GNU bug report logs - #54190
29.0.50; [PATCH] Incorrect/missing documentation for some Eshell "$" syntaxes

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Jim Porter <jporterbugs <at> gmail.com>
To: bug-gnu-emacs <at> gnu.org
Subject: 29.0.50; [PATCH] Incorrect/missing documentation for some Eshell "$"
 syntaxes
Date: Sun, 27 Feb 2022 13:34:26 -0800

[Message part 1 (text/plain, inline)]

In the documentation for lisp/eshell/esh-var.el (and 
`eshell-parse-variable-ref' in that file), it says that "$<FOO>" is a 
way of accessing the value of the variable FOO to disambiguate the 
length of the variable name, sort of like "${FOO}" in ordinary shells. 
However, that's not actually true. The correct syntax for that is:

  $"FOO"
  ;; or...
  $'FOO'

In fact, what "$<FOO>" does is to run FOO as a subcommand, writing its 
stdout to a temp file, and returning that file's name. This is (very!) 
subtly implied in the Eshell manual in the "Bugs and ideas" section, 
where it says:

  `grep python $<rpm -qa>' doesn't work, but using `*grep' does

  This happens because the `grep' Lisp function returns immediately,
  and then the asynchronous `grep' process expects to examine the
  temporary file, which has since been deleted.

Attached is a patch which updates the documentation to correctly 
describe the current behavior. Note: since this is just a documentation 
change, it might be worth pushing to the 28 branch.

[0001-Improve-correct-documentation-about-Eshell-variable-.patch (text/plain, attachment)]

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