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

[Sean Whitton]

Most people who use Eshell a lot need to be aware of the limitations of
its support for shell pipelines: primarily, that all data in pipelines
has to go through Emacs buffers, and that can be slow.  If you're
running pipelines which will move a lot of data, you should use a system
shell which will be more performant.

Currently, if Eshell is your primary shell, it's not particularly
convenient to switch to using an operating system shell when you're
preparing a pipeline that you know is going to move a lot of data.  You
would need either to switch to a shell-mode buffer, or quote and escape
your Eshell input and put something like `bash -c' in front of it.

I think that it would be good to have a way to easily toggle Eshell's
own pipelining support on and off for particular commands.  I have come
up with the attached patches.  When the new functions I've defined are
enabled, you can type

    !! foo | bar 'arg' >baz >>#<buffer *scratch*>

and it will be executed by Eshell as if you had typed

    bash -c 'foo | bar '"'"'arg'"'"' >baz' >>#<buffer *scratch*>

The idea is that you can easily toggle Eshell's pipelining on and off as
appropriate to your needs just by adding and removing the "!!" prefix.

I think that the patches I've prepared are a clean implementation of
this feature that would be good to include in Emacs.

-- 
Sean Whitton

0001-Add-eshell-restore-unexpanded-input.patch
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0002-Add-eshell-shell-command-and-eshell-expand-to-eshell.patch
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