When we speak of the command line, we are really referring to the shell. The shell is a program that takes keyboard commands and passes them to the operating system to carry out. Almost all Linux distributions supply a shell program from the GNU Project called bash. The name “bash” is an acronym for “Bourne Again SHell”, a reference to the fact bash is an enhanced replacement for sh, the original Unix shell program written by Steve Bourne.
Terminal Emulators When using a graphical user interface, we need another program called a terminal emulator to interact with the shell. If we look through our desktop menus, we will probably find one. KDE uses konsole and GNOME uses gnome-terminal, though it’s likely called simply “terminal” on our menu. There are a number of other terminal emulators available for Linux, but they all basically do the same thing; give us access to the shell. You will probably develop a preference for one or another based on the number of bells and whistles it has. Your First Keystrokes So let’s get started. Launch the terminal emulator! Once it comes up, we should see something like this:
This is called a shell prompt and it will appear whenever the shell is ready to accept input. While it may vary in appearance somewhat depending on the distribution, it will usually include your username@machinename, followed by the current working directory (more about that in a little bit) and a dollar sign. If the last character of the prompt is a pound sign (“#”) rather than a dollar sign, the terminal session has superuser privileges. This means either we are logged in as the root user or we selected a terminal emulator that provides superuser (administrative) privileges. Assuming that things are good so far, let’s try some typing. Enter some gibberish at the prompt like so:
[me@linuxbox ~]$ kaekfjaeifj
Since this command makes no sense, the shell will tell us so and give us another chance: bash: kaekfjaeifj: command not found
Command History If we press the up-arrow key, we will see that the previous command “kaekfjaeifj” reappears after the prompt. This is called command history. Most Linux distributions remember the last 1000 commands by default. Press the down-arrow key and the previous command disappears.
Cursor Movement Recall the previous command with the up-arrow key again. Now try the left and right-arrow keys. See how we can position the cursor anywhere on the command line? This makes editing commands easy.
A Few Words About Mice And Focus: While the shell is all about the keyboard, you can also use a mouse with your terminal emulator. There is a mechanism built into the X Window System (the underlying engine that makes the GUI go) that supports a quick copy and paste technique. If you highlight some text by holding down the left mouse button and dragging the mouse over it (or double clicking on a word), it is copied into a buffer maintained by X. Pressing the middle mouse button will cause the text to be pasted at the cursor location. Try it. Note: Don’t be tempted to use Ctrl-c and Ctrl-v to perform copy and paste inside a terminal window. They don’t work. These control codes have different meanings to the shell and were assigned many years before Microsoft Windows.
Your graphical desktop environment (most likely KDE or GNOME), in an effort to behave like Windows, probably has its focus policy set to “click to focus.” This means for a window to get focus (become active) you need to click on it. This is contrary to the traditional X behavior of “focus follows mouse” which means that a window gets focus just by passing the mouse over it. The window will not come to the foreground until you click on it but it will be able to receive input. Setting the focus policy to “focus follows mouse” will make the copy and paste technique even more useful. Give it a try if you can (some desktop environments such as Ubuntu’s Unity no longer support it). I think if you give it a chance you will prefer it. You will find this setting in the configuration program for your window manager.
Try Some Simple Commands Now that we have learned to type, let’s try a few simple commands. The first one is date. This command displays the current time and date.
[compose@amp ~]$ date
Sun Mar 19 04:10:43 UTC 2017
A related command is cal which, by default, displays a calendar of the current month.
To see the current amount of free space on your disk drives, enter df:
Likewise, to display the amount of free memory, enter the free command.
Ending A Terminal Session We can end a terminal session by either closing the terminal emulator window, or by entering the exit command at the shell prompt:
[me@linuxbox ~]$ exit
The Console Behind The Curtain Even if we have no terminal emulator running, several terminal sessions continue to run behind the graphical desktop. Called virtual terminals or virtual consoles, these sessions can be accessed on most Linux distributions by pressing CtrlAlt-F1 through Ctrl-Alt-F6. When a session is accessed, it presents a login prompt into which we can enter our username and password. To switch from one virtual console to another, press Alt and F1-F6. To return to the graphical desktop, press Alt-F7.
Summing Up As we begin our journey, we are introduced to the shell and see the command line for the first time and learn how to start and end a terminal session. We also see how to issue some simple commands and perform a little light command line editing. That wasn’t so scary was it?