[dzaima]

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.

[chubot]

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.

[dzaima]

OP links you to POSIX explicitly denoting it being implementation-defined - https://pubs.opengroup.org/onlinepubs/9699919799/utilities/e..., and https://pubs.opengroup.org/onlinepubs/9699919799/utilities/e... literally says "It is not possible to use echo portably across all POSIX systems unless [specific setup]"

Interestingly, that doesn't allow implementation-defined behavior for "echo -e", for which bash does have special behavior.

[chubot]

OK interesting, I knew about the -e -n flags issue, which means that any portable shell script has to use printf, not echo. Didn't quite realize that the use of backslash also forces printf ... what a mess

[gaigalas]

Here's how I deal with it using feature detection:

    case "$({
        printf %b "\\061" ||
        print -r -- 2 ||
        echo -n -e "\\063" ||
        command -p printf %b "\\064"; } 2>/dev/null)"
    in
        1)
            alias _printb1='printf %b'
            alias _printn1='printf %s'
            ;;
        2)
            alias _printb1='print -n --'
            alias _printn1='print -nr --'
            ;;
        3)
            alias _printb1='echo -n -e'
            alias _printn1='echo -n -E'
            ;;
        4)
            _printb1 () { command -p printf '%b' "$1"; }
            _printn1 () { command -p printf '%s' "$1"; }
            ;;
        '-e 3')
            _pdash () { _pd=$1; while :; do case $_pd in -*) echo -n "\\055"; _pd=${_pd#-};; *) break;; esac; done; }
            _printb1 () { _pdash "$1"; echo -n "$_pd"; }
            _printn1 () {
                local _p _r
                _pdash "$1"; _r=$_pd
                while case $_r in *"$_BS"*) :;; *) false;; esac; do
                    _p=${_r%%"$_BS"*}
                    _r=${_r#*"$_BS"}
                    echo -n "$_p$_BS$_BS"
                done
                _pdash "$_r"; _r=$_pd
                echo -n "$_r"
            }
            ;;
        *)
            exit 1;;
    esac

1) is for shells who have printf, 2) is for the ksh family, 4) is for PATH='' yash exclusively, -e 3) is for posh exclusively.

It exports two callables, _printb1 and _printf1 that escape and don't escape backslashes respectively, are immune to trailing dash gotchas and other pitfalls.

Unfortunatelly, not all shells allow overriding `echo` so I can't proper polyfill when necessary.

I'm going to extremes here, but a simpler version of this can be used for the most popular shells.

[chubot]

I think that is basically the approach modernish uses: https://github.com/modernish/modernish

But it does feel complex to me.

Personally I just write in the common subset of bash and OSH, which is very large. I never need to support say dash, since every machine that has dash also has bash, which is less broken in terms of 'echo' and so forth.

bash has its own broken-ness, but running under OSH solves that.

And OSH also supports some busybox idioms -- we learned last year that bash cannot run busybox ash scripts on Alpine

e.g. 'chdir' is an alias for 'cd' in busybox, but not in bash. And Alpine scripts use it, so bash can't run Alpine scripts, but OSH can (as of last year).

[gaigalas]

One of the reasons for using dash is speed. It's so fast, it allows people to write programs that were not viable before.

The pure shell C compiler is an example of that: https://gist.github.com/alganet/2b89c4368f8d23d033961d8a3deb...

It's portable among the most popular shells, but much faster on dash. It's obviously a gimmick, but also a way to stress the interpreters past their usual breaking points.

osh fails at it because of multi-command/block-level alias (I opened an issue with a simplified reproduction).

---

Options are good. Shell is amazing at that, lots of interpreters to serve all kinds of users and use cases.