The Shell and Commands

The shell is the command interpreter that processes what you type and executes programs. Understanding how the shell works will make you much more effective at the command line.

What is a Shell?

The shell is a program that:

• Reads commands from the terminal
• Interprets those commands
• Executes programs
• Returns output to the terminal

Common Shells

Shell	Description
bash	Bourne Again Shell - most common, default on most Linux
zsh	Z Shell - enhanced features, default on macOS
sh	Bourne Shell - original Unix shell
fish	Friendly Interactive Shell - user-friendly

To check your current shell:

Command Structure

Commands follow a consistent structure:

command [options] [arguments]

Examples:

• ls - just the command
• ls -l - command with option
• ls -l /home - command with option and argument
• ls -la /home - command with multiple options

Options and Arguments

Options (Flags)

Options modify how a command works:

• Short options: Single dash with letter (-l, -a, -h)
• Long options: Double dash with word (--all, --help)
• Combined short: Multiple letters (-la same as -l -a)

Arguments

Arguments are what the command operates on:

cat file.txt        # file.txt is the argument
cp source dest      # two arguments
mkdir new_folder    # folder name is argument

Getting Help

Every command has help available:

Method	Usage
--help	ls --help
man	man ls (manual page)
help	help cd (for built-ins)

Command History

The shell remembers your commands:

• history - show all previous commands
• Up Arrow - recall previous command
• !! - repeat last command
• !n - repeat command number n

Tab Completion

Tab completion saves time:

1. Start typing a command or filename
2. Press Tab
3. Shell completes it (if unique) or shows options

Try it in the terminal!

Key Takeaways

• The shell interprets and executes commands
• Commands follow: command [options] [arguments]
• Use --help or man for command documentation
• Tab completion and history make you faster