The most important time to enter the debugger is when a Lisp error happens. This allows you to investigate the immediate causes of the error.
However, entry to the debugger is not a normal consequence of an error. Many commands signal Lisp errors when invoked inappropriately, and during ordinary editing it would be very inconvenient to enter the debugger each time this happens. So if you want errors to enter the debugger, set the variable debug-on-error to non-nil. (The command toggle-debug-on-error provides an easy way to do this.)
This variable determines whether the debugger is called when an error is signaled and not handled. If debug-on-error is t, all kinds of errors call the debugger, except those listed in debug-ignored-errors (see below). If it is nil, none call the debugger.
The value can also be a list of error conditions (see Signaling Errors). Then the debugger is called only for error conditions in this list (except those also listed in debug-ignored-errors). For example, if you set debug-on-error to the list (void-variable), the debugger is only called for errors about a variable that has no value.
Note that eval-expression-debug-on-error overrides this variable in some cases; see below.
When this variable is non-nil, Emacs does not create an error handler around process filter functions and sentinels. Therefore, errors in these functions also invoke the debugger. See Processes.
This variable specifies errors which should not enter the debugger, regardless of the value of debug-on-error. Its value is a list of error condition symbols and/or regular expressions. If the error has any of those condition symbols, or if the error message matches any of the regular expressions, then that error does not enter the debugger.
The normal value of this variable includes user-error, as well as several errors that happen often during editing but rarely result from bugs in Lisp programs. However, “rarely” is not “never”; if your program fails with an error that matches this list, you may try changing this list to debug the error. The easiest way is usually to set debug-ignored-errors to nil.
If this variable has a non-nil value (the default), running the command eval-expression causes debug-on-error to be temporarily bound to t. See Evaluating Emacs-Lisp Expressions in The GNU Emacs Manual.
If eval-expression-debug-on-error is nil, then the value of debug-on-error is not changed during eval-expression.
Normally, errors caught by condition-case never invoke the debugger. The condition-case gets a chance to handle the error before the debugger gets a chance.
If you change debug-on-signal to a non-nil value, the debugger gets the first chance at every error, regardless of the presence of condition-case. (To invoke the debugger, the error must still fulfill the criteria specified by debug-on-error and debug-ignored-errors.)
For example, setting this variable is useful to get a backtrace from code evaluated by emacsclient’s --eval option. If Lisp code evaluated by emacsclient signals an error while this variable is non-nil, the backtrace will popup in the running Emacs.
Warning: Setting this variable to non-nil may have annoying effects. Various parts of Emacs catch errors in the normal course of affairs, and you may not even realize that errors happen there. If you need to debug code wrapped in condition-case, consider using condition-case-unless-debug (see Handling Errors).
If you set debug-on-event to a special event (see Special Events), Emacs will try to enter the debugger as soon as it receives this event, bypassing special-event-map. At present, the only supported values correspond to the signals SIGUSR1 and SIGUSR2 (this is the default). This can be helpful when inhibit-quit is set and Emacs is not otherwise responding.
If you set debug-on-message to a regular expression, Emacs will enter the debugger if it displays a matching message in the echo area. For example, this can be useful when trying to find the cause of a particular message.
To debug an error that happens during loading of the init file, use the option ‘--debug-init’. This binds debug-on-error to t while loading the init file, and bypasses the condition-case which normally catches errors in the init file.
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https://www.gnu.org/software/emacs/manual/html_node/elisp/Error-Debugging.html