Difference between terminal, shell, prompt and command line

When I started using Linux, the command line was a whole new world to me. The differences between a terminal, a shell, a prompt and a command line were absolutly lost on me.

Now, a few years later, boundaries are much clearer. This post is an attempt at clarifying them.

What is a terminal?

A terminal, or terminal emulator, is a software program that lets you interact with the command line through your shell. The important part is that this is a GUI program.

gnome-shell is the default terminal emulator on Ubuntu, while iTerm is the common choice in the Mac world (I think, I don't own a Mac). Other alternatives include rxvt, termite, terminology or terminator.

They all look like a dark screen where you type commands, but each adds its own features and configuration. terminology for example lets you play video or music directly, and have some advanced visual effects.

terminator is the one I used for a long time. It has a very useful feature that lets you split your screen in different chunks, to avoid alt-tabbing all the time and multi-task easily.

In the end, the terminal emulator is just a wrapper around your shell.

What is a shell?

The shell is the program that reads your commands (like ls, pwd, etc) and evaluate them. Shells are mostly used as REPL (Read, Eval, Print, Loop). Meaning it is waiting for you to type something in its prompt and pressing Enter. Then it will evaluate the command and display a new prompt for you to type in a new command.

The most basic shell is simply named sh, and the most common one is bash. Other shells (with names like ksh, zsh or fish) also exists and provide different features than bash.

If you're writing shell scripts, different shells are like different scripting languages (but they do share a common basis and most Unix shells you'll find today are bash-compatible). In your everyday life though, what a different shell might bring you is clever tab-completion, output coloring, smart aliases, etc.

I'm personnally using zsh. It offers a really nice tab-completion (even through distant ssh connections), and let me greatly configure what my prompt looks like. oh-my-zsh is a very popular set of plugins to enhance the default zsh experience. I don't personnaly use it, but I've seen countless person using it.

In the end, as all shells are just scripting languages, they are all able to do pretty much the same thing. bash as the huge advantage of being a standard and available almost on any machine, while zsh has a large library of plugins you can download and install.

In the end, the shell is where you'll spend most of your time. You should invest some time into configuring it so you get faster on the command line.

What is a prompt?

The prompt is simply the line that is displayed in your shell when you type a command. The default is usually something like user@host:/path/ $. This gets displayed at the start of each line and reminds you of who the current user is, on which machine and in which path.

After a while, you're so used to it that you stop seeing it. But such a basic prompt is not very useful. I strongly encourage you to customise it to your needs. As it will get displayd on every new command, it is a nice place to display useful information.

You can even add color to it, and also make use of what is known in zsh as the RPROMPT, the prompt on the right.

In my config, I display the path with a different color depending if I currently have writing rights in it or not. I also color the @ in red if the previous command returned an error code.

When I'm in a git repository, I also change the $ symbol to a ±, and color it differently depending on the status of my current index (green if clean, red if untracked files). I also display the current branch and tag in the right prompt, as well as symbols to tell me if I need to push or pull.

I spend a lot of time in the command line and in git repositories, and I don't want to type git status or git branch too often. It is much better when this information is displayed on my prompt. If you choose your colors well, your brain will be able to parse the information really fast.

In the end, the shell is the line that gets displayed whenever you need to type a command. It is the best place to put output of commands you often type, so you no longer have to.

What is a command line?

When someone tells you to "type something in the command line", it actually means "type something in your prompt, which is displayed by your shell, which is loaded by your terminal emulator". The command line is all this.

At that moment, we don't really care about what your prompt looks like or if you're using ksh inside rxvt. All we need to know is the output of the command.

Wrapping up

Hopefully, this made this a lot clearer for some of you. Next time, I'll try to talk about tmux, which is a new layer between the terminal and the shell.

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