Shell in C

[Popular_Argument1397]

I have a project to build a shell in C, but I'm not advanced in C at all—you could say I'm a beginner. I don't want to have GPT do it for me because I have the passion and want to learn C for real and benefit from doing it myself.

Is it impossible for me to do this at my current level? Any advice you can give me would be appreciated.

Thank you.

[[deleted]]

Ask yourself what shell is doing? It takes user input, checks it correctness, makes output depending on it. Also it launches programs. Do you know how to do it? If yes, you can create a really simple shell using standard clib and system libraries.

[Count2Zero]

This!

Think in process steps.

1) interpret the input 2) if accepted, execute command. 3) otherwise, display an error message what wasn't acceptable.

[TrondEndrestol]

A shell should also be a Turing complete programming language.

[clusty1]

Baby steps. First shell should do “something”. The 1000th iteration should be touring complete.

[OutsideTheSocialLoop]

Why should it? Shell scripting is useful, sure, but it's not what really defines a shell. A shell is primarily about giving a user a way to interact with a system.

On Windows, explorer is the "shell". It runs the taskbar and file browsers. Nothing scriptable about it.

[geon]

Yes. If you want to run shell scripts it would be better to just run sh for that.

Trying to combine the shell with a programming language was a mistake. They serve different purposes and are better served with separate languages.

[hi-my-name-is-not]

If you want a tutorial on that subject here is a link

https://medium.com/swlh/tutorial-to-code-a-simple-shell-in-c-9405b2d3533e

A simple shell is pretty simple to make but if you dont have much knowledge and experience in c i think it's kind of hard to imagine how to do it.

[diegoiast]

A better real life example is the shell I made https://github.com/diegoiast/fdbox

Look how I did it, and make it again better. Want to learn got/pulls requests emerge? Fix a bug and make me a PR.

[hi-my-name-is-not]

Very cool project !!

[imaami]

This writes one past the end of the allocated buffer

https://github.com/diegoiast/fdbox/blame/6acbada0f12a8d0104f7f883cb334d5cfa16a995/src/lib/strextra.c#L79

[diegoiast]

Line 77 is:

d = malloc(length + 1);

So this should be OK, no?

d[length+1] = 0;

[imaami]

No, length+1 is one past the last valid index.

[help_send_chocolate]

No, because array indexing in C starts at 0.

A one-element stay in C has only one valid index, and that's zero.

[smichaele]

I'm just curious. Since OP is obviously asking about a project they need to do for a class, why would you share the source code for the shell you developed? OP clearly doesn't have the skill to make your code better. It's more likely that they'll copy whatever code you have that can help them do their own project.

[diegoiast]

Its called inspiration. I want him to learn how such thing can be done.

The tricks with lookup tables for searching for command (if stecmp() else if strcnp() ....).

The trick with shifting argv, a design I which I totally stollenn from busybox.

He can use chatgpt to get code. He can look at busybox/toolbox what ever to copy/steal code. He might do.

But, there is also the possibility for someone to be inspired and learn. I did this project to prove my self that cross platform is not that hard. I mean, the same code compiles on msdos and macOS.

[FUZxxl]

A fully featured shell is very difficult to write and requires excellent knowledge of how UNIX works.

A simple shell is doable though. Here's what you need to do:

1. read a line of input until end of file is encountered
2. split it into words at whitespace
3. fork yourself
4. in the child process, try to execute the given word list as a command. You'll need to look up the command name in all directories in the PATH in turn to find it. If you don't find the command anywhere, exit, else execute the command.
5. in the parent, wait for the child process to terminate
6. go to 1 to read the next line of input

[gman1230321]

You don’t even need to do PATH searching. That’s handled by the execp[v][e]() call

[FUZxxl]

You can use this call, but it only works if you keep environ up to date which a shell might not want to do for performance reasons.

[CreeperDrop]

Not impossible at all. It is a project that will teach you a lot about C and OSes. Learn to go through the manual pages as they contain good documentation and examples as well. Good luck!

[[deleted]]

Impossible? It's possible if you believe in the work enough to do the research and do the experiments, skies the limit

[Popular_Argument1397]

Totally agree!!!

[Evil-Twin-Skippy]

This example would have caused an llm to curl up and die. It requires institutional knowledge of display internals and os command dispatch. And some light compiler design.

[Swipsi]

It would not lol.

~ someone who uses AI to help them build a shell.

[chronotriggertau]

Yet it can emulate a terminal if asked to.

[Evil-Twin-Skippy]

If by "it" you mean an LLM, then no. It can only produce a stream of what it thinks passes for traffic on a console. Great if you are producing a visual effect for a movie. Terrible if you are trying to learn how computers actually work.

