I have recently come across a useful addition for my Eshell prompt. I have a love of a simple prompt, but one thing I think it is worthwhile to add is a warning when the previous command exited with a non-zero exit status.

This will be the first time we replace the definition of a function. Thanks to Lisp being great, this is effortless. 

;;(require 'esh-io)
(defun mak::last-command-status ()
  (interactive)
  (let ((c eshell-last-command-status))
    (when (and c (not (eq 0 c)))
      (format "?%d " c))))

(defun mak::eshell-prompt ()
  (interactive)
  (concat (if (not (eq 0 (current-column)))
              "\n"
            "")
          (mak::last-command-status)
          (mak::ghost-or-percent)
          " "))

Then we update the prompt regex to match our new prompt format: 

(setq eshell-prompt-regexp (format "^\(?[0-9]+ \)\?%s " (mak::ghost-or-percent)))

And take the new prompt for a spin. 

👻 false
?1 👻 false
?1 👻 true
👻 true
👻 false
?1 👻 true
👻

There we go. I don't need the current directory in the prompt, because I can find that in a heartbeat. I don't need the current Git branch in the prompt, because I do everything with Magit. This will be useful in that while I can find the information if I need, checking the exit status is something I never think to do.

One glitch that testing revealed is that if I run a command that exits with a non-zero status, kill the Eshell buffer, and create a new Eshell buffer, the prompt will be ?1 👻. This indicates that Eshell does not clear the eshell-last-command-status variable on initialization. I'm not going to fix this in my code, because I feel like it should be fixed in Eshell. Maybe I'll submit a bug report for it when I have some free time.