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Hello, the beautiful people.

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So in the last video you saw how to redirect standard output.

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And in this video, we're going to be picking that up and continuing by learning how to redirect standard

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error and standard input.

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So let's start with standard error.

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How do you go about redirecting that?

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Well, standard error works in very much the same way as standard output.

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Now, the number for the standard error data stream is the number two.

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So to redirect standard error, you would type cat and then instead of one and then a greater than sign,

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which is what we use for standard output.

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You just have a two because the data stream number four, standard error is the number two.

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And this would redirect the standard error from the cat command to error text and to append the standard

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error to whatever is already in error text, you would do the two arrows.

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So remember the difference between the one arrow and the two arrows is the one arrow will delete everything

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that's in the file before append before writing to it.

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So it will truncate the file and two arrows will append or add to what's already there.

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So let's demonstrate this.

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The cat command doesn't have the k option.

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So if I do cat give it the k option and just type blah.

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Right.

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We're going to get an error and it's going to say invalid option k, try cat help for more information.

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So this is an error message.

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So therefore it was output on the standard error data stream.

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And the standard error data stream is by default connected to our terminal and therefore we see it in

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our terminal.

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But let's redirect it to error text so I can press the up arrow key to save myself some typing and I'm

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going to type the number two and then a greater than sine and say redirect that to error text.

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So I'm saying run this and redirect the, the, the standard error to text.

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So when I press enter, we first of all, we noticed that nothing popped out on our terminal, which

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is a good sign.

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And when we open up our files, we can now see there's a file called Error Text.

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And when I open it, we can see that indeed the error message has been sent to that location.

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A common use for this kind, for redirecting standard error is to keep track of log messages coming

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from web servers and things like that.

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But a common mistake is to use just one arrow like we have here, because if I keep doing that, what

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that means is it will delete what's in the file before before it sends standard error again.

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So if I keep doing that, okay, imagine a new error happens every time.

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If I keep doing that and open error text.

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Only the most recent error has been saved.

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But if I instead of having one arrow, I have two arrows.

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Now I'm going to append each error on top of each other.

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Okay.

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Which is exactly what you want with a log file, because you want to be able to scroll back and see

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what's happened in the past.

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So it's open it again.

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Now we can see that the errors are being added on top of each other.

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So that's the difference between having the one arrow and the two arrows.

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The two arrows appends onto what was already there, and that's really what you need for standard error.

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Now you can, of course, redirect standard error and standard output at the same time.

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So for example, we could type the command and redirect our output to output text.

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Remember that standard output is data stream number one.

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And we could also redirect standard error to another place called error text.

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And we could make sure that we've, we append both of them by making sure there's two arrows at both

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places.

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And then when we type something like Linux is amazing and then we cancel when we open up our file browser

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here, we will see that we now have two files and output.

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TXT output txt contains Linux as amazing and standard error.

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The error text doesn't contain anything because there were actually no errors from this output.

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It was a perfectly valid command, there were no errors required, but that is how you would go about

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doing it.

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So this is how we would be redirecting standard output to output tcd.

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We can tell that because the number one is here and standard error to it file called error dot txt.

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Now we can see what the number and the arrows that standard error would be appended and so would output

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txt.

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Now we actually remember you actually don't need the one output at txt, so you could actually just

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do get rid of that and do the say.

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Linux is very amazing and when I look at that, we're going to see output at TXT has had its text appended

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and again there will still be no errors in error txt because there weren't any errors.

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But you can see here that without using the number one, this will still work, but you do still need

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the number two for standard error because that's shortcut isn't available.

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So you can only get rid of the number one for the standard output data stream.

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Okay.

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Brilliant.

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So you now understand how to redirect standard output and standard error.

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So how about standard input?

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Well, actually, that's relatively straightforward.

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At the moment, the cat command is looking for standard input from the keyboard.

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But what if we told it to get standard input from a file instead?

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So let's make a file called input text and type in there the words Hello world.

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Okay, so what we're going to do is we're going to use the cat command and we're going to output that

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now to a file called input text.

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Okay.

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So we're going to make a file we're going to put in there Hello World.

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Now when I cancel that, we have now effectively created a file called input txt.

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And in there we have hello world.

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Okay, so you can see how you can use redirection to create files like this as well.

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Now let's use now let's try and read from that file using the cat command again.

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How can we make the contents of this file, this input text file be read into standard input for the

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cat command so that we don't have to type it later on, let's say.