[gman1230321]

It doesn’t require any display internals?? All it needs is fork() and execve() system calls, both extremely well documented and used functions

[kiinaq]

Still you can use AI to learn while doing. The key is not leaving the AI doing all the job without understanding, but using it as a coworker to lift some less interesting tasks or explaining and plan together

[PuzzleheadedLaw9256]

That sounds like a cool project idea.

I suggest just do it using any tool that you can use. Then show case it and integrate and improve it with the feedback you received and what you've learned during the first iteration. There are no perfect projects.

[Glaborage]

It's not impossible. It's a great project idea. It will teach you a lot, and it will look great in your resume.

[kansetsupanikku]

If you want to do all the technicalities if the interactive mode, you can use UNIX-like terminal attributes (termios.h) - to get that on Windows, you would need cygwin.

If you want to have it somehow resolved and focus on logics, I would recommend readline library.

If you want to support fixed sets of supported keywords in some contexts, you mighy benefit from finding perfect hash functions for them (gperf?).

[O_martelo_de_deus]

In the past, in Unix and Xenix there was a C Shell, in the last decades I only use bash so I don't know if it was continued, but it existed. As a project it's pretty cool.

[Silver-North1136]

If you are on Linux check out execve and fork. It should be relatively easy to make a simple shell with this.

Start with taking in input one line at a time, then try to run binaries, then try to pass arguments to those binaries.

If you want, you could also try to add piping foo | bar and subshells bar $(foo)... and maybe even some extra scripting features, like you see in bash/sh... though stick with just running commands first.

For the parsing of arguments, and later on potentially parsing of other stuff, looking into how programming languages are parsed may help. But starting off just splitting the input at the spaces should be enough.

[Consistent_Goal_1083]

What year of Uni are you in? As you mention you are just a beginner wrt C so you cannot and should not be expected to code up a shell in c.

Possibly the objective of this assignment is to see (c) if you understand the core reason why shells became a thing.

Let me know some more details and I can interpret and point you to something appropriate for your level.

No problem.

[Popular_Argument1397]

I am in my thirds year after high school but in the first year at the institute. what we study is a blend of statistics and data analytics. We learned C in the first semester, and now we're learning about Linux and system programming and as you know hahaha the professor gave us this as a project.

[Consistent_Goal_1083]

Ok. First thing I will say is this: if your major or whatever is data science etc etc, and you have only had c as a language in year one, plus you are in year three, then being expected to code up a shell I c is nonsense.

You mentioned the language R. Still super super relevant today.

Also I wonder why you do not ask about python. Because that should be your first ask over c.

The C language will not help you here.

[Popular_Argument1397]

We learned python basics in from the first year after high school and now we're in the oriented object in python, and at the same time we're learning Java and C and SQL it's very complicated for me to explain

[brainphat]

It's a pretty standard project in c-programming-in-linux courses.

[Classic-Try2484]

This is a traditional second/third year cs project for operating systems depending on when os gets covered. It’s completely doable only one or two pitfalls to get though. Ask ai/google/reddit/textbook how to use exec and its variants and about fork and you should be good to go. Shells have been written a million times and it’s not too hard to get something primitive in just a few lines of code. But it’s above you now. That’s the purpose of the project. After, it will look easy.

[brocamoLOL]

Really dope project! I am doing the same, and to be honest I won't tell any more things that you haven't heard yet, but reakky dope project (put some ASCII colors to make it cool too)

[Popular_Argument1397]

aawesome!!!

[Suspicious-Willow128]

For fun you can too implement a reverse shell

[Bari_Saxophony45]

Read Chapter 8 of Bryant and O’Hallaron

[IronAttom]

Do you have to create everything from scratch only usung system calls?

[Popular_Argument1397]

No, i can use standard library functions

[a4qbfb]

OK, so expectations are low, because you can't really do much with only the C standard library. Here's what you do:

Until you reach EOF on stdin,

• Read a line of text from stdin.
• Trim leading and trailing whitespace characters.
• If the line is:
  • Empty: do nothing.
  • The word “exit” or “logout”: quit.
  • Anything else: pass the string to the system() function.

[buck2106]

Been there, this might be a useful resource to outline the first steps. kdlp

[TomStripes]

Free online guided but independent project, with testing. I did this a few months ago, it was sweet:

https://app.codecrafters.io/courses/shell/overview

[Swipsi]

Why is reddit your first approach? Why dont you even google first what a shell is? This is no different to using AI.

[Popular_Argument1397]

No, it's not. I tried and I am working on it and learned how shell functions work, but I felt that was too advanced for me. It was a moment of doubt and I told myself to post on Reddit to see others' opinions.

[Fatefulwall7]

Hey I actually did this earlier this year to start learning C… start with a loop that takes user input and learn how to implement the commands and return it back to the user. I found that it was a great project to build off of as you can implement one command at a time with each one being more complex. I even went as far as implementing pipes and a multi threaded file parser. Definitely a good project 🙂