The "vcgencmd" command is very useful and allows you to display all manner of system information like for example the clock speed of the CPU, the voltage the CPU is running at and the temperature of the system core.
To find out the clock speed of the ARM processor type "vcgencmd measure_clock arm" at the command line prompt and press Enter:
pi@raspberrypi ~ $ vcgencmd measure_clock arm_
This should return a result similar to the following, which in my case shows that the ARM processor is running at 700 MHz (700,000,000 Hz):
You can also measure your system's core frequency by typing "vcgencmd measure_clock core" at the command line prompt and press Enter:
pi@raspberrypi ~ $ vcgencmd measure_clock core_
This should return a result similar to the following, which in my case shows that the system core is running at 250 MHz (250,000,000 Hz):
To measure the voltage your system is running at type "vcgencmd measure_volts" at the command line prompt and press Enter:
pi@raspberrypi ~ $ vcgencmd measure_volts_
This should return a result similar to the following, which in my case shows that the system is running at 1.20 volts:
Finally you can measure your system's core temperature by typing "vcgencmd measure_temp" at the command line prompt and press Enter:
pi@raspberrypi ~ $ vcgencmd measure_temp_
This should return a result similar to the following, which in my case shows that the system is running at at temperature of 48.2 deg C:
All the above commands are very useful for monitoring your system, especially if you wish to overclock your Raspberry Pi. Overclocking simply means running your system at a higher frequency and/or voltage than it was designed for in order to increase performance, but an overclocked system will inevitably run hotter and possibly be less reliable than a non-overclocked system.
To start experimenting with overclocking run the Raspberry Pi configuration utility by typing "sudo raspi-config" at the command line prompt and pressing Enter:
pi@raspberrypi ~ $ sudo raspi-config_
Use the arrow keys to move down to the "overclock" command and press Enter:
You will then see the following warning message:
Press Enter to dismiss this warning and you will be presented with a number of overclocking options to choose from:
Select your desired option and press Enter. Your will receive conformation of your selection (in my case I selected "Medium"):
Then you will be returned to the Raspi-config main screen. When you select "Finish" and press Enter you will be asked if you want to reboot now:
Select "Yes" and let your Raspberry Pi reboot. Once your Raspberry Pi has rebooted and you have signed back in you can use the "vcgencmd" commands to check the frequency, voltage and temperature of your system.
Please note however that your system will only use the overclocking settings if required. In other words when the system is "idle" CPU frequency and voltage will be normal, but under load the overclocking settings will be used. You can see this if you run the "vcgencmd" commands when the system is "idle" and then run them again while you have a resource hungry application running in the background.
Also note that even if overclocking settings are enabled, holding down the "Shift" key while the system is rebooting will cause overclocking to be disabled temporarily until the system is rebooted again
A final word of warning - overclocking can cause your SD memory card to become corrupted, so if you want to experiment with overclocking make sure to back up your memory card first...