How to determine the AUCTex TeX engine?

Question

How can I find out which TeX engine my AUCTeX uses? How can I change the TeX engine my AUCTeX uses (specifically, (a) to LuaTeX, (b) to XeTeX)? Following this post, I tried to inspect the value of the variable TeX-engine, however this variable isn't defined in my environment, even though AUCTeX works fine.

Answer 1

This variable is not defined (or set) until AUCTeX is loaded. On your system, it seems to be deferred until it is first needed ("autoloaded" in emacs slang).

More precisely:

• you start emacs, some initialization happens during which emacs notes that LaTeX-mode is defined in some .el file in the AUCTeX package;
• if you try to inspect TeX-engine at this point, the variable has never been defined;
• if you now open a .tex file, emacs turns on LaTeX-mode, which is not loaded yet. So emacs loads the AUCTeX package, and now TeX-engine is defined;
• at this point, you can inspect TeX-engine from any buffer of the emacs session (it will have a default value in all buffers, and a buffer-local value in AUCTeX buffers).

For your second question, you can use the command TeX-engine-set, which will apply the setting to the current buffer. To apply the setting to the default value (for all buffers), set the variable through the customize interface.