efaq: Debugging a customization file
5.4 How do I debug a ‘.emacs’ file?
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Start Emacs with the ‘-debug-init’ command-line option. This enables
the Emacs Lisp debugger before evaluating your ‘.emacs’ file, and places
you in the debugger if something goes wrong. The top line in the
‘trace-back’ buffer will be the error message, and the second or third
line of that buffer will display the Lisp code from your ‘.emacs’ file
that caused the problem.
You can also evaluate an individual function or argument to a
function in your ‘.emacs’ file by moving the cursor to the end of the
function or argument and typing ‘C-x C-e’ (‘M-x eval-last-sexp’).
Use ‘C-h v’ (‘M-x describe-variable’) to check the value of variables
which you are trying to set or use.