Linux command line for you and me | Hacker News Reader

Linux command line for you and me

lym.readthedocs.io

218 pointsrex_lupi35 comments

TIL that mac has an executable `cd` command:

   $ cat /usr/bin/cd
   
   #!/bin/sh
   # $FreeBSD: src/usr.bin/alias/generic.sh,v 1.2 2005/10/24 22:32:19 cperciva 
   Exp $
   # This file is in the public domain.
   builtin `echo ${0##*/} | tr \[:upper:] \[:lower:]` ${1+"$@"}

Although it didn't behave as expected when invoked ...

Required by POSIX, and it does exactly what it's supposed to do: Changes directory in the process (not the parent shell). This isn't completely useless -- you get an exit status and potentially error messages out.

(And yes, I'm the cperciva in that version tag.)

> And yes, I'm the cperciva in that version tag.

Makes my day that you could discover this post - Thanks so much for chiming in !

You never cease to amaze me, Colin. Big fan of yours. Keep rocking.

Something to note which wasn't mentioned here is that `cd` is not a binary executable, but instead a shell command. You won't find it anywhere in /bin, /sbin and so on. That's also why `sudo cd ` doesn't work, `sudo` will try to execute `cd` as root but it's not an executable so it won't find it. (tip: do `sudo -i` instead)

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

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.

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.

> 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

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?

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

These are always fun and informative. One of the cool options that a lot of people don't know about the "cd" command is the :

    cd -

This takes you to the last directory you were in (not the same as cd .. which takes you one level up)

pushd / popd are also useful in scripts. For some reason they don't seem to be very well known and I rarely see them in practice.

  $ pwd
  /foo
 
 $ pushd /bar && pwd
  /bar

  $ pushd /quux && pwd
  /quux

  $ popd && pwd
  /bar

  $ popd && pwd
  /foo

So it's basically a stack.

Pushd / popd are not POSIX and some shells like ash/dash don't support them. I would avoid them unless you are targeting a specific shell you know supports them (ex: bash).

"targeting" seems like an odd thing to say there. This is an interactive shell command. Does the shell you are using interactively support it seems like a more interesting concern? We don't have to be portable typing commands into a shell. :-)

Never seen {push,pop}d in a shell script.

GP was specifically talking about scripts, and I see the utility.

As an interactive feature, I use the "autopushd" option in zsh, which make all "cd" behave like pushd, and you can also use "cd -n" where "n" is the number of levels you want to go up the stack (so "cd -" is equivalent to "cd -1")

oh wow. How did I not see that? Sorry!

Nice basic introduction. For anyone wanting a comprehensive overview of each command, with examples (and generally more readable than man pages), the computerhope site is good. For example here's wc:

https://www.computerhope.com/unix/uwc.htm

It's UN*X. The one thing it actually does good is having documentation on the system, instead of some weirdo's web page covered in ads. Type man <cmd> or info <cmd> in the shell. Also, for most of the world population, this will be a few seconds faster since it avoids loading a web page.

Very nicely written and comprehensive website. It is handy as we often work with docker and linux command line these days.

Probably not popular to say here, but it's ironic that the key benefit of computers is the ability to distance the user from complexity and mind-defeating details, yet computers themselves don't - yet? - offer said type of benefits.

What are you talking about? Of course they do! Try performing any of the operations listed in this article, or making a reply to this comment, by only twiddling the state of the transistors in your CPU.

I've created my own list, which is a more condensed version: https://github.com/r0f1/linuxhelp

Anyone knows of something similar for MacOs?

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