Basics
The shell operates under some fairly foreign principles when compared to the rest of the software you might use on a daily basis. It doesn't work how you expect — Ctrl+C doesn't copy and Ctrl+V doesn't paste, for instance — but it's got plenty of conventions that are carried through into other tools you might end up using, so it's worth taking the time to play around a bit. Try entering this into your shell:
$ echo "Hello!"
Hello!
note
Only type the text after the $. Then, press enter, and you should see the
same output on your screen as you see in the text above.
The $ here represents your prompt. It might look a little different in
your terminal, but many default prompts end in a $, so that's the character most
commonly used to represent it in documentation.
echo simply repeats whatever you give it. Despite what it looks like, there's
actually a lot going on here. Try the next two commands:
$ echo "Hello," "world!"
Hello, world!
$ echo "Hello, world!"
Hello, world!
These two commands are different, but they produce the same result. This comes
down to two very important concepts in the shell that you'll come into contact with
again and again: commands and arguments. In the above, we can say that
you ran the command echo "Hello!", but we can also say that the command
you ran was echo, and the argument was Hello!. This is the difference
between echo "Hello," "world!" and echo "Hello, world!". The first has just
two arguments, and the second has just one. The command echo just happens to
display all of its arguments together, putting a space in between them.
By the way, the quotes are strictly optional. All of the commands below have the same output. Try them!
$ echo "Hello," "world!"
Hello, world!
$ echo Hello, "world!"
Hello, world!
$ echo "Hello, world!"
Hello, world!
$ echo Hello, world!
Hello, world!
Even though these are all the same, it's generally considered good practice to quote arguments. To see why this is, try this command, and guess what the output might be:
$ echo Sorry. All the microchips are gone. I got #hungry.
You'll notice that the final #hungry doesn't display! What's going on here? It
turns out that the shell treats everything in a line after the # character to be a
comment: It's entirely ignored. You can use this to write out prose descriptions
of what your commands do:
$ echo "Hello" # Print a greeting to the display.
This command has three parts: There's the command itself (echo), the argument
(Hello), and the comment (everything after the #). What if you really want
to echo a #, though? Quoting will help you here:
$ echo "Sorry. All the microchips are gone. I got #hungry."
Sorry. All the microchips are gone. I got #hungry.
These ideas of arguments, comments, and commands make up very much of the
command line that you'll be interacting with, but you'll be using some more
commands than just echo. Go on to navigation for some
more useful techniques.