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In this lesson you're going to learn about the different types of input and output and how to redirect

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and control those types of input and output.

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So as always I have a local terminal up on my machine and I'm going to move into our working folder

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for this class and into the project folder.

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Next I'm going to bring up our virtual machine

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now that the virtual machine is up I'm going to connect to it from here I'm going to move into the shared

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folder of forward slash vagrant.

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The name I'm going to use for our demo script today is user dash demo 0 8.

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SH I'm going to add the shebang here and give a path to the bash executable and I'll just add a short

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header to our file

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so far on this course.

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The only type of input and output redirection that you used has been in the form of a pipe you've already

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written scripts that redirect the standard output from one command and send it as standard input into

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another.

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By separating those commands with the pipe you've used that method to string together several commands

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in order to generate a random password for example you also used to send that random password to the

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password command.

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Let's take a step back and talk about the different types of input and output.

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First off there are three types of IO.

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They are standard input standard output and standard error by default standard input comes from the

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keyboard.

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For example the read command accept standard input and you use the read command to collect information

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from a user typing something in at their keyboard.

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You also know however that standard input doesn't have to come from a keyboard.

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Again you use the pipes to take the output generated by one command and use that as the standard input

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for another command.

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Beyond standard input there is standard output and standard error.

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But if both standard output and standard error are displayed to the screen.

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Let's start out by redirecting the standard output of a command into a file and you can do that by using

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the greater than sign.

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By the way I often see Standard out abbreviated as STV out we're going to be using a file several times

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throughout the script so I'm just going to assign that file name to a variable.

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I'm going to call that variable file and then I'm going to say that that file is going to live and forward

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slash temp and we'll call the name of that file data.

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Let's use a command here that we've already used in a previous exercise.

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We'll use the head command dash and one will output the very first line of a file and we'll just look

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at the first line of the ETSI password file.

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And also the output of this command the standard output of this command and to the file.

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Now remember by default standard output is displayed to the screen.

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But since we're using the greater than symbol after the command it is redirecting that output away from

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the screen and into the file we specified after the greater than sign.

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So let's see what happens when we actually execute this script.

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So I'm going to save my changes.

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Make sure that this is executable

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and then execute the script.

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As you can see no output was generated by our script and that's pretty much to be expected because we

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told the output of the head command that's in the script to go to a file.

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Now let's look at that file.

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Ok that seems to be the output of head dash in one ETSI password and we can actually confirm this by

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just running the command here on the command line and sure enough that is the output of the command

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that's in our script and that output was not displayed to the screen but instead went to the file where

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we redirected that output to be super clear this output redirection is not part of any one command.

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It works with all commands.

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While we're on the command line let's try it out with let's say the ID command.

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So we'll use ID dash you when you use this in a previous project and we'll send the standard output

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to a file that will decide to call ID.

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Again this file name could be anything.

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So no output goes to the screen.

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However if we look at the contents of the ID file with the cat command we see the output that would

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be generated by that command.

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Again we can do this with any command.

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Let's use echo

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if we can to you I.D. file Sure enough the output of the equil command is in the file that we redirected

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that output to.

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By the way file permissions are in play here too.

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So if you try to redirect output into a file where you don't have write permissions you're going to

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get an error.

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So let's back up here with our echo command and let's try to write this and say forward slash you ID

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and hit enter.

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And sure enough we get a permission denied error.

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Let's look at the permissions on forward slash.

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There's set 2 5 5 5 or read execute read execute execute with the owner of route and the group of Route

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Well we're vagrant and we're in the group of vagrants So we have no right permissions so we can't create

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files there.

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We can't do that either through redirection or using an editor or any other means we just simply don't

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have permissions.

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I just wanted to point out that if you get a permission denied error when you're working with redirection

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take it at face value.

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You don't have permissions there.

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Kate let's jump back into our script.

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Ok now we know that the greater than sign redirects standard output while to redirect standard input

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use the less than sign

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against standard input is often abbreviated as TV.

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I meant let's use an example with the read command will do read line do an input redirection and then

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supply a file and we'll just use the same file that we created with the head command.

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Now you use the read command to accept standard input from a user by using imper redirection you are

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now accepting standard input from a file.

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The command reads one line of standard input.

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When someone is typing at the keyboard and they hit enter that represents one line of standard input.

