POSIX Is Not a Shell

https://alganet.github.io/blog/2026-06-28-12-POSIX-Is-Not-A-Shell.html

by gaigalas • 24 days ago

56 64 24 days ago

alganet.github.io

echoangle3• 24 days ago

Pretty bad argument. If it’s not defined by POSIX, it’s not POSIX compatible if you rely on a specific behavior.

If you only use defined behavior and it works, it is compatible.

It’s like saying C99 isn’t a compiler. True, but you can still write C99 code, right?

smitty1e• 24 days ago

C99 isn’t a compiler.

Sure, but the pojt here is that if we say "Write in X" we generally understand it to mean "Treat X like a standard and don't get too colloquial with the stylings."

Pedantry is worthwhile, but it can be a diminishing returns game.

eqvinox2• 24 days ago

Feels like you missed the point.

On the example of 'echo \n' - it's not defined in POSIX, therefore a script written in "POSIX shell" must simply never hit that case.

TFA kinda implies you can't target POSIX shell. That's silly, of course you can. The question is, what tools are there to check for compliance. Whether running on 14 shells is a good such tool - idk. Something specifically searching for POSIX violations might be better.

Muhammad523• 24 days ago

This post is nice: the writer first explains a problem, using a simple example. In the next section, they reflect a bit about the problem, and then they casually mention two tools they built. In my opinion, this is amazing: you sponsor you project, while also making the problem it solves clear: use their tool to test how portable your code is

jmclnx• 24 days ago

Will not build without docker, so I am out of luck. This tells me this is not portable, even to some Linuxes.

Joker_vD2• 24 days ago

Strict POSIX conformance is arguably worse. I mean, have you seen what it advises for shebangs? First of all:

    The shell reads its input from a file (see sh), from the -c option or from the system() and popen() functions defined in the System Interfaces volume of POSIX.1-2017. If the first line of a file of shell commands starts with the characters "#!", the results are unspecified.

Ah, so shebangs are not required to be supported, already a great start.

    Applications should note that the standard PATH to the shell cannot be assumed to be either /bin/sh or /usr/bin/sh, and should be determined by interrogation of the PATH returned by getconf PATH, ensuring that the returned pathname is an absolute pathname and not a shell built-in. [...]

    Furthermore, on systems that support executable scripts (the "#!" construct), it is recommended that applications using executable scripts install them using getconf PATH to determine the shell pathname and update the "#!" script appropriately as it is being installed (for example, with sed). For example:

        #
        # Installation time script to install correct POSIX shell pathname
        #
        # Get list of paths to check
        #
        Sifs=$IFS
        Sifs_set=${IFS+y}
        IFS=:
        set -- $(getconf PATH)
        if [ "$Sifs_set" = y ]
        then
            IFS=$Sifs
        else
            unset IFS
        fi
        #
        # Check each path for 'sh'
        #
        for i
        do
            if [ -x "${i}"/sh ]
            then
                Pshell=${i}/sh
            fi
        done
        #
        # This is the list of scripts to update. They should be of the
        # form '${name}.source' and will be transformed to '${name}'.
        # Each script should begin:
        #
        # #!INSTALLSHELLPATH
        #
        scripts="a b c"
        #
        # Transform each script
        #
        for i in ${scripts}
        do
            sed -e "s|INSTALLSHELLPATH|${Pshell}|" < ${i}.source > ${i}
        done

Marvelous. What a robust foundation of useful and hard-to-misuse utilities.

gaigalas21• 24 days ago

Author here.

It definitely builds outside docker. It's a musl-cross-make toolchain, you can procure the dependencies locally if you don't like the Docker recipes.

Feel free to open an issue if you feel like that's a challenge. Likely, you can get it to work but checksum reproducibility will be hard without a controlled environment like docker.

sdovan1• 24 days ago

If your environment is POSIX, testing scripts with tool written in POSIX shell, like shellspec[1], might also be a choice.

[1] https://shellspec.info/

paulddraper2• 24 days ago

When someone says "write it in POSIX shell for portability," they mean well.
POSIX is a specification. Not a program. The thing that actually runs your script is bash, dash, ash, ksh, yash, or one of a dozen others.

