Ubuntu uses Psensor to monitor hardware Temperature

Psensor is a tool that monitors hardware temperatures and is well integrated with Unity. It supports High Temperature alarms using desktop notification systems and has application indicators. Monitoring of the following hardware temperatures is supported: motherboard and CPU temperature NvidiaGPU temperature experimental ATI/AMDGPUs temperature monitoring, which is not enabled in UbuntuPPA. Hard Disk temperature fan speed CPU usage first install monitoring cpu motherboard fan and other required packages: sudoapt-getinstalllm-s

Psensor is a tool that monitors hardware temperatures and is well integrated with Unity. It supports High Temperature alarms using desktop notification systems and has application indicators.

The following hardware temperatures can be monitored:

Motherboard and CPU Temperature
Nvidia GPU Temperature
Experimental ATI/AMD GPUs temperature monitoring, not enabled in Ubuntu PPA.
Hard Disk Temperature
Fan speed
CPU usage

First install the package required for monitoring the cpu motherboard fan:

Sudo apt-get install lm-sensors hddtemp
Sudo sensors-detect
Reply "yes.
Restart or use the following command to load the module required by sensor.
Sudo service module-init-tools start
Sudo apt-get install psensor

Or install it through stable PPA.

Sudo add-apt-repository ppa: jfi/ppa
Sudo apt-get update
Sudo apt-get install psensor

There is also an unstable Ubuntu PPA, which may contain bugs.

Sudo add-apt-repository ppa: jfi/psensor-unstable
Sudo apt-get update
Sudo apt-get install psensor