RFC: expand $'' within heredocs

RFC: expand $'' within heredocs

[Eric Cook]

Hey all,

bash 5.2[1] recently released and a change i found of interest is allowing $'' to be expanded
within heredocs. $'' is currently the only expansion that uses the `$' operator that doesn't
expand within heredocs(even $[] does); so i can see that edgecase being confusing in addition
to the feature just being useful.
cat <<eof
hello$'\t'world
eof
...


[1] https://lists.gnu.org/archive/html/help-bash/2022-09/msg00002.html
Not to bandwagon other shells, i just like the idea.

Re: RFC: expand $'' within heredocs

[Mikael Magnusson]

On 9/28/22, Eric Cook <llua@gmx.com> wrote:
> Hey all,
>
> bash 5.2[1] recently released and a change i found of interest is allowing
> $'' to be expanded
> within heredocs. $'' is currently the only expansion that uses the `$'
> operator that doesn't
> expand within heredocs(even $[] does); so i can see that edgecase being
> confusing in addition
> to the feature just being useful.
> cat <<eof
> hello$'\t'world
> eof
> ...

$'foo' is technically a form of quoting, not expansion, so it is
consistent the way it is now. Eg, there is no $ operator in play, the
$' together form an operator (if you want to call it that) which
begins this particular form of quotes.

% cat << EOF
"foo" 'foo' $'foo'
EOF
"foo" 'foo' $'foo'


-- 
Mikael Magnusson

Re: RFC: expand $'' within heredocs

[Jun T]

> 2022/09/29 5:30, Eric Cook <llua@gmx.com> wrote:
> 
> cat <<eof
> hello$'\t'world
> eof

I think bash-5.2 does not substitute $'\t' by a TAB in this case.

Here-document is something like a string quoted by "...". All of
bash-5.1, 5.2 and zsh do not replace $'\t' by a TAB in 
  $ echo -E "foo$'\t'bar"

In the following example:
  $ a=$'\tHello'
  $ cat <<EOF
  ${a#$'\t'}
  EOF
5.2 removes the TAB (as zsh does) and just outputs 'Hello',
while 5.1 does not remove the TAB.