getting a process buffer for an eshell buffer

Question

To get a process buffer an eshell buffer I thought I used to be able to run:

(get-buffer-process (current-buffer))

In Emacs 25 this no longer works. Specifically "GNU Emacs 25.1.50.2 (x86_64-unknown-linux-gnu, GTK+ Version 3.10.8) of 2015-11-30"

Here is what I do. I start an eshell with M-x eshell. Then with that as the only window or buffer around I run

M-x eval

Eval: (get-buffer-process (current-buffer))

And the result is nil.

I am using this in the code for realgud, an emacs front-end to various debuggers. It needs to interact with the debugger like see if the debugger is running or has terminated, send commands to it and run hooks when output appears.

How would I do this?

Answer 1

Eshell has not a process-id of its own. It is solely running on the base of emacs-lisp functions within emacs.

Nevertheless, you can start processes from an eshell buffer. The eshell buffer becomes the process-buffer for the started processes.

The following eshell session shows what is going on with processes in the eshell buffer.

Welcome to the Emacs shell

~ $ emacs-version 
GNU Emacs 24.5.1 (x86_64-unknown-cygwin, GTK+ Version 3.14.13)
 of 2015-06-23 on desktop-new
~ $ get-buffer-process (current-buffer)
~ $ /usr/bin/xterm &
[xterm<1>] 5168
~ $ get-buffer-process (current-buffer)
#<process xterm<1>>
~ $ /usr/local/bin/xpdf.exe &
[xpdf.exe] 6464
~ $ get-buffer-process (current-buffer)
#<process xpdf.exe>
~ $ process-buffer (get-process "xpdf.exe")
#<buffer *eshell*>
~ $ process-buffer (get-process "xterm<1>")
#<buffer *eshell*>
~ $ remove-if-not #'(lambda (proc) (equal (process-buffer proc) (current-buffer))) (process-list)
(#<process xpdf.exe> #<process xterm<1>>)

Note, that I have deleted some lines that may only cause confusion (e.g., font messages for the started processes).

Side note: It is rather interesting that both started background processes have the same eshell buffer as process buffer, i.e., the process of a process buffer is not unique. It looks like get-buffer-process gets only the last started process for the buffer.