Is there some free software that can show CPU temperature in real time for OS X El Capitan?
Any way to have CPU temperature in the menu bar? doesn't provide such an option.
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Since nobody suggested it yet, you can try Macs Fan Control. I'm using it for MBP on El Capitan 10.11.5, and it shows real temperature. http://www.crystalidea.com/macs-fan-control
smcFanControl has been good for me. It is simple but perfectly does the one thing I need: providing me the temperature.
If you have homebrew, you can use
brew cask install smcfancontrol to install the program.
However, as others noted, it may not perform perfectly so if that is the case for you, you can use https://www.derman.com/iMac-Fan-Control, which is also good.
I think your best option, and the one I use, is iStat Menu.
There are hundreds of different configurations and sensors you can use to monitor every aspect of your Mac. See the image below for a small example. This is not freeware but you can try it out for 14 days for free to see if it works for you.