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So let's
talk about su.

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Why would you use su? Well, SU is a switch user
and it allows you to open a shell as a specific user.

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For instance, if you use SU lisa,
then you will be prompted for a password,

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which is a password of user lisa
and open a shell as usual lisa.

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If you do this with super user
privileges, you won't be prompted for a password.

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If you use SU without any further
arguments, it will prompt for the root password

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and open a root shell. Now, when
you use su, you open a subshell.

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That means that not all the environment variables are set
as a target user. That might not be what you want.

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For that reason, it might
be better to use su.

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SU opens a login shell, which
means that environment variables are set as

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a target user and you really
appear to be the target user.

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User root can use SU to
become any user without entering a password.

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Regular users can use SU also,
but they need the target user password.

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Let me show
you how it works.

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Here I am in a root shell and in
this machine I do have a couple of users.

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The use case for user su is if you
have administrative access, you use SU lynda, for instance,

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which allows you to become user lynda without entering
a password or whatsoever. Now why would you want

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to become user Lynda? Let me type who
am I to verify that I really am?

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Well, for instance, to verify that
you have set up permissions the right

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way and user lynda can do
whatever that user wants to do.

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It's also possible to do this from
a shell that is not a root shell.

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Then either you use sulinda and enter
the password for user, which is getting

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you in, or you use your
sudo superpowers using sudo su minus linda.

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And then you're not prompted for the
password of the target user, but for your

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own user password, because that is what
you need to invoke your sudo superpowers.