C Shell

This is a simple shell-like command interpreter. It supports basic operations like executing programs (using execvp internally) and changing directories with cd (using chdir). Implementation of additional features is planned.

This shell is not POSIX-compliant and does not have any scripting capabilities. That's mainly because it does not have a complex parser. Instead, single lines are read from stdin and split at delimiters like or tabs. Only a limited set of special characters like | are treated differently.

GNU shell documentation that covers the basic features of bash can be found here. Implementing some of these features might be educational, too.

Roadmap of Future Features

• implement two-way pipes with | (pipe system call)
• implement file IO redirection with > (open instead of pipes)
• implement setenv in child process, i.e. in cmds like VAR=val echo $VAR prints val
• implement shell variable expansion via getenv (like echo $HOME prints /home/daniel)
• implement background processes with & (don't wait on child process)

---

• implement arbitrary-length pipes
• implement redirection of fd 2 (stderr like echo 'err' 2> out)
• implement redirection of fd 2 to fd 1 (like echo 'err' > out 2>&1)
• implement export builtin which sets a var in the parent
• implement wildcard expansion (look at glob in glob.h)
• implement aliasing of commands
• support a settings file:
  • aliases
  • exported variables
  • $PS1

Some Useful Documentation

Pipes:

int pipe_fds[2];
pipe(pipe_fds);  // puts returns [4, 5] in pipe_fds

To create a pipe ls | wc, we need to:

1. create a pipe with pipe. Imagine that this creates file descriptors 4 and 5.
2. fork twice (once for ls and once for wc).

parent: pid 1000, stdin: terminal, stdout, terminal
---- fork twice ----
parent: pid 1000, stdin: terminal, stdout: terminal
child 1: pid 2000, stdin: terminal, stdout: terminal (for `ls`)
child 2: pid 3000, stdin: terminal, stdout: terminal (for `wc`)

3. In child 1, run dup2(pipe_fds[1], 1) and exec ls
4. In child 2, run dup2(pipe_fds[0], 0) and exec wc
5. Close both ends of the pipe in the parent process.

• Write a fork_and_exec_with_io(cmd* cmd, int stdout_fd, int stdin_fd) that forks and execs the specified command and changes its stdin and stdout as needed

• Use fork_and_exec_with_io to implement pipes with 2 elements.

License

All code in this repository is MIT-licensed (see license file).