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by qudat 30 days ago
This is quite a timely post compared to: https://bower.sh/posix-shell-is-all-you-need

This post was thought provoking, I wonder, is the hidden argument here that the posix spec for a shell is not well specified if there is so much variance between the implementations?

Or is the fundamental issue simply a matter of history? Both?
2 comments
gaigalas 30 days ago
Most shell families precede the spec. Both bash and ksh had features at the time the spec was written that are not in it. It's weird.

My main point is that following the spec doesn't guarantee shell scripts will be portable, which is a common misconception.

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qudat 30 days ago
> My main point is that following the spec doesn't guarantee shell scripts will be portable, which is a common misconception.

That feels like a failure in the spec. Your example illustrates it: echo has unspecified behavior that literally prevents it from being portable.

Is it possible portability is just not a feature of posix?

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SAI_Peregrinus 29 days ago
POSIX specifies a common subset of functionality of several different shells (and C libraries) that existed when it was created, and that any compliant shell (or C library) will share. Some functions like `echo` are guaranteed to be present but don't have fully-specified behavior, because the shells that POSIX was created to be the common functional subset of had different behaviors when POSIX was created.

POSIX doesn't guarantee portability. POSIX only guarantees that the things it specifies will work in a POSIX-compliant environment. POSIX makes no guarantees about portability to non-POSIX-compliant environments like Windows' shells or MSVC, the Fish shell, etc. POSIX makes no guarantees about the things it doesn't specify.

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gaigalas 30 days ago
I honestly don't know! What I know is that there is an intersection of features that work with polyfills and tweaks in all shells. That intersection has a lot of the spec, but also has a lot that is not specified anywhere.

I started to build something here 12 years ago: https://github.com/Mosai/workshop. Then here 2 years ago: https://github.com/alganet/coral

Now I'm attacking it from another angle, trying to document what I learned methodically and share the tools I made for me with everyone else.

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chasil 30 days ago
A real problem with bash in particular is that it had many incompatible features which were masked with bash's POSIX mode.

This mode is not set when called as #!/bin/bash but is enabled for #!/bin/sh

The full list of legacy behaviors that are altered in POSIX mode are in a URL at the bottom of the bash manual page:

https://tiswww.case.edu/php/chet/bash/POSIX

Ksh doesn't do this.

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