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How does `cd` work?

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Re: How does `cd` work?

#71

Post by emmanuel_1234 »

clord wrote: post #66
jsnell wrote: post #6

As a historical curiosity, in the very first versions of Unix chdir was a normal command rather than a shell builtin. You see, at that point Unix had no fork system call. There were multiple processes, but they were created statically at startup rather than on-demand. Running a command in the shell would cause the command to replace the shell in the address space of the process, and the process quitting would put the…

This is a really common way for software to become better. The pattern of development: - Make A - Make B better - A now broken because it had dependencies on B - (maybe) Realize a fundamental pattern that unifies A and B. - (maybe) Unify principles so that A and B become lemmas. This is a very scientific way of working. The truth emerges as the byproduct of keeping all the plates spinning in your model. As long as yo…

This is such a cool way of presenting it, I'll reuse that. Thanks!

Re: How does `cd` work?

#72

Post by 205guy »

c22 wrote: post #60
fishtank wrote: post #56

Earlier quoted context omitted.

Everybody who comes to know about systems calls will have learned the concept for the first time. Some people come to that knowledge by becoming a web developer, gaining some rudimentary understanding of the command line, typing `which cd`, and eventually reading the bash source. The fact that this is even possible is a testament to the author's curiosity and the value of free software. Others come to that knowledge…

I think gp is not ragging on the blog's author for being a novice at Unix systems internals but lamenting the " I looked into this but then it all seemed so complicated so I stopped" attitude that is present in this as well as the sibling posts on sudo and ls. I would much prefer to read three posts delving deeper into the inner workings of any one of these commands than the existing three muddled surface level treat…

Yes, thanks for putting it more succinctly than I could. I got the impression the author set out to understand 'cd' (which I would love to read about) then didn't, and I don't think we can tell from her article how much she got out of it. I quoted the part about higher-level languages because it's been my experience that a lot of those libraries are ultimately implemented in C somewhere down at the bottom as well.

I will say I did learn about some shell along the way and the bit about processes was interesting and starting to get at the core of 'cd'. It just seems like there are many other pieces of the puzzle, and I encourage the author to keep researching and keep writing about them.

Re: How does `cd` work?

#73

Post by lokedhs »

disconnected wrote: post #16
ataylor32 wrote: post #5

Random piece of info about cd: cd.. (no space between "cd" and "..") works on Windows

It's been like that since the DOS days, IIRC. You can simulate that behavior on bash with an alias such as this one: alias cd..="cd .."

I believe that you also don't need the space when you specify a path:

    cd\foo\x
The cmd parser in Windows is the most bizarre piece of software in common use. I don't believe there is a single person that actually understands how it works completely.

Re: How does `cd` work?

#74

Post by colejohnson66 »

gowld wrote: post #20
qubex wrote: post #12

Earlier quoted context omitted.

There's no reason why one shouldn't be able to invoke the chdir system call and implement the same behaviour.

Allowing any process to arbitrarily modify the state of another process would be a disaster for stability and security.

Isn’t that what a debugger does?

Re: How does `cd` work?

#77

Post by kbob »

LukeShu wrote: post #48
zokier wrote: post #31

Earlier quoted context omitted.

> If /usr/bin is on a case-insensitive filesystem, and you execvp("CD", ...) That seems like fairly rare edge case, considering that UNIX typically had case-sensitive file-systems, or rather handled filenames as opaque blobs. I wonder what actual system caused the need for case folding? Maybe HFS?

The `tr` bit isn't actually in the FreeBSD version of the script[1], so I'm assuming it's a macOS HFS thing. https://github.com/freebsd/freebsd/blob/master/usr.bin/alias...

Mac filesystems case-fold by default, so the script could be invoked as cd, CD, Cd, or CD. Only the lowercase cd is shadowed by the builtin.

Re: How does `cd` work?

#78

Post by danso »

_zachs wrote: post #55

Not one character in the article is actually devoted to how `cd` works...it's just the story of how the author found out where to find the code for it. An article much more worthy of the title "How does `cd` work?" would maybe, you know, actually go through the code that makes `cd` work.

Huh? The author’s explanation may not be thorough but it does talk about the use of a system call and ‘chdir’

Re: How does `cd` work?

#79

Post by wodenokoto »

kazinator wrote: post #69

> The cd builtin is invoked as part of the Bash shell. > The Bash shell invokes the chdir function. Nope! Not quite. Bash: ~$ ls -ld foo lrwxrwxrwx 1 kaz kaz 4 Mar 1 6:50 foo -> /etc ~$ cd foo ~/foo$ pwd /home/kaz/foo ~/foo$ cd .. ~$ pwd /home/kaz Raw chdir and getcwd syscalls: $ pwd /home/kaz $ txr This is the TXR Lisp interactive listener of TXR 190. Quit with :quit or Ctrl-D on empty line. Ctrl-X ? for cheatsheet.…

She says that it is all a ton of functions that sanitize input and implement logic before calling chdir.

So the article does point out that CD isn't a direct alias for chdir

Re: How does `cd` work?

#80

Post by Someone »

colejohnson66 wrote: post #74
gowld wrote: post #20

Earlier quoted context omitted.

Allowing any process to arbitrarily modify the state of another process would be a disaster for stability and security.

Isn’t that what a debugger does?

Yes, and it is a disaster for stability and security. The designers of dtrace worked hard on the stability part. See, for example, the section on DIF safety in https://www.cs.princeton.edu/courses/archive/fall04/cos518/p....
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