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Well, the standard input stream has the number of zero.

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So what we can do is we can type cat then the number zero and instead of the greater than sine, which

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is like an output, you actually use the less than sine we say then input, text or as a shortcut because

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the standard input stream is the only way that you can actually input.

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You don't even need the zero, you just put the less than sign right there.

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Now when we do this, we see the hello world message pop up on the screen.

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Now, this is because Cat has read the file from standard input and outputted it to standard output,

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which is by default connected to our terminal.

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Now here's a mini challenge.

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Okay?

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I want you to get CAT to read standard input from input text and redirect that output to hello text

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and make it so that that happens.

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Okay.

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So go ahead and give that a go.

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Pause the video now and try to get CAT to read standard input from input text and redirect the output

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to hello text.

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Then come back to see how you did.

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Did you manage?

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It was, here's how I would go about it.

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I would first redirect cat's standard input so that it comes from input text.

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So we're just going to do like that.

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And then I would output it to a file called Hello Dot Text.

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Now to make it easier to read, I might add the stream numbers.

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So I might say, okay, I'm going to redirect into stream zero input text and I'm going to redirect

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from stream one, which is standard output to hello text.

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It's totally up to you which one you use.

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This might be a way you do it.

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It might not be.

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But just remember that zero is standard input, one is standard output and two is standard error.

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Okay.

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And here we can see that we're reading input text into standard input.

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It's quite easy to see that quite, quite graphically.

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And we're writing from standard output into hello text.

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Okay.

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So if we go ahead and do that now, we should be able to see that in fact, yes, we now have a file

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called Hello Text and when we open it we see hello world.

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Isn't that amazing?

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That's how you can redirect data all around your computer.

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Now, here's one more mind blowing thing.

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Everything in Linux is actually treated the same as a file, even terminals.

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So let me open up another terminal.

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I'm going to use the shortcut control Orton T and I'm going to open them right next to one another.

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So I'm going to put one over here and I'm going to put one over here.

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Okay.

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Now in the other terminal, this one over this new one on the right, I'm going to run the t y command.

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Now, the t command will actually tell us where this terminal is located, where on the file system

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it is.

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And we see it comes back with slash dev, slash RTS, slash one.

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Okay.

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Now what I'm going to do is I'm going to redirect the, the, the content from input text.

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So just like this and instead of writing it to a file, I'm going to write it to this location, slash

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dev, slash RTS, slash one.

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So slash dev, slash RTS, slash one.

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And when I press enter.

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Oh my goodness.

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We can see that the data has been passed from one terminal to another.

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It's kind of like a really rubbish instant messaging system right now.

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Well, this isn't particularly useful in itself.

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It shows how redirection can be used to pass data around your entire computer and even over computer

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networks.

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I've heard.

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So that's pretty amazing.

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So how awesome you now know everything you need to know about redirection in Linux and the beauty of

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the Linux system should begin to be making itself clear.

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Now, as a quick recap, you learn that standard input, standard output and standard error are streams

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of data and like normal water streams, their flow comes from somewhere and goes to somewhere.

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Redirection is all about controlling that, where using redirection you can control where standard output

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and standard error streams go and you can control where standard input comes from.

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You also learn that these data streams have numbers associated with them.

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Standard input is stream number zero.

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Standard output is stream number one and standard error is stream number two.

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You then learned about the different symbols that can be used for redirection.

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They are basically the greater than and less than signs.

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Just remember that if you're redirecting output, one greater than sign means that the file will be

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erased or truncated before you write to it.

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So if you don't want to lose the data that's in there, make sure to use to greater than science to

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append data to the file.

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In other words, add data to what's already there.

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Now there are a few more cool things that you can do with redirection, but they're less common than

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what you've learned in these last few videos.

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So if you're curious, I've put a link where you can learn more about redirection in the lecture resources.

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But one question that must be burning in your head right now is How can I redirect the output from one

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command to the input of another?

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Surely there must be a way.

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Well, that process is known as piping, and it is, in my opinion, that that's the crown jewel of

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the Linux command line.

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Piping is probably the most important thing that the command line lets you do because it takes all of

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the commands that do one thing very, very, very well and allows you to tie them together seamlessly

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to do basically anything that you want.

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Mastering piping will definitely give you superpowers.

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So to master the power of piping and gain super powers.

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I'll see you in the next video.