Question

If I run M-x shell in emacs to get a terminal, it knows where to wrap lines automatically. For example, the output of ls is formatted into columns that fit the window properly.

My problem is if I then split the window vertically with C-x 3, shell-mode still thinks the window fills the whole frame. The result is ugly wrapping of command output. Is there a way to let shell-mode know it has to update the screen width?

EDIT:

Using HN’s answer below, I came up with this fix:

(defun my-resize-window ()
  "Reset the COLUMNS environment variable to the current width of the window."
  (interactive)
  (let ((proc (get-buffer-process (current-buffer)))
        (str (format "export COLUMNS=%s" (window-width))))
    (funcall comint-input-sender proc str)))

(defun my-shell-mode-hook ()
  (local-set-key "\C-cw" 'my-resize-window))

Answer 1

I’m a bit late to the party, but COLUMNS isn’t the way to do this. Here is an excerpt from my .emacs:

(defun comint-fix-window-size ()
  "Change process window size."
  (when (derived-mode-p 'comint-mode)
    (set-process-window-size (get-buffer-process (current-buffer))
                         (window-height)
                         (window-width))))

(defun my-shell-mode-hook ()
  ;; add this hook as buffer local, so it runs once per window.
  (add-hook 'window-configuration-change-hook 'comint-fix-window-size nil t))

(add-hook 'shell-mode-hook 'my-shell-mode-hook)

Unlike exporting COLUMNS each time, this method doesn’t require you to be in
bash at the moment and it doesn’t spam your session with blank prompts. This
code should probably be in comint itself, maybe I will file a bug report.

EDIT: If you modify window-configuration-change-hook in a buffer-local-way
the hook runs once per window, as opposed once per frame.