The Linux Command Line (2012)
Part I. Learning the Shell
Chapter 1. What Is the Shell?
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 that 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. A number of other terminal emulators are available for Linux, but they all do basically 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, you should see something like this:
[me@linuxbox ˜]$
This is called a shell prompt, and it appears 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 hash mark (#) rather than a dollar sign, the terminal session has superuser privileges. This means that either we are logged in as the root user or we’ve 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 tells us so and gives us another chance:
bash: kaekfjaeifj: command not found
[me@linuxbox ˜]$
Command History
If we press the up-arrow key, we see that the previous command kaekfjaeifj reappears after the prompt. This is called command history. Most Linux distributions remember the last 500 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. A mechanism built into the X Window System (the underlying engine that makes the GUI go) 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 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.
Don’t be tempted to use ctrl-C and ctrl-V to perform copy and paste inside a terminal window. They don’t work. For the shell, these control codes have different meanings that were assigned many years before Microsoft Windows came on the scene.
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 it. This is contrary to the traditional X behavior of “focus follows mouse,” which means that a window gets focus when the mouse just passes over it. The window will not come to the foreground until you click it, but it will be able to receive input. Setting the focus policy to “focus follows mouse” will make using terminal windows easier. Give it a try. 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:
[me@linuxbox ˜]$ date
Thu Oct 25 13:51:54 EDT 2012
A related command is cal, which, by default, displays a calendar of the current month:
[me@linuxbox ˜]$ cal
October 2012
Su Mo Tu We Th Fr Sa
1 2 3 4 5 6
7 8 9 10 11 12 13
14 15 16 17 18 19 20
21 22 23 24 25 26 27
28 29 30 31
To see the current amount of free space on your disk drives, enter df:
[me@linuxbox ˜]$ df
Filesystem 1K-blocks Used Available Use% Mounted on
/dev/sda2 15115452 5012392 9949716 34% /
/dev/sda5 59631908 26545424 30008432 47% /home
/dev/sda1 147764 17370 122765 13% /boot
tmpfs 256856 0 256856 0% /dev/shm
Likewise, to display the amount of free memory, enter the free command:
[me@linuxbox ˜]$ free
total used free shared buffers cached
Mem: 513712 503976 9736 0 5312 122916
-/+ buffers/cache: 375748 137964
Swap: 1052248 104712 947536
Ending a Terminal Session
We can end a terminal session by either closing the terminal emulator window or 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 ctrl-alt-F1 through ctrl-alt-F6 on most systems. 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.