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- In this video, you will learn

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about I/O Redirection and Piping.

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So, let's start with redirection.

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Redirection is used to manipulate

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input and output of commands.

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And there are three ways of doing it:

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standard input, also known as the zero.

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The zero is for the file descriptor,

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and the file descriptor is
something used internally

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by the Linux kernel to
take care of redirection.

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And the zero is always
your standard input.

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And the standard input normally represents

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your keyboard attached to your computer.

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And if you want to get
information from somewhere else,

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well, there is a smaller than redirector.

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Now, the thing with the
smaller than redirector

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is that you don't need it that often,

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because many commands don't
need a smaller than redirector

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to take their input from
a file for instance,

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but this would give you something

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sort smaller than etc services.

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And I can only say, if you would tell me,

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Hey we can do that without a smaller than.

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And I would say, yeah, you are right.

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But standard input redirectors
are just not used that often.

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That's not the case for
standard output redirector.

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The standard output redirector,

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normally your standard output.

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That's your screen, your terminal
attached to your computer

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and results of commands
are sent to your computer.

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And you can redirect it
using a greater than sign.

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So if you use ls greater then to ~ myfile

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then it'll create myfile
in your home directory.

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Do notice that a single greater than sign

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will override destination
if it already exists.

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if you don't want to override but append

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then you use a double redirect.

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That will add it to the end of the file.

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Also convenient is the option
to redirect standard error.

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We have seen that on a
couple of occasions already.

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In order to redirect standard
error, you use 2 greater than

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in which the 2 is the file descriptor 2

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and the greater than will send it

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wherever you want to send it.

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Like right here, for
instance, where we use

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2 greater than dev null. dev
null is a discard device.

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Anything you send to the dev
null device will be discarded.

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You won't see it anymore.

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And this is a perfect
way to make sure that you

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don't see any error messages
while running commands.

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And here we have another example

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which is using ampersand greater than.

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Now what is ampersand greater than?

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That will represent the standard output

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as well as the standard error.

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Now that is stuff that you
normally don't want to write

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to the dev null device, but you might want

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to capture all that
information in a specific file.

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And then this is how you
do it. Before we move over

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and talk about piping, let's
go check out these examples.

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So, right. Let me start using

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sort smaller than etc services

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to use etc services as the
thing that we want sort.

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And yeah, the smaller than
is just redirecting the

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the input from the
keyboards to etc services.

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Right. As I was telling you before

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that's not a very
convincing example, I know.

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But ls greater than, lsfile
is a more convincing example.

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What do we get? We get
no output on screen.

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We get output in lsfile.

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Which is the list of files just generated.

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Now, if I use who greater than to lsfile

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then I'm doing an, a standard
output redirection again.

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And after checking the contents of lsfile

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you have seen that who
command has overwritten.

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And if you don't want
to override but append

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then you should use a double redirect.

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The double redirect, as you
can see is adding the output

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of the command to the
file we just created.

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Now, how about the error messages?

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Well, let me do grab a root on etc start.

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That's an example that we've seen before.

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Many, many permission
denied is a directory

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which are error messages.

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And if you don't care
about the error messages.

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2 greater than dev null.

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And that will make sure that you only see

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the regular output. Now, what is happening

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if you make that ampersand greater than.

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Ampersand greater than dev null

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is showing nothing at
all. That might be useful

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for processes that are
supposed to be running

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in the background

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but not for interactive
commands, like grep.

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So let me create outfile and
write the output to outfile.

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And there we can see the, the output

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including the error messages.

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We have the file descriptor 1 as well as

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as the file descriptor 2

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both being sent to outfile.
Let's go check out piping.

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So a pipe is used to send
the output of one command

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to be used as input for a second command.

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This also is something that we have

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already seen a couple of times.

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Like ps aux, grab http. Where ps aux is

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showing a lot of running
processes and grep http is

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filtering out the lines
that contain the text http.

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In combination with the pipe,
you might see the tee command.

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Now, what is the tee command?

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The tee command combines
redirection and piping.

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It allows you to write
the output to somewhere

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and at the same time, use it
as input for another command

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like ps aux, tee psfile, pipe grep ssh.

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Let me show you these examples.

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So to start with ps aux, ps aux.

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Which by the way, is
another BSD style commands.

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This command is showing a list

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of all the processes currently running.

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And if you want to verify whether

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or not SSH is running well,
this is what you can do.

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This is an ordinary pipe
as we have already seen.

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Now, how does that work with, with tee?

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Well, I am going to pipe to tee.

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psfile or whatever you wanna call it.

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And then I'm going to pipe to grep SSH.

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Here we can see the
result of the grep SSH,

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but at the same time,
it has created psfile.

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So in psfile, we have the ps aux

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and the grep SSH is working on the ps aux.

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The tee is really like
the uppercase letter T.

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It's sending you two different directions

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to a file as well as to the pipe

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where it is further treated by grep SSH.

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This tee can be handy

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if you want to have an
additional opportunity to

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analyze what exactly was happening

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before you applied the
command after the pipe.

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And that's all, let's continue.