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- All right, as I told you,

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Linux is all about using commands.

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So before we are starting
to learn how to work

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with many, many commands
that are available on Linux,

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first a few generic words.

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The first thing to realize is
that Linux is case sensitive.

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That means that really it makes sense

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if you use upper or lowercase,

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and I can tell you all commands

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that you are going to
work with are lowercase.

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So make sure that you are typing lowercase

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because otherwise you
will get an error message.

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Also in Linux commands,
go together with options.

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An option is something that
you put behind the name

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of the command to change
the behavior of the command.

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And options exist as long
options or as short options.

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Long options will always start

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with dash dash and short
options normally start

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with a single dash.

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If you use, for instance, uname--help

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then --help is the long option.

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And if you use ls -lrt,

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the -lrt is referring
to three short options;

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the L, the R, and the T.

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And if you wanna know
which options are available

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you can use --help

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on any command to learn how to use it.

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Let's do a quick demo
before we are moving over

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to the next video.

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All right, let me demonstrate
the use of these options.

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So ls --help.

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I really like the --help

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because that's printing the
options that are available

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and oh boy, that's a lot.

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We will take care of that for later.

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But what I want to show you is

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that ls -- help is showing help.

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You know what, let's do that on mkdir,

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mkdir is for make directory.

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At least it fits on one screen.

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I'll tell you about the help text later.

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What I want you to realize right now is

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if we are using mkdir-help
with a single minus instead

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of a double minus, then I'm
getting invalid option H.

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So the single minus is
interpreting H and E and L and P

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and that is invalid.

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So we need a double minus.

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Let me also show you the uppercase thing.

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If I would type LS in
uppercase, then what do we get?

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We get LS command not found.

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In some cases your distribution

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will tell you, "a LS command not found.

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"Did you want to run LS in all lowercase?"

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In this case, you can see
that is not happening.

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So the message here is in Linux,

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mind the use of uppercase and lowercase.