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Before we can cover redirecting standard error we need to talk about file descriptors a file descriptor

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is simply a number that represents an open file for us humans it's easier for us to reference files

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by name but it's easier for computers to reference them by number by default.

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Every new process starts with three open file descriptors.

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They are file descriptors zero which is standard input file descriptor one which is standard output

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and file descriptor 2 which is standard error.

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You may be thinking hold on by default standard input comes from my keyboard but my keyboard isn't a

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file and by default standard output and standard error are displayed to my screen but my screen isn't

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a file either.

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On one level that's true but Linux represents practically everything as a file.

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Perhaps a more accurate description of a file descriptor is that it's a way that a program interacts

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with files or to other resources that work like files.

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These other resources include devices such as keyboards terminals and so on.

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This abstraction of treating almost everything like a file allows you to do some really powerful things

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like take the standard output of one command that would normally be displayed to your screen and use

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it as input to another command.

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Of course I'm talking about pipes in this case so file descriptors are like pointers to sources of data

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or places that data can be written things like keyboards file screens and so on.

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OK back to I O so far in this lesson we've been using the implicit way of redirecting input and output.

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For example when we redirect standard input into a command file descriptor zero is assumed.

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Let's look at the contents of a file on our system.

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Now let's use that file as standard input let's say to the read command.

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So we're going to read into the variable x the contents of Etsy Santos release.

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Now if we echo dollar sign X we'll see the contents of that file.

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So sure enough that input redirection work.

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And that's the implicit way to do it.

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If you want to be explicit specify the file descriptor number right before the redirection operator.

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So the previous redirection is the same thing is this read x 0 less than sign Etsy Santos release and

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hit enter.

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And if we echo X sure enough it does the same thing.

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The reason it's the same is because if you don't supply a file descriptor then zero is assumed for standard

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input.

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Notice that there is no space between the file descriptor zero.

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In this case and the redirection operator the less than sine in this case this is very very important.

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If you leave a space then what you thought was a file descriptor number that you were specifying ends

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up being an argument to the command before the redirection operator so quickly.

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This is wrong.

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Read x 0 with a space when you intend to do redirection

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and we should see an error.

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And sure enough 0 is not a valid identifier that is actually coming from the read command it thinks

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we specified read x 0 then let's give it some bogus data here and then hit enter.

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And sure enough we get the same situation.

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0 is not a valid identifier.

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However this is correct read x 0 with outer space less than sine Etsy Santos release.

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Now if we look at the value of x sure enough it's assigned to the contents of that file.

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Let's be explicit with standard output redirection.

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First let's do this.

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Echo you I.D. to the file.

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Let's call it you I.D. the contents of that file.

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Sure enough 1000 that's her you.

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No big deal.

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So that was the implicit way.

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If no file descriptor is given.

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Then file descriptor 1 is assumed when using the greater than symbol the previous command is the exact

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same thing as this.

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If we specify a 1 as a file descriptor right before the greater than sign it enter and if we look at

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the contents of that file it's the same thing because those two forms of redirection are the same.

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Again do not use a space when supplying a file descriptor.

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Let's see what happens when you do that with the Echo command here.

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So we'll do your I.D. and then 1 and specify space.

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Now this is the wrong way to do this.

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Just want to make that clear.

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Now let's use the greater than symbol to the UI file.

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So now if we catch the contents of your I.D. we see we get one thousand which is what the UI variable

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expands to and 1 because one was treated as an argument to echo.

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So if we do this without redirection we'll see that that is what is displayed to our screen by convention

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normal program output is sent to standard output while error messages are sent to standard error when

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you're working at the command prompt you may not even notice the difference because again by default

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they are both displayed to the screen.

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Let's make a program generate an error.

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First let's get some more information about the head command.

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If we read the synopsis we see that file is in brackets and there's also three periods or an ellipsis

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after that which means we can supply multiple files to the head command and here's what that looks like.

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We'll just do head dash in one to read the first line of the Etsy password file and we'll also read

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the first line of the Etsy hosts file.

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So when we do this head tells us which follows displaying what lines from shows the password file the

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first line from that then it shows Etsy hosts file and the first line from that.

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Now let's use a file that doesn't exist.

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So we'll supply something we'll name it fake file and hit enter.

