Can Emacs render shell escape codes stored in a GNU Screen session log file?

Question

The GNU Screen utility has the ability to log all shell output to a file, a useful feature to log the output of a long running build system, for instance.

• The log file generated by GNU Screen contains the terminal escape sequences that were output during the session.
• Although I could filter them out with some tool, I'd like to keep these escape sequences inside the log file as they color the output and help pin-point things like compiler warning and error messages.
• But I want to be able to view the file inside Emacs to process it, search for things, etc...

I'd like to visit that log file in Emacs and get Emacs render the escape code the way the shell originally did. Now what is seen is the escape sequences. Is there an Emacs major or minor mode that can do that?

Something like term-mode that would only render the text but without launching a terminal process?

• I was able to render the escape sequence by visiting the screen log file in fundamental-mode first, then launching the eshell-mode and then executing (term-mode). But that's not ideal as it gets in the way by starting an active shell in the buffer.
• If something already exists that'd be great, otherwise maybe it would be possible to write a major mode that acts just as a text editing mode but renders the escape sequences like term-mode does. Any suggestions?

Answer 1

There's a function called ansi-color-apply-on-region that can almost do what you want. It's not interactive, but I was able to create an interactive wrapper and apply it to a region with escape characters in it.

(defun u/ansi-color-apply-on-region (begin end)
  (interactive "r")
  (ansi-color-apply-on-region begin end t))

Highlight a region with terminal escape sequences in it, and hit M-x u/ansi-color-apply-on-region to test it out. Maybe you can build something on top of this.

Answer 2

Perhaps what follows will be useful to you. I use a variant of this in my emacs setup. (I don't use screen but another program that logs screen output.)

As is, it instructs emacs, whenever you open a file which filename looks like screenlog.n with n a number, to enable rendering ANSI sequences in color. (It does not process cursor movement and the like, which I prefer anyway.)

(add-hook 'text-mode-hook
          (lambda ()
        (when (string-match "screenlog\.[0-9]*$" (buffer-name))
              (progn
        (format-decode-buffer 'backspace-overstrike)
        (format-decode-buffer 'ansi-colors)
        (hide-dos-eol)
        (face-remap-add-relative 'default '((:foreground "white" :background "black")))
        ))))

It has some drawbacks, e.g. you cannot save buffers (you get "Sorry, 'ansi-colors' format is read-only."), but it allows what you asked for, rendering shell escape codes.