Stéphane (not "M") Chazelas is a genius. I have written him several times over the past 30 years, encouraging him to write a book about shell, POSIX, and other topics that he is an expert in. He writes very well too. I fear we will all be the poorer if/when he retires.

"M." is the French abbreviation for "monsieur", the equivalent of "Mr." in English. So "M. Chazelas" is the correct way to politely refer to Stéphane Chazelas by his last name, since he's French. (Not that writing "Mr. Chazalas" would be wrong either when writing in English, but "M. Chazelas" is entirely correct).