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So as says had cannot open forward slash fake file for reading there's no such file a directory even

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though you can't tell by looking at the output there is both standard output and standard error generated

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here.

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Let's redirect standard output to a file run the same command the greater than sign.

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We'll call this head dot out since standard error was not redirected.

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It was sent to the screen and of course that error message is that file cannot be found and the normal

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standard output is saved into this file red dot out.

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Let's redirect standard error and the way to do that is to specify its file descriptor of 2.

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So we use the same command head dash in one Etsy password.

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It's a host this file that doesn't exist and then specify a file descriptor too without any spaces.

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We'll use the greater than sign and then give a file to redirect this standard error to.

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We'll just call this head dot e r r.

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So now what comes to our screen is the standard output while the standard error goes to the file.

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So let's look at the file.

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Okay.

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That's where the error message went.

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Let's go ahead and get rid of these files for a quick head dot out and head Dot.

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E r r we can even get real fancy here and redirect standard output to one file while redirecting standard

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error to another.

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So let's do this head dash in one Etsy password.

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Etsy hosts some file that doesn't exist.

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We'll redirect standard output to head out and standard error to head our art.

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OK no output was displayed to the screen.

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Let's look at the standard output because it's in this file and now we can look at the standard error

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because it's saved in this other file.

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You can probably see that this could be helpful in some situations.

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If you were just concerned about errors that were being generated well you could redirect all the error

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messages coming from standard error into a file for example by the way appending output is probably

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what you would expect using to greater than signs assume standard out.

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But even when you use a specific file descriptor such as file descriptor 2 for standard error it still

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opens to the file.

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So let's execute our command again here.

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What we're going to change is adding the greater than symbol so we have to greater than greater than.

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And then a path to a file so we'll hit enter and let's do this again will hit enter a couple other times

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here and then catch the contents of head dot E R.

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And sure enough we have several repetitions of the error message because we repeated that file multiple

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times using append instead of overwrite what if you want to send standard output and standard error

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to the same place.

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There are a couple of different ways to accomplish this.

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I'm going to show you the older syntax first and then the new syntax right after that.

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So again we'll just use our example command here.

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We're going to send the output to head up both to greater than Ampersand 1 and hit enter.

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So if I can head both you're going to see the standard output and the standard error both in that one

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file.

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So why does that work.

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Well first off you understand that greater than sine head both is a redirecting standard output to the

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head that both file.

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That's still the same a number that comes directly before a redirect operator is a file descriptor so

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to greater than means redirect standard error.

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The new piece is the ampersand symbol normally with the redirection of file follows the redirection

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operator.

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However if you want to use a file descriptor instead of a file name use the ampersand symbol.

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So instead of redirecting standard error to a file it is being redirected to standard input.

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If you were to omit the ampersand then one would be treated as a file named one.

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So the command you see here on your screen head dash in one NC password and he hosts fake file greater

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than header both to greater than Ampersand one sends the standard output of head to the file name head

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both and append the standard error to the standard output since Standard Error is redirected to standard

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output and standard output is redirected to head that both all output will be written to head both.

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I'd like to point out here that spacing is important as well.

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Do not use a space after the redirection operator and the ampersand.

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OK let's get rid of this file head dot both.

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If that seems a little confusing and hard to follow or too complicated.

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No problem.

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You're not alone.

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Since this is a common practice new syntax was added to bash to redirect both standard input and standard

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output that syntax is Ampersand greater than and then a path to a file.

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So let's do our same command here had dash one couple of files that actually exist and one that doesn't.

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And then we're going to use this new syntax Ampersand greater than sine and there's no spaces there

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and then provide a path to a file and that path is just going to be our current directory at head dot

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both.

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That's much prettier and probably easier to understand in my opinion.

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And if we look at head both.

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Sure enough the standard output and standard error both end up there.

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Again using double greater than signs appends to the file so let's prove that real quick.

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We'll just do this.

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Adding other greater than sign here so it's Ampersand greater than greater than that.

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Enter.

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And now if we cat had both we'll see two sections of output because we've ran the command twice and

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the second time we appended to head.

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Both

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if you think back to when you first started using pipes you learned that a pipe takes the standard output

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of one command and uses it as standard input for another command.