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Of course when dealing with files a newline character will cause read to stop.

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So the variable line should contain the first line of the file.

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In this example there is only one line in the file.

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Let's prove that by echoing it to the screen will say that the line variable contains the contents of

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the line variable here and hit enter.

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Let's save our changes and execute the script.

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Stepping through this short script we used output redirection to send the first line of the ETSI password

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file into a file named Ford slash temp forge slash data.

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That file was then used as standard input to the read command which assigned the value to the line variable.

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Finally the line variable was displayed to standard output using the echo command.

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And that gives you what you see on your screen here redirecting standard input from a file works with

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any command that accepts standard input.

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Take the password command as an example.

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Let's put some text into a file and we can do that with the echo command will just echo out some information

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here.

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Normally that would be displayed to our screen but we're going to use output redirection to put that

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output in a file called P-A s w o r d.

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So we'll look at the contents of that file sure enough that contains the word secret.

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Now that use that file a standard input to the password command.

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So we need root privileges to do this we're going to use Sea-Doo password with a standard option and

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we need to supply a user while I have the Einstein user set up on this system.

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So I'm going to use that user and then I'm going to redirect in the standard input that lives in the

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file password.

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So we're going to redirect the contents of the file name password as standard input into the P.A. accessed

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at the command.

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It says the passwords were changed for the user Einstein and I can verify this by just switching users

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and I'm going 200 s e c r t and hit enter.

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And sure enough it accepts the password so that works.

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So the first method you used was with a pipe but now you can also use this input redirection with the

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less than sign and remember the difference between those methods is that in a pipe the standard input

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comes from the output of a command.

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Whereas with this less than symbol the standard input comes from a file.

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So that's the difference.

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You use a pipe when you want to take output from a command as input into another command but you can

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use this redirection when you want to redirect the contents of a file into a command.

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It's important to know that using a single grader inside to perform a redirection creates a new file.

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If that file doesn't exist or it overwrites the contents of that file if it does exist.

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So right now the password file contains the word secret.

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If we perform a redirection to that file it will get overridden.

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So we'll do this will just echo the word new and to the password file.

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So now when we look at the contents of that file it contains new notes.

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Demonstrate this in our script.

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So now we're going to take the first three lines of the ETSI password file and send that into this file.

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Going to echo a blank line to the screen and then now show the contents of that file.

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Hey let's save our changes and execute the script.

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The first time we performed a redirection to the script we took the first line at the ETSI password

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file and put it into a while.

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The next time we perform a redirection to that file we took the first three lines of the password file

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and wrote it to that file.

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So here you can see that the contents of temp data contained three lines not four lines it didn't append

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to it.

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It Over wrote the contents of that file.

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So what if you don't want to overwrite a file but you want to add or append to it.

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In that case you use double.

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Greater than symbols.

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So let's look at the contents of our password file here in our current directory.

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It contains just one line and that line says new.

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Now let's add another line to the file.

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We'll do this echo another line and then we'll use two greater than symbols.

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And by the way they can have a space between them they should be together.

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And we're going to provide the name of that file that we're going to redirect this output into the name

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of that Folles ph SS w o r d.

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So now when we look at the contents of this file we should see two lines.

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And sure enough we do the original contents which was new and then the new contents which is another

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dash line.

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Let's keep adding to this file by using one of our password generation techniques.

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We'll just take the current date take the output of that as input to the Shah 256 some command and then

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we'll just take the first 10 characters that that command generates and then we'll appended to the P

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S S W O R D file.

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Now the file contains that extra data.

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Now I think it's pretty darn awesome how you can combine different types of input and output redirection

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all on the same command line.

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So here we use pipes and output redirection.

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Let's set this as an example of IO redirection to our script.

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So now we're going to redirect the standard out to a file appending to that file.

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So let's just echo a couple of random numbers in there.

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And then we'll just repeat this line.

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So I missed a dollar sign in front of this variable and went to place it there.

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Now I'll just add a blank line and then echo the contents of that file.

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Let's save our changes and execute the script.

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OK so originally the contents of temp data was one line.

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It was overwritten by three lines.

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And then we performed and append the output redirection.

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And so now that has 5 lines you can see the random numbers that we generated and appended to the file.

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So we've covered redirecting standard input and standard output.