Updated 2018-07-03: Based on the reddit comments, I’m now using the correct output devices.
Updated 2019-03-21: Here you can find my Rust port of the shell.
In this post, we will write a minimalistic shell for UNIX(-like) operating systems in the Go programming language and it only takes about 60 lines of code. You should be a little bit familiar with Go (e.g. how to build a simple project) and the basic usage of a UNIX shell.
UNIX is very simple, it just needs a genius to understand its simplicity. - Dennis Ritchie
Of course, I’m not a genius, and I’m not even sure if Dennis Ritchie meant to include the userspace tools. Furthermore, a shell is only one small part (and compared to the kernel, it’s really an easy part) of a fully functional operating system, but I hope at the end of this post, you are just as astonished as I was, how simple it is to write a shell once you understood the concepts behind it.
» What is a shell?
Definitions are always difficult. I would define a shell as the basic user interface to your operating system. You can input commands to the shell and receive the corresponding output. When you need more information or a better definition, you can look up the Wikipedia article.
Some examples of shells are:
The graphical user interfaces of Gnome and Windows are shells but most IT related people (or at least I) will refer to a text-based one when talking about shells, e.g. the first two in this list. Of course, this example will describe a simple and non-graphical shell.
In fact, the functionality is explained as: give an input command and receive the output of this command. An example? Run the program
ls to list the content of a directory.
That’s it, super simple. Let’s start!
» The input loop
To execute a command, we have to accept inputs. These inputs are done by us, humans, using keyboards ;-)
The keyboard is our standard input device (
os.Stdin) and we can create a reader to access it. Each time, we press the enter key, a new line is created. The new line is denoted by
\n. While pressing the enter key, everything written is stored in the variable
Let’s put this in a
main function of our
main.go file, and by adding a
for loop around the
ReadString function, we can input commands continuously. When an error occurs while reading the input, we will print it to the standard error device (
os.Stderr). If we would just use
fmt.Println without specifying the output device, the error would be directed to the standard output device (
os.Stdout). This would not change the functionality of the shell itself, but separate devices allow easy filtering of the output for further processing.
» Executing Commands
Now, we want to execute the entered command. Let’s create a new function
execInput for this, which takes a string as an argument. First, we have to remove the newline control character
\n at the end of the input.
Next, we prepare the command with
exec.Command(input) and assign the corresponding output and error device for this command. Finally, the prepared command is processed with
» First Prototype
We complete our
main function by adding a fancy input indicator (
>) at the top of the loop, and by adding the new
execInput function at the bottom of the loop.
It’s time for a first test run. Build and run the shell with
go run main.go. You should see the input indicator
> and be able to write something. For example, we could run the
Wow, it works! The program
ls was executed and gave us the content of the current directory. You can exit the shell just like most other programs with the key combination
Let’s get the list in long format with
It’s not working anymore. This is because our shell tries to run the program
ls -l, which is not found. The program is just
-l is a so-called argument, which is parsed by the program itself. Currently, we don’t distinguish between the command and the arguments. To fix this, we have to modify the
execLine function and split the input on each space.
The program name is now stored in
args and the arguments in the subsequent indices. Running
ls -l now works as expected.
» Change Directory (cd)
Now we are able to run commands with an arbitrary number of arguments. To have a set of functionality which is necessary for a minimal usability, there is only one thing left (at least according to my opinion). You might already come across this while playing with the shell: you can’t change the directory with the
No, this is definitely not the content of my root directory. Why does the
cd command not work? When you know, it’s easy: there is no real
cd program, the functionality is a built-in command of the shell.
Again, we have to modify the
execInput function. Just after the
Split function, we add a
switch statement on the first argument (the command to execute) which is stored in
args. When the command is
cd, we check if there are subsequent arguments, otherwise, we can not change to a not given directory (in most other shells, you would then change to your home directory). When there is a subsequent argument in
args (which stores the path), we change the directory with
os.Chdir(args). At the end of case block, we return the
execInput function to stop further processing of this built-in command.
Because it is so simple, we will just add a built-in
exit command right below the
cd block, which stops our shell (an alternative to using
Yes, the following output looks more like my root directory.
That’s it. We have written a simple shell :-)
» Considered improvements
When you are not already bored by this, you can try to improve your shell. Here is some inspiration:
- Modify the input indicator:
- add the working directory
- add the machine’s hostname
- add the current user
- Browse your input history with the up/down keys
We reached the end of this post and I hope you enjoyed it. I think, when you understand the concepts behind it, it’s quite simple.
Go is also one of the more simple programming languages, which helped us to get to the results faster. We didn’t have to do any low-level stuff as managing the memory ourselves. Rob Pike and Ken Thompson, who created Go together with Robert Griesemer, also worked on the creation of UNIX, so I think writing a shell in Go is a nice combination.
As I’m always learning too, please just contact me whenever you find something which should be improved.
» Full Source-Code
Below is the full source-code. You can also check the repository, but the code there might already have diverged from the code presented in this post.