Deleting the shell prompt in Shell Mode

Deleting the shell prompt in Shell Mode

[Raffaele Ricciardi]

I am using the function `shell-snarf-envar' to read environment
variables in Shell Mode.  However, `shell-snarf-envar' leaves trace of
its passage, whilst I would rather have it act invisibly.  For instance,
calling:

     (shell-snarf-envar "SHELL")

leaves the following output (the first line belongs to my custom
prompt):

     ~/Downloads
     $ printenv "SHELL"
     /bin/bash

Therefore I have written a wrapper function to delete the shell
interaction performed by `shell-snarf-envar':

     (defun stealth-shell-snarf-envar/rr ($var)
       "Like `shell-snarf-envar', but leaves no trace."
       ;; fixme: does not delete the prompt.
       (lexical-let (($start (point)))
         (prog1
             (shell-snarf-envar "PS1")
           (delete-region $start (point)))))

But as the comment says, the above function does not delete the prompt,
as I would like it to do.

I have read that Shell Mode keeps track of prompts with text properties,
therefore I have run `shell-snarf-envar' with Edebug to understand its
inner workings, but Edebug has hung each time, leaving me without a clue
about how to detect shell prompts.

Thank you.