bug#52999: 29.0.50; [PATCH] `eshell-eval-using-options' should follow POSIX/GNU argument conventions

[Jim Porter]

Currently, `eshell-eval-using-options' doesn't follow POSIX/GNU argument 
conventions[1], resulting in some confusing behavior. To see this in 
action, the easiest way is probably to make a small patch to 
`eshell-do-ls' in lisp/eshell/em-ls.el: just comment out the line that says,

  :external "ls"

and then eval the function again. (This is necessary so that eshell 
doesn't fall back to the system's `ls' command when it gets confused.) 
Then open `eshell' and run:

  ls '-I*.el'

Instead of what you'd expect (a directory listing that ignores Emacs 
Lisp files), instead you get a directory listing of *all* the files in 
the long listing format. That's because `eshell-eval-using-options' 
assumes that all the characters after the "-" are names of short 
options, rather than a single short option followed by its value. You 
can see a similar problem with:

  ls '--ignore=*.el'

In this case, `eshell-eval-using-options' looks for an option named 
"ignore=*.el" instead of an option named "ignore" followed by its value.

I've attached a patch with tests to fix this and use the POSIX/GNU 
argument conventions, supporting both the above cases. However, I should 
mention that this is a slightly incompatible change. A small number of 
existing eshell commands work like `ls -I', and their behavior is now a 
bit different. Previously, you could do the following,

  ls -Ia '*.el'

to list all the files in a directory, excluding Emacs Lisp files. Now, 
you have to spell that as:

  ls -aI '*.el'
  # or...
  ls -aI'*.el'

I think that's ok though, since I can't imagine anyone *wanting* the old 
behavior. It surprised me quite a bit when I stumbled across it, and 
worse, it only crops up sometimes, since eshell transparently falls back 
to the real commands when it gets confused.

For completeness, the following commands+options are affected:

  sudo -u
  du -d
  ls -I

[1] https://www.gnu.org/software/libc/manual/html_node/Argument-Syntax.html

0001-Follow-POSIX-GNU-argument-conventions-for-eshell-eva.patch
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