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I launch a shell in my session with M-x shell and then I launch a python script that produces a lot of output to stdout---so many lines of output that my emacs session beomes quite slow.

Is there a command I can run to ask my buffer to be fixed length and automatically delete lines older than N? I have tried looking this up, but don't even know what terms to use in my query beyond "automatically delete," "circular buffer" and "fixed length" which were not helpful.

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    You are looking for M-x comint-truncate-buffer. Ref: stackoverflow.com/questions/25862743/… – Mittenchops Jul 20 '20 at 5:46
  • No, this doesn't do what I want. The buffer grows beyond 1024 soon after doing this. I'm looking for a fixed length not greater than X, not a one-off chop to that length. – Mittenchops Jul 20 '20 at 5:53
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comint-buffer-maximum-size is a variable defined in ‘comint.el’.
Its value is 1024

Documentation:
The maximum size in lines for Comint buffers.
Comint buffers are truncated from the top to be no greater than this number, if
the function ‘comint-truncate-buffer’ is on ‘comint-output-filter-functions’.

You can customize this variable.
(defun my-shell-mode-hook ()
  "Custom `shell-mode' behaviours."
  (add-hook 'comint-output-filter-functions 'comint-truncate-buffer nil :local))

(add-hook 'shell-mode-hook 'my-shell-mode-hook)
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  • I have this defined and when used this, with the above, it clipped my buffer to 1024 lines. However, as the script kept running, it then proceeded to grow without bound. I can M-x comint-truncate-buffer again, manually, but this does not seem to fix the length at 1024 automatically as the buffer grows. – Mittenchops Jul 20 '20 at 23:32
  • You would need to create a new shell buffer for shell-mode-hook to be called. You could use M-: (my-shell-mode-hook) RET in an existing buffer to set that config. Either way, C-h v comint-output-filter-functions in the shell buffer should include comint-truncate-buffer. Can you confirm whether you still have the problem after you've verified that the output filter is present for the shell buffer in question? – phils Jul 20 '20 at 23:47

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