Add eshell-shell-command and eshell-expand-to-eshell-shell-commandseries/unexpanded-input-v2

* lisp/eshell/esh-mode.el (eshell-shell-command,
eshell-expand-to-eshell-shell-command): Define new functions.
Register eshell-expand-to-eshell-shell-command as a customization
option for eshell-expand-input-functions, and add it by default.
* etc/NEWS:
* doc/misc/eshell.texi: Document the new syntax.

1 files changed, 35 insertions, 0 deletions

diff --git a/doc/misc/eshell.texi b/doc/misc/eshell.texi
index a87dd4308c5..7e9233c09e1 100644
--- a/doc/misc/eshell.texi
+++ b/doc/misc/eshell.texi
@@ -875,6 +875,7 @@ is equivalent to entering the value of @code{var} at the prompt.}
 @menu
 * Dollars Expansion::
 * Globbing::
+* Running Shell Pipelines Natively::
 @end menu
 
 @node Dollars Expansion
@@ -945,6 +946,40 @@ Eshell see the Customize@footnote{@xref{Easy Customization, , , emacs,
 The GNU Emacs Manual}.}
 groups ``eshell-glob'' and ``eshell-pred''.
 
+@node Running Shell Pipelines Natively
+@section Running Shell Pipelines Natively
+When constructing shell pipelines that will move a lot of data, it is
+a good idea to bypass Eshell's own pipelining support and use the
+operating system shell's instead.  This is especially relevant when
+executing commands on a remote machine using Eshell's Tramp
+integration: using the remote shell's pipelining avoids copying the
+data which will flow through the pipeline to local Emacs buffers and
+then right back again.
+
+To quickly construct a command for the native shell, prefix your
+Eshell input with the string @code{||}.  For example,
+
+@example
+|| cat *.ogg | my-cool-decoder >file
+@end example
+
+Executing this command will not copy all the data in the *.ogg files
+into Emacs buffers, as would normally happen with Eshell's own
+pipelining.
+
+As this feature is implemented as an expansion, your original input
+will be replaced with something like this:
+
+@example
+eshell-shell-command \"cat *.ogg | my-cool-decoder >file\"
+@end example
+
+This makes it clear what Eshell is doing, but has the disadvantage of
+being harder to edit, especially for complex pipelines with a lot of
+quoting.  If you would prefer that the @code{||}-prefixed input be
+restored, customize the variable @code{eshell-input-filter-functions}
+to include the function @code{eshell-restore-unexpanded-input}.
+
 @node Input/Output
 @chapter Input/Output
 Since Eshell does not communicate with a terminal like most command