To display the amount of free memory enter the following command in the terminal
free
To display the amount of free memory in megabytes enter the following command in the terminal
free --mega
To display the amount of free memory in gigabytes enter the following command in the terminal
free --giga
To display the amount of free memory in human format enter the following command in the terminal
free -h
Why would I use this command when I have options in the GUI?
If you don't have access to a remote desktop for example because you are checking the amount of free memory available on a remote server, you can connect and login to that machine using ssh and putty and use the free command.