Whatever you type after the shell prompt and before pressing Enter
is called a _command line_. It’s a line of text that commands the
computer to do something. The Debian default shell offers several
features to make entering command lines easy.
You can scroll up to previous commands to run them again, or you can
modify them slightly and _then_ run them again. Try this: Enter any
command, such as whoami; then press the Up Arrow key. The whoami
command will reappear at the prompt. You can then press Enter to run
whoami a second time.
If you’ve entered several commands, you can keep pressing the Up
Arrow key to go back through them. This feature is handy if you’re
doing the same thing several times, or if you type a command
incorrectly and want to go back to fix it. You can press the Down
Arrow key to move in the other direction, toward your more recent
commands. If there are no more commands to move to, the computer
will beep.
You can also move around on the command line to make changes. The
easiest way is with the Left and Right Arrow keys. Try typing
whoasmi instead of whoami, and then use the Left Arrow key to move
back to the s. You can erase the s with the Backspace or Delete
keys.
There are more advanced features as well (no need to memorize them
all now, though). Try pressing Ctrl-a. This moves you to the
beginning of the line. Ctrl-k (the k stands for “kill”) deletes all
characters until the end of the line; try it from the middle of the
command line. Using Ctrl-a followed by Ctrl-k, you can delete the
entire command line. Ctrl-y pastes the last thing you killed,
reinserting it at the current cursor position (y stands for “yank,”
as in “yank it back”). Ctrl-e will move the cursor to the end of the
command line.
Go ahead and play around with command-line editing to get a feel for
it. Experiment.