“When someone says ‘write it in ECMAScript,’ they mean well.

“ECMAScript is specification. Not a program. The thing that actually runs your script is Node.js, Bun, Deno, Rhino, or one of a dozen others.”

See how silly that sounds?

gaigalas21• 24 days ago

No one says "Let's write ECMAScript for portability". If someone did, I would probably be writing about that too.

hnfong• 24 days ago

Hey guys... I have a wonderful idea.

... Let's write ECMAScript for portability.

paulddraper2• 23 days ago

Are you serious?

People say that all the time.

People use JavaScript everywhere.

dzaima3• 24 days ago

ECMAScript has a pretty massive amount of fully-specified behavior though; the things that differ between those implementations is nearly-entirely limited to fresh additions like `require` or whatever.

The echo thing would be like if ECMAScript allowed stuff like `"123" == 123` to give either false or true; and then indeed many things would probably break if moved across implementations.

C is the closer comparison, and indeed much software that could easily be portable (and might claim it is) often depends on implementation-specific things like 8-bit bytes, 32-bit int, assuming int8_t/etc in stdint.h exist, twos complement (before C23 at least), arithmetic shift right, etc.

17186274402• 22 days ago

Because both POSIX and C are descriptive rather than prescriptive. When people have different ideas, and others already depend on both behaviour, that's the best you can do, without breaking compatibility.

JdeBP• 24 days ago

Stéphane Chazelas would agree,

* https://unix.stackexchange.com/a/496642/5132

because the problem here is educating people with slipshod ideas about 'sh' being 'the POSIX shell' or (worse) 'the Bourne shell'. Both M. Chazelas and M. Gaigalas are making the point that 'sh' is a language that one aims to write in, not one of the many programs that sort of, sometimes, if invoked in the right way, implement that language; and a subordinate point that people are generally very poor about doing that when yet they insist that they are writing 'POSIX shell script'.

Fun facts: Standardization led by existing practice is not a simple process.

* https://unix.stackexchange.com/a/493743/5132

POSIX/SUS standardization is not a static thing. The long discussed thorny issue of echo has now subtly changed from all of those explanations given about it over the years, because in 2024 the standard was changed.

* https://pubs.opengroup.org/onlinepubs/9799919799/utilities/e...

M. Chazelas's own quite famous 2013 StackExchange answer on the subject has not yet been updated with the change that now incorporates -e and -E into the rule. Amusingly, it was M. Chazelas that raised defect 1222 that caused this change.

* https://unix.stackexchange.com/a/65819/5132

* https://www.austingroupbugs.net/view.php?id=1222

wpollock2• 24 days ago

Stéphane (not "M") Chazelas is a genius. I have written him several times over the past 30 years, encouraging him to write a book about shell, POSIX, and other topics that he is an expert in. He writes very well too. I fear we will all be the poorer if/when he retires.

rmunn• 24 days ago

"M." is the French abbreviation for "monsieur", the equivalent of "Mr." in English. So "M. Chazelas" is the correct way to politely refer to Stéphane Chazelas by his last name, since he's French. (Not that writing "Mr. Chazalas" would be wrong either when writing in English, but "M. Chazelas" is entirely correct).

kevin_thibedeau• 24 days ago

Solaris most definitely has a POSIX shell at /usr/xpg4/bin/sh which behaves differently than OG Bourne at /bin/sh.

gaigalas21• 24 days ago

You can run this pup to find out what's underneath!

https://www.in-ulm.de/~mascheck/various/whatshell/

qudat2• 24 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?

gaigalas21• 24 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.

qudat2• 24 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?

chasil5• 24 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.

croemer3• 24 days ago

Inconsistent: claims /bin/sh on Alpine is dash in one paragraph, BusyBox ash in another.

Smells a lot of AI writing.

gaigalas21• 24 days ago

They are the same, kinda! ash and dash are from the Almquist family.

https://en.wikipedia.org/wiki/Almquist_shell

It's just poetic license.

croemer3• 24 days ago

They are not the same. For example, `[[` apparently works on Alpine's ash but not on Debian's dash. BusyBox ash has some bash compatibility options enabled in Alpine.

