jncraton / shell

Shell

A minimal shell for x64 Linux systems.

This implementation strives to be as simple as possible to understand from top to bottom. It can be challenging to understand where user applications end and the kernel begins, especially with middleware such as libc present. This shell therefore includes no external libraries, including libc. A limited number of x64 Linux system calls are provided via syscall.h. These are:

• System call 0 - read
• System call 1 - write
• System call 57 - fork
• System call 59 - execve
• System call 60 - exit
• System call 61 - wait4

It turns out that using just these six system calls, we can implement a basic shell.

What is a shell?

At its most basic level, a shell is simply a program that exposes the operating system to a user. Shells can be graphical or they can use a command-line interface. A basic shell should allow a user to launch programs on the system.

A simple CLI shell may operate as a read-eval-print loop. It accepts input from a user, evaluates it, then displays any results of the evalation. In order to implement a REPL, we need a loop and the read and write system calls.

Running Programs

In order to run additional programs on POSIX systems, we need to perform two system calls:

1. fork - Split the current process (our shell) into two processes
2. execve - Replace one process (the child) with another program

The parent shell should then loop waiting for the child to finish execution. This can be implemented using the wait4 system call.

Builtin Functions

In order for our shell to be able to terminate, we need a builtin to close the program. We'll use exit for this purpose. The exit builtin should call the exit system call to terminate the shell.

It is also helpful to be able to change the working directory that child processes will use. We can change the working directory using the cd builtin. This should call the chdir system call to change the working directory of the shell.

Task

You should complete the shell program by making the noted modifications to shell.c. You may not #include any libraries to complete this program. Necessary changes are indicated with TODO comments.

Once complete, you should be able to run your shell on an x64 Linux system and perform output like the following:

> cd /
> /bin/pwd
/
> /bin/echo Hello, world!
Hello, world!
> exit

You should even be able to run interactive programs such as /usr/bin/vi.

Note that this simple shell implementation does not support the PATH environment variable, so programs like ls need to be executed as /bin/ls using their absolute path.