cd is a good example of something that has to be a built-in command, because it makes the shell itself do something (change its current working directory). Other commands in that category are umask, the job control ones (fg, bg, wait, ...), the ones setting variables (read, printf), and the control flow ones (if, while, break, return, exit, ...).

A lot of commands are also built-in for convenience but don't strictly need to be.

> cd is a good example of something that has to be a built-in command

Theoretically you could also have a cd that would IPC the shell to change the pwd but I guess you could also say that is overengineering.

That sounds like a weird way to implement a built-in command (the IPC handler still needs to be built-in, so you didn't really remove it from the shell). And then if course that binary would be available globally but only work if you're using the shell it's built for...

Yeah, that is why it isn't done. I am just pointing out the pointless fact that you could write a cd command that wasn't itself built-in.

You could hook up to the parent process with ptrace or the equivalent debugging API of your OS of choice and forcibly change the directory. Stackoverflow even has an implementation: https://unix.stackexchange.com/a/114262/81005

And yet ...

https://unix.stackexchange.com/questions/50058/what-is-the-p...

Yes, it's idiotic.

The question and highest rated reply there are very interesting and worth reading (at least for trivia value), thanks for that.

I don't understand what you meant is idiotic though

As that comment said, it’s there so utilities can use it for checking the return of “chdir()”. It’s at best a name conflict with a shell command with a well established past and use. At worst, it probably enables some bad code smells or outright malicious code.

I've seen questions about sudo (for instance) that ask why does "sudo chdir" not work.

By idiotic I think it's a mistake to blur the line between shell builtins and external commands.

We can use the `compgen -b` (`help cd` to get short help or `man bash-builtins` to read man pages) to list all internal bash commands. For example, we can use the `type -a cmd1` or `command -V cmd1` to see if `cmd1` is built-in or external binary.

Don’t forget “dirs -v”. I’ve tried to use Powershell a few times and always tap out because it doesn’t come with the dirs command. It’s so fundamental to my workflow a shell without it or without an easy way to implement it is a no go.

I'm curious if this is true for every alternative new niche shells like murex, oil, nushell etc?

An executable of 'cd' wouldn't do anything useful, it could only change its own idea of working directory then exit, the shell or other executable that invoked it would be unchanged.

why couldn't it call an api in the shell to change directory?

What would that improve? And what happens if you call it from another shell?

I never implied that it was better or worse but that it was possible

Many platforms will ship a (mostly) useless cd binary.