bug#60846: 29.0.60; [PATCH] Add more documentation about Eshell command invocation

[Eli Zaretskii <eliz@gnu.org>]

> Date: Sun, 15 Jan 2023 18:51:25 -0800
> From: Jim Porter <jporterbugs@gmail.com>
> 
> The Eshell manual isn't as thorough as it could be about how users 
> should invoke commands. While there's a reasonable amount of 
> documentation about the details, it never directly describes how to run 
> a simple command.
> 
> Attached is a patch to remedy this. I also corrected the documentation 
> about how Eshell picks what command to run in command form; previously, 
> it stated that ordinary Lisp functions had higher priority than external 
> commands, which isn't actually the case (unless you set 
> 'eshell-prefer-lisp-functions').

Thanks.

> Since this is purely a documentation change, I'd like to merge this to 
> the Emacs 29 branch.

That's fine.

> +Unlike regular system shells, Eshell never invokes kernel functions
> +directly, such as @code{exec(3)}.  Instead, it uses the Lisp functions
> +available in the Emacs Lisp library.  It does this by transforming the
> +input line into a callable Lisp form.@footnote{To see the Lisp form
> +that will be invoked, type: @samp{eshell-parse-command 'echo hello'}}

This should use the @kbd markup, like any command the user should
type.  I also suggest to say explicitly that you mean to type this at
the Eshell prompt, as it wasn't clear when I read it.

> +The command can be either an Elisp function or an external command.
> +Eshell looks for the command in the following order:

Here I would add a few useful index entries

  @cindex order of looking for commands
  @cindex command look up, order

> +@vindex eshell-prefer-lisp-functions
> +If you would prefer to use ordinary Lisp functions over external
> +programs, set the option @code{eshell-prefer-lisp-functions} to
> +@code{t}.

I'm guessing this swaps the order of the two last candidates, but the
text doesn't say that explicitly for some reason.  Just saying
"prefer" is not enough when you have more than 2 candidates.

> +In addition, you can @emph{combine} command forms and Lisp forms
> +together into single statements, letting you use whatever form is the
> +most convenient for expressing your intentions.
> +
> +@example
> +~ $ ls *.patch > (format-time-string "%F.log")
> +@end example

Either explain here the meaning of redirecting into a Lisp form, or
add a cross-reference to where it is explained in detail.

> +specify an argument of some other data type, you can use a
> +@ref{Invocation, Lisp form}:

This kind of cross-references is usually a bad idea in Texinfo: it
looks nifty in HTML, but reads awkwardly in all other formats.  It is
better to use the slightly wordier alternative:

  specify an argument of some other data type, you can use a Lisp form
  (@pxref{Invocation}):