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In a previous lesson we use the if statement in a positive form meaning if this expression is true then

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do something in this lesson we're going to use the logic of if this expression is not true then do something

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you're also going to learn how to determine if a given command failed or succeeded by checking the exit

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status of the command as well as how to control the exit status of your scripts.

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You can follow along with me right now if you'd like or you can wait for the practice exercise where

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you'll get to apply all the concepts you've learned so far.

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The first thing I'm going to do is open up a command line section of my local machine and move into

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our working folder.

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I'm here I'm going to go into the project folder that we're working on which is local users.

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Now I'm going to start the virtual machine and connect to it.

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Now that I'm inside the virtual machine I'm going to C.D into the shared directory of slash vagrant

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and I'm going to start working on this script for this lesson which is tell users which stands for a

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local user demo.

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Oh three got sh horse.

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We start out our shell scripts the exact same way every time.

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We supply the interpreter to be used on the shebang line here.

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Now we're going to specify a goal for our script.

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So this script is just going to display the ID and user name of the user that's executing the script.

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And we're also going to display to the screen if the user is the vagrant user or not.

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So we're going to test for the vagrant user.

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I'm going to go ahead and put the steps in as comments and then fill in the code as we go.

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So the first step we want to do is display the new ID.

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The next thing we're going to do is only display a message if the ID does not match 1000.

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From there we're just going to display the username then we're going to test if the username command

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succeeded from them and demonstrate how you can use a string tests conditional.

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Next we're going to test for not equal or the string.

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So let's go back to the top of our script here and start filling in some code.

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So the first thing I want to do is display the ID as you already know the U.

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ID is a shell variable that is set by default.

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So we can just display that we're going to use double quotes because we're going to have a variable

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in here that we want to get expanded

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Of course when we access a variable we access it with a dollar sign in opening curly brace the variable

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name and a closing curly brace it let's just write and quit our changes here and then execute the script

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and see what happens.

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Remember the script has to have an executable bitset on it before it can be executed so we're going

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to chmod the script.

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Now we're going to execute it with Dot Ford slash l user demo 0 3.

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SH K and displays r you ID which is 1000.

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Now let's get back to the script and continue editing our script here.

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Now what we want to do is only display a message if the ID does not match 1000.

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So we're going to have a variable called you ID to test for.

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And we're going to set that to one thousand by the way there is this concept called dry which stands

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for.

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Don't repeat yourself.

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We're going to apply that concept in this script through the use of a variable.

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We're going to reference this you id more than once so instead of hard coding that you ID in multiple

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places in the script you're going to specify that value in one place.

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If you ever want to change that you ID you only have to change it in one place and not go searching

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all throughout the entire script and change it in multiple places in a small script like we're writing

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here it might not look like that such a big deal but when you create larger more complex scripts you're

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going to be glad that you followed the dry principle.

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OK so let's test for this you idea with an if statement.

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We remember that we use the F word followed by a space double opening brackets followed by a space.

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We're going to look at the ID and we're going to see if it's actually not equal to the idea that we're

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going to test for

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if the ID is not equal to the ID to test for then we're going to display your ID does not match you

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ID that we're testing for.

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Let's say we don't want the user to go any farther unless they have the ID of 1000 in order to stop

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the execution of the script.

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We're going to use the exit command by the way we can supply an exit status after the exit command.

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We're going to supply one here.

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The reason why we're going to specify one is because this script does not successfully execute all the

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way to the bottom as it should.

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So we're going to want to exit with a non zero status.

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So by convention when a script or a program execute successfully it returns an exit status of 0 if it

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doesn't execute successfully for whatever reason it returns a non zero exit status.

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A lot of times that exit status is one.

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However it could be two.

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It could be 99 it could be 244.

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You can use a variety of different exit statuses if you want.

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Let me close out this statement I'll show you an example.

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Let me write and quit real quick.

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So let's look at a command that we're going to use We're going to be using the user add command to add

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users.

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So let's take a look at its man page.

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I'm going to search for exit status by Ford slash exit and press enter.

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So here it's called them exit values the user add command exits with the following values 0 6 success.

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You can actually test for these exit statuses and we'll get to that in just a minute.

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So for example exit status of nine means the user name is already in use.

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So if you're writing a script and you run the user add command and you get an exit status of nine then

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you can know that that user name is in use and it's not for some other reason such as an invalid command

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syntax which would be an error status of two.

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So again we're going to use the exit show built in.

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And let's get some help about that.

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Let's see what help is available for exit.

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So as you can see you can use exit without any options because the in there is in brackets and that

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means it's optional.

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So you can just type exit and exits the shell if you want to supply an exit status you just supply it

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to exit exit 1 for example like we're using in our script.

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If you omit that status then the exit status is that of the last command executed.

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Sometimes that's what you want and sometimes that's not what you want.

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So if you want to be explicit then specify a exit status following the exit shell built in.

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Let's get back to our script real quick here.

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So now we've gone over the exit status.

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So let's review where this dash in e came from.

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So if your ID is dash any which means not equal to the ID to test for then we're going to execute this

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code.

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Hopefully you remember that you can get information about these tests by running help test.

