Re: bug: nested for loop body is executed once!

[Lawrence Velázquez <larryv@xxxxxxx>]

On Thu, Aug 12, 2021, at 11:13 PM, Daniil Iaitskov wrote:
> I just spot a following bug on Big Sur zsh 5.8 (x86_64-apple-darwin20.0)

It's not a bug.

> > $ K="1 2 3"
> > $ for i in $(for j in $K; do echo "ddd/$j" ; done) ; do echo  "$i" ; done
> > ddd/1
> > 2
> > 3
> 
> I would expect following output
> > ddd/1
> > ddd/2
> > ddd/3

By default, zsh does not word-split unquoted parameter expansions;
this behavior differs from most Bourne-adjacent shells.  Observe
that your "inner" loop actually only loops once, with $j taking on
the entire value of $K:

    % K="1 2 3"
    % for j in $K; do echo "<ddd/$j>"; done
    <ddd/1 2 3>

However, zsh *does* word-split unquoted command substitutions, so
the output of $(for j in $K; do echo "ddd/$j" ; done) is split into
'ddd/1', '2', and '3', and $i takes on each value in turn.

> > $ for i in $(for j in $(echo 1 2 3); do echo "ddd/$j" ; done) ; do echo  "$i" ; done
> 
> produce expected output:
> > ddd/1
> > ddd/2
> > ddd/3

In this example, $(echo 1 2 3) is word-split because it is a command
substitution.  Thus, $j takes on the values '1', '2', and '3', as
you expected.

> I don't know if this is a feature due to some legacy optimizations.
> I mostly use BASH and this behavior differs from BASH.
> BASH behaves exactly as I expect.

Actually, many zsh users consider the word-splitting of unquoted
parameter expansions to be a misfeature, which zsh's default behavior
remedies.  In any case, it's very much intentional.  You can obtain
word-splitting behavior with the ${=foo} form:

    % K="1 2 3"
    % for j in ${=K}; do echo "<ddd/$j>"; done
    <ddd/1>
    <ddd/2>
    <ddd/3>

You can also set SH_WORD_SPLIT, or rewrite the code to use an array.
If you're running code that relies on word-splitting (a POSIX script,
perhaps), you can run it under sh emulation.

> Wow! Are you still using just a mailing list for bug tracking?!

Yes.

-- 
vq