We'll be exploring the Linux command line using the Raspberry Pi as a baseline system. Let's go over the basic options for getting to a shell/command line in a terminal.
The easiest thing you can do is to just plug a display and keyboard into your Pi, running Raspbian, and work right on the console.
This just means that the "terminal" you're using to talk to the shell on your Pi is provided directly by the Linux kernel. You'll need a USB keyboard (mouse optional) and an HDMI display (or an adapter for other kinds of display).
Another option which requires a little more tinkering (but can save you the need for an extra keyboard and monitor) is to hook up a console cable to the Pi from another computer. Check out our guide on using a console cable, which comes complete with instructions for installing a terminal emulator on Mac and Windows machines.
The console cable connects to a couple pins on the Pi, and converts the text into a standard "serial" USB port, which is supported by tons of free software for Mac, Linux, and Windows like PuTTY, minicom, or screen.
This is called running the Raspberry Pi headless - because you are not using the 'monitor (head)' as a console.
If your Pi is configured to start a graphical desktop when it boots, or if you just prefer to use one, you'll probably be looking at something like this:
Look for a program called "LXTerminal" and fire it up. There should be an icon on the desktop:
...or you can find it under "Accessories" in the main menu:
Most of the time, when professional nerds need to access the command line on a remote Linux system, they rely on a standard protocol called SSH, which stands for Secure SHell.
SSH clients (the Pi is the SSH 'host') are available for all the major operating systems. It's easy to make the Raspberry Pi speak SSH once its connected to Ethernet or WiFi, and like using a console cable, it can save you cluttering your workspace with extra monitors and keyboards. However, you need to have the Pi running and connected to WiFi first, which is a bit of a chicken-and-egg problem if you don't already have a way to console in.