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This means that standard error doesn't flow through the pipe.

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Let's demonstrate this first.

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Let's take a look at the cat man page

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want to point out this dash in option that numbers.

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All output lines so if we go back to our head command here without any redirection it looks like it

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specifies six lines.

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That's the password routes a blank line as he hosts one that starts with a 127 and then heads 1 2 3

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4 5 6.

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However if we pipe this to cat and use the dash in option Kat only counts five lines.

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You see first standard error is displayed to the screen because it was not passed through the pipe.

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The standard output of the head command was passed through the pipe as the input to the cat command.

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The cat command numbered each file it received a standard input.

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It's important to know that about pipes that you're only getting the standard output going through the

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pipe.

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Now this could be exactly the way you want things to work when you're working with pipes or it might

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not be.

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If you want to force all the output of a command through the pipe then you need to append the standard

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error to the standard input.

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So one way we can do this is specifying a file descriptor of 2 which is for standard error and then

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redirect that into Ampersand 1 which represents file descriptor 1 which in turn represents a standard

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output.

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So now that standard error is going to standard output and all the standard output goes through the

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pipe then cat should count all of our lines let's hit enter and see if that happens.

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Sure enough it counts all six of our lines.

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Now there's a shorthand for this.

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It's pipe ampersand.

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So instead of supplying all this food supplied pipe Ampersand it combines the standard error and standard

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output into one and then passes that through the pipe.

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So we get the same result.

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OK.

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We've covered a lot here on the command line.

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Let's get back to our script and really do a mini recap of what we've been talking about and let's documented

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in our script so we have something to look back to if we want to later.

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So I'm going to edit our file and we're just going to be really repeating some of the examples above.

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But this time using file descriptors

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so we'll redirect standard into to a program using file descriptor 0

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we'll just display this back

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and now what we'll do is redirect standard out to a file using file descriptor one over writing that

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file.

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So let's do head dash in three let's see C password will be explicit here in use one and then give the

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path to the file

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let's just stop right here and say make changes and execute our script and see what happens so far more

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or less we get a repeat of what happens at the top of the script.

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We get line containing the first line of data.

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Now we get the contents of temp data containing three lines as well.

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Even though we use the explicit form of redirection by supplying file descriptors.

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So again it works the exact same way.

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Implicit or explicit.

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Let's go ahead and continue editing our script here

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by the way a standard error is abbreviated STV E R and standard error is a file descriptor too.

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So we're going to use this error file a couple of times.

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Let's create a new variable as well as a new fall for this error file r e r r file.

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Let's give this temp data dot e r r

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will give this file something that's going to generate an era file that doesn't exist.

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We'll redirect that error message into the error file.

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OK.

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Let's save our changes execute our script and we get the standard output to our screen.

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Let's see what the contents of this temp data e r file are.

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Sure enough that's the error message that was generated by the head command that we redirected into

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that file so that's doing exactly what we specified.

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Let's get back into our script and keep on going here.

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Now let's redirect the standard out and the standard error to a file we'll use our same head command

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here.

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Only this time we're going to use the new syntax the ampersand greater than sign and send that to a

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file and now we'll just display the contents here.

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Can't see where changes and execute our script.

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And sure enough everything is displayed to our screen there.

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So the standard output as well a standard error were both combined.

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Now let's redirect standard output and standard error through a pipe.

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We'll print a blank line and then we'll use our head command here

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we'll use the new syntax if you will the latest syntax of pipe Ampersand and we'll pipe that to the

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cat command with the dash in.

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That's going to number the standard input that cat receives.

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So we'll see where changes and execute our script.

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And sure enough the error message of cannot open fake file gets passed through the pipe to cat and gets

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numbered.

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So we've appended standard error to standard output.

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Let's do the opposite which is to append standard output to standard error.

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So if we take something like the echo command and just echo the word error pipe that into cat with a

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dash in option it just shows one line because one line came through a standard input into cat.

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So now let's do the same thing here.

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Echo Error.

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But this time let's use the redirect symbol.

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The ampersand which specifies a file descriptor and we'll use file descriptor too and pipe that to cat.

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Now remember that the ampersand that follows the redirection operator is used when you want to use a

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file descriptor instead of a file name.

