shell is a program that takes keyword commdands and passes them to
the operating system to carry out.when you use GUI,KDE uses konsole and GNOME uses
gnome-terminal.
$ is called a shell prompt and it will appear whenever the shell is ready to accept input.
# means you logged in as the root user or the terminal emulator that provides superuser privileges.
df: see the current amount of free space on your disk drives.
free:display the amount fo free memory.