bug#54227: 29.0.50; [PATCH] Inconsistencies with Eshell variable interpolation

[Jim Porter]

On 3/3/2022 11:50 AM, Eli Zaretskii wrote:
> Thanks, this now LGTM.

Thanks.

I found one more issue with the code though: the subscript operator 
doesn't work on subcommands. For example, from "emacs -Q --eval '(eshell)'":

  ~ $ echo ${*echo -e "hi\nbye"}[0]
  ("hi" "bye")

Since `${COMMAND}' forms split the output line-by-line, you'd expect 
this to say "hi", but the subscript operator is a no-op in this case. 
This was previously documented to work in the docstring for 
`eshell-apply-indices':

  For example, to retrieve the second element of a user's record in
  '/etc/passwd', the variable reference would look like:

    ${grep johnw /etc/passwd}[: 2]

I also updated the manual to indicate that this is possible (though I 
didn't provide any direct examples), since I thought this already worked 
based on that docstring.

Attached is a patch with some tests for this.

Just a note: using subscript on `$<COMMAND>' forms is probably not 
super-useful (at least not currently), though I added support for it 
anyway for consistency and future improvement. Since the result of that 
form is the name of a temp file, there's not much reason to do something 
like `$<COMMAND>[0]'. However in the future, if the subscript operator 
were more advanced, you could do something like `$<COMMAND>[/ *]' to 
split the file name by directory separators. (The "*" is a suggested 
feature in the "Bugs and ideas" section to return the whole list.)

0001-Support-applying-indices-to-more-Eshell-dollar-expan.patch
Description: Text document