The past couple of days the fan on my MacBook Pro has been constantly on and battery usage down to around a third. Launching Activity Monitor, it showed 3 Ruby processes all at 100% CPU usage. The solution was to force quit these processes and within seconds, the fan slowed up and the battery indicator went up. Force quitting these processes didn’t affect anything I was doing with Rails or IRB so I guess they were just stray processes.
If anyone else gets the same problem, just launch Activity Monitor which lives in the /Applications/Utilities folder, click the CPU tab if not already selected, select processes with a process name of “ruby”, click “Quit Process” and then select “Force Quit”. Just selecting “Quit” wouldn’t work for me hence the “Force Quit”.
I hope this saves someone else the head-scratching as to why their Mac is running hot.