2	`declare -g`, negative array indices
4.3	`declare -n` (namerefs)
4.4	`${var@operator}` transformations
5.0	`EPOCHSECONDS`, improved arrays

macOS ships with Bash 3.2 due to licensing. Install Bash 5.x with Homebrew:

brew install bash

Interactive vs Non-Interactive¶

Interactive shell: You type commands, shell responds

$ ls
Documents  Downloads  Pictures
$ echo "Hello"
Hello

Non-interactive shell: Runs a script without user input

#!/bin/bash
echo "This runs automatically"

This distinction matters for configuration files (covered in Dotfiles).

Login shell: First shell when you log in

Opens when you SSH into a server
Reads .bash_profile or .profile

Non-login shell: Shells started after login

Opening a new terminal tab
Running bash from another shell
Reads .bashrc

Check if current shell is a login shell:

shopt -q login_shell && echo "Login" || echo "Non-login"

Basic Command Structure¶

Commands follow this pattern:

command [options] [arguments]

Examples:

ls                      # command only
ls -l                   # command + option
ls -l /home             # command + option + argument
ls -la /home /tmp       # command + options + multiple arguments

Options¶

Short options start with single dash:

ls -l           # long format
ls -a           # show hidden files
ls -la          # combined: -l and -a

Long options start with double dash:

ls --long       # same as -l
ls --all        # same as -a

Arguments¶

Arguments are the targets of the command:

cat file.txt            # file.txt is the argument
cp source.txt dest.txt  # two arguments

Getting Help¶

man Pages¶

The manual (man) provides detailed documentation:

man ls

Navigate with:

Space or f - page forward
b - page backward
/pattern - search forward
n - next search result
q - quit

--help Flag¶

Most commands support --help:

ls --help

help Built-in¶

For shell built-ins, use help:

help cd
help echo

type Command¶

Find out what a command is:

type ls

ls is /bin/ls

type cd

cd is a shell builtin

type ll

ll is aliased to `ls -l'

Command Execution¶

When you type a command, bash:

Parses the input (splits into words)
Expands variables, globs, etc.
Searches for the command:
Aliases
Functions
Built-ins
External programs (via PATH)
Executes the command
Returns output and exit status

Exit Status¶

Every command returns an exit status:

0 = success
1-255 = failure (specific meaning varies)

Check the last command's exit status:

ls /
echo $?

ls /nonexistent
echo $?

ls: /nonexistent: No such file or directory
1

Running Multiple Commands¶

Sequential (`;`)¶

Run commands one after another, regardless of success:

echo "first"; echo "second"; echo "third"

Conditional AND (`&&`)¶

Run next command only if previous succeeded:

mkdir mydir && cd mydir

Conditional OR (`||`)¶

Run next command only if previous failed:

cd mydir || mkdir mydir

Combining¶

cd mydir || mkdir mydir && echo "In mydir"

Comments¶

Comments start with #:

# This is a comment
echo "Hello"  # This is an inline comment

Try It¶

Check your current shell and version:
```
echo $SHELL
bash --version
```
Explore the type command:
```
type echo
type type
type ls
```
Practice getting help:
```
man bash
help help
ls --help
```
Test exit codes:
```
true; echo $?
false; echo $?
```

Summary¶

A shell interprets commands; a terminal displays them
Bash is the most common shell on Linux
Commands follow command [options] [arguments]
Exit status 0 means success; non-zero means failure
Use man, --help, and help for documentation