It's ironic that a post about shell differences glosses over this. Maybe not surprising that slop is sloppy.

chasil5• 24 days ago

The author has neglected the pdksh family of shells which includes all of Android, so this is a sizable oversight.

This would include the MirBSD mksh, and oksh from OpenBSD.

They are much smaller than ksh93 and bash, and I would suggest that their syntax extensions be given preference, as they originally represented ksh88 (from which the POSIX shell standard was derived).

gaigalas21• 24 days ago

These are all in the shell-versions toolkit I recommended.

    docker run -it --rm alganet/shell-versions:all /opt/mksh_R59c/bin/mksh -c 'echo $KSH_VERSION'
    docker run -it --rm alganet/shell-versions:all /opt/oksh_7.9/bin/oksh -c 'echo $KSH_VERSION'
    docker run -it --rm alganet/shell-versions:all /opt/loksh_7.9/bin/loksh -c 'echo $KSH_VERSION'

I omitted them from the post because this is not about the history of ksh and the pdksh codebase.

chasil5• 24 days ago

The POSIX commentary mentions the Korn shell ten times, including particular behavior of the 1988 ksh release. Bash is not mentioned.

It is easier to understand the POSIX standard with a ksh focus, particularly ksh88.

https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V...

XYen0n• 24 days ago

Usually I prefer to use `#!/usr/bin/env bash`.

chubot4• 24 days ago

FWIW Oils has an option to prevent the ambiguity:

    osh-0.37$ echo "c:\new"
    c:\new

    osh-0.37$ shopt --set no_parse_backslash
    osh-0.37$ echo "c:\new"
      echo "c:\new"
              ^
    [ interactive ]:6: Invalid char escape in double quoted string (OILS-ERR-12)

It really should be

    echo "c:\\new"  # with two backslashes

That is an unambiguous program that works in every shell. In a well-written shell program, the only things that should follow a single backslash in a double quoted string are

    \ " ` $

(Although I found that this option is only on in ysh, not in shopt --set strict:all ... arguably that should be changed)

Nine Reasons to Use OSH - https://oils.pub/osh.html

gaigalas21• 24 days ago

yash also has something similar, ECHO_STYLE var.

https://magicant.github.io/yash/doc/_echo.html

Additionally, the ksh family has `print`, which solves some of the issues.

And the story repeats all over again, each shell solving the problem in a different way :) That's one of the main reasons I went for solving the portability problem from the scripting side, not the interpreter side.

dzaima3• 24 days ago

That seems to be be an entirely-different question - `echo "c:\\new"` still differs in behavior between bash and dash - dash parses backslashes in both the double-quoted string, and then echo does another backslash parsing pass, still printing a newline; whereas bash prints a backslash + n.

chubot4• 23 days ago

OK interesting, yeah I think dash is just plain broken ...

    $ dash -c 'echo "c:\\new"'
    c:
    ew
    $ busybox ash -c 'echo "c:\\new"'
    c:\new

It requires 4 backslashes:

    $ dash -c 'echo "c:\\\\new"'
    c:\new
    $ busybox ash -c 'echo "c:\\\\new"'
    c:\\new

I am not sure this is a matter of "undefined behavior in POSIX" -- I think it might just be dash being wildly unconformant, which I have seen in other cases.

It's one of the least POSIX compliant shells. It is derived from the same codebase as busybox ash, but busybox receives more maintenance.

---

In any case, it is pretty sad that sh is in such poor shape than the default /bin/sh on Debian has different behavior in this basic case.

I built OSH to be a set of semantics agreed upon by many shells. I don't think any cross-shell test suites like our spec tests had existed in the past - https://oils.pub/release/0.37.0/quality.html

But there is little coordination among shell authors, and no real motivation to fix the gaps. In contrast, there is A LOT of coordination among JavaScript engine authors, mostly because there are people paid to work on them.

hulitu• 22 days ago

POSIX is a specification. Not a program. The thing that actually runs your script is bash, dash, ash, ksh, yash, or one of a dozen others. They each implement POSIX with their own gaps, extensions, and historical accidents.

Blaming POSIX, because a program does not implement it, is childish.

gaigalas21• 22 days ago

Have you ever tried to implement it?

POSIX Is Not a Shell

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