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And here we're doing a arithmetic test means we're testing numbers here we have dash Q Which corresponds

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to equal dashi and he is not equal Dasch LTE is less than Nashe L E is less than or equal G is greater

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than and dash G is greater than or equal to.

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So we're using dash any do tests for not equal.

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Now let's go ahead and executer script and see what happens.

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Says your ID is 1000 and no messages were displayed to the screen because you I.D. does not equal 1000

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is actually false.

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Now let's see what happens when we execute the script with the different user ID.

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Well one way to do that is to execute it with Sulu privileges and that will run the script with root

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user privileges.

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And as we know the root user always has the UI ID of 0.

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So this time you get your you ID is zero your ID does not match 1000.

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So the if statement evaluates to if 0 and not equal 1000 then do something which has to echo your ID

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does not match 1000 OK continue working on the script.

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Now let's display the username

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as you probably remember the dollar sign parentheses syntax is used to capture the output of a command.

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And we're going to send the output of that command and store it into the variable user underscore name.

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Now what we want to do is test if that command succeeded and how we do this of course is with an if

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statement.

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So Bash has some special variables that it sets.

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And one of these special variables is a dollar sign questionmark.

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And of course as you know you can encapsulate that and curly braces so dollar sign opening Crilley race

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questionmark closing curly brace is actually a special variable that holds the exit status of the most

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recently executed command.

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So the most recently executed command in our script is ID dash you in.

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So the exit status of that script is going to be stored in dollar question mark.

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Now we can use dollar question mark to do some comparisons here.

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Make a decision.

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So if dollar question mark does not equal zero then what we want to do is tell the person that executing

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this script that the ID command did not execute successfully and we're going to cause our script to

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exit as well as to exit 1 and then close our if statement if we get past this if statement that means

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the ID command did successfully complete.

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So we can display their username.

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So let's execute this script on the command line and also do some demonstrations where the dollar question

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marks special variable.

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So now the output of the script is your you ideas 1000 your user name is vagrant.

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And as you remember the username is a vagrant echo command was directly after the if statement.

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So the if statement did not execute.

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So that means the ID command actually executed properly.

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So let's test this out let's do id dash you in which we know is a good command to get the user name

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of the current user.

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Again the special variable dollar questionmark contains the exit code.

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Let's look at it.

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So Will do.

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Echo dollar question mark and the exit status of the ID command is zero.

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Now let's make the ID command fail in some way I happen to know that dash X is not a valid option to

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ID so let's run that command and see what happens.

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So we get an invalid option dash x.

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So now let's look at the exit status which is stored in dollar question mark.

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So now you can see that one is the exit status of that command that failed.

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So again by convention it's a non zero exit status and this case is one.

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Again it could be two it could be three it could be one hundred thirty nine.

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In our user add example that we were looking at earlier.

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So again if we want to test for success we'll probably want to test if it does not equal 0 because that's

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the convention.

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Kate let's continue with our script.

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Instead of testing numbers let's do a test against the string.

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So let's have a user name to test for

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we'll use an IF statement to perform a test.

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So this reads if the user name equals the user name to test for if that turns out to be true then what

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we want to do is echo your user name matches.

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The user name that we're testing for

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it's important to point out that the equal sign can either be an assignment operator or a test operator

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depending on the context.

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If you look at the variable assignment user name to test for that equal sign is used as assignment where

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assigning the value of vagrant into the value of user name to test for.

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However when we're doing a test inside these double brackets then it's a test operator.

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It's not assigning user name to test for to user name it's comparing them it's testing them by the way

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sometimes you'll see a double equal sign and let me put this in the script to show you here.

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So sometimes you will see that when you use the double equal sign operator the string to the right of

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the operator is used as a pattern and pattern matching is performed.

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So we're not looking for a pattern we're looking for an exact match so we'll just use one equal sign.

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However some people even use the double equal signs when they don't use a different pattern if they're

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also using an exact match.

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So be aware that sometimes you'll see a single equal sign and sometimes you will see a double equal

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sign.

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We're going to stick with a single equal sign in this case.

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OK let's exit over script and see what happens when we execute it.

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Now

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those are you ideas.

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Thousand are you using name is vagrant and our user name matches Veiga.

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Well let's run this with Sue

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are you ID is 0.

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Your I.D. does not match 1000.

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And again if you remember we had an exit statement in our script and we exit it as we want.

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So let's check the exit status and see if that's true.

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And sure enough the exit status of our script is one.

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Hey let's get back to our script and finish it up.

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Now let's test for inequality using an if statement with a string.

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So we'll do if double brackets

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username is not equal to user name to test for.

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Then we're going to echo your user name does not match user name to test for and then we're going to

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exit with one and we're going to end the statement with ephi.

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And finally if we make it all the way to the end of the script we can use exit and we can be very explicit

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about the exit status that we want to use we'll use 0 to say our script completed successfully.

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Remember that if you don't specify an exit status or exit code the exit status of the most recently

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previously executed command will be used as the exit status of your script or as the exit status of

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the exit command.

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Hey that wraps it up for this script.

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Let's save our changes and execute at the command line.

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So you can see the output on the screen and let's check our exit status here.

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We have an exit status of 0 which means by convention that our script completed successfully.