bug#46351: 28.0.50; Add convenient way to bypass Eshell's own pipelining

[Sean Whitton <spwhitton@spwhitton.name>]

Hello Michael,

On Sat 25 Dec 2021 at 02:51PM +01, Michael Albinus wrote:

> Yep, the eshell history is there. However, it looks a little bit weird,
> when I have applied '|| cat .emacs | grep a'. The history shows me
> 'eshell-shell-command "cat .emacs | grep a"' instead. I understand
> what's meant, but is this convenient for an occasional eshell user?

Yeah.  Actually, when I prepared the first version of this series, the
unexpanded version got stored in the history, so I think something has
changed about Eshell history recording in the past year.

> Thinking more about, perhaps we could solve it a little bit
> differently. Eshell knows how to invoke external commands explicitly, by
> prefixing the command with a star like '*ls'. What if we do it the same
> with pipes? That means, '*|' would mean to call an external pipe. So my
> command above would be 'cat .emacs *| grep a'. And an external pipe
> shall convert all other commands in that line into external commands as
> well, like '*cat .emacs *| *grep a'.
>
> By this you wouldn't need your restore-unexpanded-input patch any more, yes?

This is an intriguing suggestion.  It would need to be implemented in
`eshell-parse-pipeline, rather than `eshell-expand-input-functions',
because parsing is required to determine which *| are unquoted, etc.  We
also require *> and *< redirections, also in `eshell-parse-pipeline'.

So, I guess when A *| B is parsed, it would be rewritten to a single
call to `shell-file-name' in the way that `eshell-shell-command' does.
I am not sure how feasible it will be to implement that within the
existing structure of `eshell-parse-pipeline', but I can give it a try.

I have one unresolved design question.  A disadvantage of your proposal
compared to mine is that it is much more laborious to convert an
existing pipeline to use the external shell: you have to replace each |
with *| and > with *>, etc., instead of just putting || at the
beginning.

I think it's important that it is easy to switch this feature on and
off, because it is often only after you start figuring out a pipeline
that you realise you ought to bypass Eshell's pipelining.

One option would be to add an entry to `eshell-expand-input-functions'
which converts all | to *| and > to *> when the input begins with ||.
But that is too simple, because you need to parse the input to know
which | to replace ... any ideas?  I don't think it can be done inside
`eshell-parse-command'.

-- 
Sean Whitton