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And to be super clear this is exactly the same as this one greater than Ampersand 2.

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Now the reason cat didn't put a line number in front of error is because the output of error never got

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to cat because it was redirected to standard error so echo Error generated standard error.

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So we forced echo to really generate standard error.

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Now why would you ever want to do this mainly so that you can make your scripts conform to the standard

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convention of sending error messages to standard error.

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If you find yourself writing an exit command in a script and echoing some information just before that.

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That's a really good sign that you need to send the output of those echo commands to standard error.

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So let's put a demonstration of how to send output to standard error.

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We'll be using this in a future script of our so we'll do echo.

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This is standard error and then do this greater than Ampersand to write or changes and execute our script

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now let's execute our script and redirect standard error to a file that we'll call ya.

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So now if we can't e are the standard error generated by our script is in that file before we wrap up

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this lesson I want to cover the null device that no device has a special file that throws away whatever

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is sent to it.

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Some people call this the bit bucket so if you don't want to see output on your screen and you don't

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want to save that output to a file either then redirect that output to the null device which is located

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at Forward slash Dev forward slash no.

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So here's an example.

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Had dash in one that's a password as he hosts fake file we'll just run this command and remind ourselves

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what this does.

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Now let's discard the standard output so we'll just send the standard output to Dev no.

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So also we're left with this standard error.

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Now we can do the same thing here and just throw away the standard error by doing to greater then and

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then the path to the node device.

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OK.

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Now we don't see any error messages.

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Now let's throw away all the output generated by the command and we know how to do that because we can

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use this Ampersand greater than sign to combined standard output and standard error and we'll redirect

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that to death.

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No.

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So no output is generated.

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You might wonder when you would want to do such a thing well.

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Here is the use case if you're executing a command in your script and you don't want the user running

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the script to see that output then send it to DAB.

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No.

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And remember if you need to know that a command succeeded or not simply check its exit status.

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You don't need the output to determine if it succeeded or failed.

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So we can do this echo dollar sign question mark and we get an exit status of one which is non-zero

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so something went wrong but if we make the command succeed for example take away that file that doesn't

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exist and then we check the return status.

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We get an exit status zero so we know that the command succeeded.

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Now let's finish up our little script here by adding these examples.

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Let's discard standard out

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and then let's discard standard error

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so we'll soon follow the too to death.

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No.

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And finally we'll just do an example here of discarding both a standard out and standard error

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we'll put a message here to try to show that we're doing that.

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And one thing I like to do is clean up after myself.

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I don't like to leave temporary files just laying around so I remember that we created two files that

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we were going to remove here will remove file and then we also created an error file that we can remove.

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And by the way this would be a good place to send to Dev.

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No.

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So if for some reason the arm command fails maybe we really don't care because it's a temporary file.

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We can just throw that out put away if we want to.

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So let's do that.

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As an example in this case.

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OK.

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We're almost done here we'll just exit on this file and then execute it and see what happens.

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So here's the example where we discarded standard output and we're only left with the error message.

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We did the reverse by discarding standard error so that only left the standard output.

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And then finally we discarded both standard out and standard error.

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And of course nothing was displayed to the screen in this lesson you learned a lot about input and output

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redirection.

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You learn that each new process starts with three open file descriptors.

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They are file descriptor 0 for standard input file descriptor 1 for standard output and file descriptor

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2 for standard error.

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You learn that the greater than signed redirects standard output to a file and that it creates the file

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if it doesn't exist or if that file does exist it truncate that file overwriting it.

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You learn that you have to use double greater than signs to append to a file without overwriting it.

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If no file descriptor is specified for output redirection file descriptor one is assumed that's why

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you had to specify the file descriptor to to redirect standard error.

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You learn that you can redirect both standard input and standard output to a file by using the ampersand

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greater than sine syntax.

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If you want to redirect both standard input and standard output through a pipe use the pipe Ampersand

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syntax.

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From there you moved on to redirecting standard input from a file into a command by using the less than

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sine.

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Finally you learned how to use the null device by redirecting output to Dev.

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No.

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I know we covered a lot in this lesson.

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If this is your first time being exposed to input and output redirection I would actually encourage

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you to watch this video again.

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Let it soak in for now and maybe come back to it tomorrow or in a day or two.