How do you open a command line?

It is called terminal.

You can launch your terminal by pressing Ctrl+Alt+T or search from dash by pressing Meta/Super/Windows button and type terminal.

If you're using Gnome, Terminal is probably under Accessories in your Application menu, or you can also try Alt F2 to bring up a launcher widget into which you can type "t-e-r-m-i..." to see all the terminal options.

Basic command line.

Enjoy your command line !

Press Ctrl Alt T on the keyboard. If you prefer, there should be something called Terminal in your programs menu. You can search for it by pressing "Windows" key and typing "terminal". Remember, commands in Linux are case sensitive (so upper- or lower-case letters matter).

Command line can be accessed in couple of ways.

One is by using terminal emulator . The default terminal emulator for Ubuntu ( with default Unity interface ) is gnome-terminal. It can be accessed by either pressing CtrlAltT or by pressing the Ubuntu icon on the launcher and typing in the word "Terminal" or "gnome-terminal"

As you can see in the image, I've multiple terminal apps available, and indeed one can install more than one terminal emulator.

Another way to access command line is through the virtual console, called TTY. There are 6 virtual consoles available. Number of each console corresponds to the number of Fx key, where x is the number. For instance, to access TTY1, press CtrlAltF1 . To return to graphical environment , press AltF7, or repeatedly press AltArrow Left/Right until you exit.

There are other, rather hackish ways to access command line , but for the 99% of the day-to-day tasks terminal emulator and tty are sufficient.

Note , when it comes to command-line, terminology may get confusing.