Install the lm-sensors and the hddtemp apps.
$ sudo apt install lm-sensors
$ sudo apt install hddtemp
Run sensors-detect to see what sensors are available.
$ sudo sensors-detect
$ sudo /etc/init.d/kmod start
Run the sensors app to get temperature info on core systems.
$ sensors
Run hddtemp to get temperature info on hard drive.
$ hddtemp /dev/sda
To monitor temperature for a period and at a given interval (default is 2 seconds), run sensors with the watch command.
$ watch sensors
