Writing a Toy Shell in C

TODO:

Write an introduction about recently writing a toy shell

TODO:

Caveat by saying this isn’t a full overview of writing a shell, but rather just a fun overview of what I’ve done.

What is a shell

At the end of the day, the purpose of a computer is to run programs. Right now, you’re reading this in your web browser, which is just a very large and complicated programs. While nowadays many programs have graphical interfaces, and get started by using a desktop shortcut, we can also start programs using a command line interface, from our terminal.

Inside of a terminal, you’re presented with a prompt, and can start programs to do different things. For example, I can start my web browser from the terminal:

>> firefox

I can also run simpler programs, like creating an empty file:

>> touch foo.txt

What’s happening under the hood is that the terminal is running a single program, called a shell, which processes these commands. The shell reads in commands, parses them, and executes them. Most often, these commands ask the shell to start other programs. Naturally, after that program is finished executing, we want to go back to the shell. We may also want that program to run in the background, and continue working in our shell. The shell needs to take care of this orchestration, launching programs in such a way that the shell sticks around to continue processing our commands.

The shell also redirects the output of the programs it launches in a certain way. Usually, we want the shell to print out whatever the program prints out. For example, if you list all the files in a directory, you want to see the result:

>> ls
a.txt b.txt c.txt

Sometimes, you want to redirect the output of a program to a file, which we can easily do inside of the shell as well:

>> ls > files.txt

So, in summary, a shell is just a program with a simple command-line interface, and which lets us launch other programs, through a simple command language.

Bash

TODO:

Explain what the bash language is

TODO:

Explain what subset of bash was implemented.

Architecture

TODO:

Explain the general architecture.

Lexer

Parser

Compiler

Interpreter

Producing Effects

Conclusion