CAIRO — Hosni Mubarak, the former president of Egypt, was back in court in Cairo on Saturday, appearing healthy, and grinning and waving as his trial resumed on charges related to the killings of about 900 protesters during the 2011 uprising that led to his ouster.

Mr. Mubarak, 85, was sentenced to life in prison for failing to stop the killings, but his sentence was overturned on appeal this year, and a retrial was ordered.