Lesson 3 of 29 beginner 6 min read

The shell explained

Key takeaways The shell is the program that reads what you type and runs it. It shows you a prompt, waits for a line, and hands the result back. Almost everything you type follows one pattern — a command, then options that change how it behaves, then arguments it acts on. Learn that pattern once and every command reads the same way.

In the last lesson we said the command line gives you speed, precision, and remote control. This lesson introduces the thing that actually does that work: the shell. Understanding it turns a wall of cryptic text into a small set of predictable pieces.

Terminal vs. shell

These two words get used interchangeably, but they’re different things.

The terminal is the window — the app on your screen that displays text and passes your keystrokes along. On Linux it might be called GNOME Terminal or Konsole; on macOS, Terminal or iTerm.

The shell is the interpreter running inside that window. It reads each line you type, works out what you meant, runs the right program, and prints the result. Common shells are bash, zsh, and sh.

A useful picture: the terminal is the phone, the shell is the person you’re talking to. The same shell can run inside many different terminals, and one terminal can run different shells — they’re separate jobs.

The prompt

When a shell is ready for a command, it shows a prompt — a bit of text ending in a symbol, with the cursor waiting after it. A typical prompt packs in some useful context:

matt@raspberrypi:~/gophertrunk$

Read left to right, that’s the user (matt), the host (raspberrypi), and the current directory (~/gophertrunk, where ~ means your home folder). Prompts vary between systems, but most show some version of who and where you are.

The final symbol follows a widespread convention:

$ — you’re a normal, everyday user.
# — you’re root, the all-powerful administrator, where a typo can wreck the system. Seeing # is a signal to slow down (more on this in sudo & becoming root safely).

Throughout this course, example commands start with $ to show they’re typed at the prompt. You don’t type the $ yourself — just the part after it.

Anatomy of a command

Here’s the single most useful idea in this whole path. Almost every command follows the same shape:

command [options] [arguments]

The square brackets just mean “optional” — plenty of commands work with none. Let’s break down a real one:

$ ls -l /home

ls is the command — the program to run (here, “list files”).
-l is an option (also called a flag) — it changes how the command behaves, in this case asking for a long, detailed listing.
/home is an argument — what the command acts on, here the directory to list.

Change the argument and ls lists somewhere else; change the option and it lists the same place differently. That separation — what to do, how to do it, what to do it to — is the same for nearly every command you’ll meet.

A few details that come up constantly:

Short vs. long options. Many options have a terse form and a readable form that do the same thing: -a and --all, or -h and --help. Short options can often be bundled, so ls -la is the same as ls -l -a.
Options that take a value. Some options expect their own argument right after them, like head -n 5 file.txt (show the first 5 lines).
Spaces separate everything. The shell splits your line on spaces, so a filename with a space needs quoting — cat "my notes.txt" — or the shell reads it as two arguments.

Built-ins vs. programs

Not every command is a separate program. A handful are built into the shell itself — cd (change directory) is the classic example, because changing the shell’s own directory is something only the shell can do.

Most commands, though, are separate programs living as files on disk — ls, cat, grep and the rest. When you type one, the shell searches a list of directories called your PATH to find the matching program, then runs it. That’s why you can type ls instead of its full location: it’s on the PATH. We’ll unpack PATH properly in environment variables, and you’ll meet your first real commands in your first commands.

Common shells

You’ll almost certainly be using one of these:

bash — the default on most Linux distributions and the one most tutorials assume.
zsh — the default on modern macOS, with fancier completion and prompts.
sh — a smaller, older shell that scripts often target for maximum portability.

The good news: for everything in this course they’re broadly compatible. The prompt, the command pattern, and the everyday commands all work the same way. When a difference actually matters, we’ll call it out.

Quick check: in ls -l /home, which part is the option (flag)?

Recap

The terminal is the window; the shell (bash, zsh, sh) is the interpreter inside it that reads and runs your commands.
The prompt shows who and where you are, ending in $ for a normal user or # for root.
Nearly every command follows command [options] [arguments] — what to run, how, and on what.
Options come in short (-a) and long (--all) forms, and some take a value.
A few commands (like cd) are built into the shell; most are programs the shell finds on your PATH.

Next up: getting a shell

Frequently asked questions

What is a shell in Linux?

The shell is the program that reads the commands you type and runs them. It sits between you and the operating system — you type a line, the shell figures out what you meant, launches the right program (or runs a built-in), and shows you the result. Bash and zsh are the two you’ll meet most.

What is the difference between a terminal and a shell?

The terminal is the window (or app) that shows text and takes your keystrokes; the shell is the interpreter running inside it that actually reads and executes your commands. One is the screen, the other is the brain. You can run the same shell in many different terminals.

What do the parts of a command mean?

Almost every command follows the pattern command [options] [arguments]. The command is the program to run, options (also called flags, like -l) change how it behaves, and arguments are what it acts on, like a filename or path. In ls -l /home, ls is the command, -l is the option, and /home is the argument.

What is the difference between bash and zsh?

Both are shells, and for everything in this course they behave the same way. Bash is the default on most Linux systems; zsh is the default on modern macOS. Zsh adds nicer tab-completion and prompts, but the commands and patterns you learn here work in either.