Conditional Statements
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>> Hello Cybrarians and welcome back to
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the Linux plus course here at the Cybrary,
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I'm your instructor Rob Geelz.
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In today's lesson, we're going to be
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talking about conditional statements.
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Upon completion of this lesson,
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you're going to be able to explain
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the purpose of conditional statements,
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as well as differentiate between
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if statements and case statements,
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and then we'll see in our demo at the end of the lesson
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how we can use conditional statements in scripts.
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Conditional statements are sometimes
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known as logic statements,
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and they allow you to test
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>> for specific conditions like,
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>> does a file exist,
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does the user enter any input,
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is the input correct,
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and maybe we can just test
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the result of a mathematical expression.
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Now, based on the result of the test,
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the condition statement can
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branch and perform different actions,
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and we'll see that a little bit later in this lesson.
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Now, there are two types of
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conditional statements that are used in BASH,
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and that is the if statement and the case statement,
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and we're going to look at each statement type
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in depth in the coming slides.
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First of all, if statements.
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If statements use
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the conditional tests that we talked about in
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the last lesson to direct
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script execution based on exit status.
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What does that mean?
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Well, remember when we're doing expressions and
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we're testing things and trying to get the condition,
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we're trying to determine
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whether or not something evaluates to true,
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or zero, or false, or one.
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If we have a tested condition evaluates to true,
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then we want to run commands
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>> immediately under that test.
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>> If it evaluates to false,
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we want to do something else,
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and we can do two different something else's.
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We have two additional logic statements
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we can use in BASH.
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The first one is the l statement,
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and this is pretty cut and dry,
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if this is true,
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do this, if that is true,
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do something else, so if else.
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But we also have the concept of the elif statement,
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which allows you to test
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another condition, so we can say,
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if this is true, do this,
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if that is true,
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do this, else, do a third thing.
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Then in all cases,
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all if statements end with fi,
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which is just if backwards,
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but that's how you close an if statement.
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We can see an example of this on
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the bottom right-hand corner of the image here,
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and what this is doing,
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remember we have those square brackets,
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so what we're doing is we're doing a
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numerical condition test,
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and what is actually happening here is it's
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expanding out this variable user integer,
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and it's dividing that by two.
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Now, it's actually doing modular division,
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that's what that percent sign is there,
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and what that means is it's just trying to
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determine if there's any remainder.
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If we divide a number by two and we
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have no remainder, what does that tell us?
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It tells us that the number is even.
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So user integer divided by two with no remainder,
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or equal zero,
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then we can echo out the number is even.
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But if there is a remainder,
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then we know that the number is odd.
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If the number divided by two has no remainder,
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then echo the number is even else, because it's false,
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echo the number is odd,
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and then we finalize or close that with fi.
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Now, a case statement is
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just a more elegant conditional statement,
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but it is really useful when you have to test
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multiple conditions and perform
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a different action or command for each,
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it's just a little bit easier to read as well.
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Let's take an example of testing the day of the week.
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We could write a test statement to do this.
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We can say case day in,
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and then what will happen is,
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it'll expand that variable day
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and determine what day of the week it is,
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and based on what day of the week
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it is, it will display output.
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For example, if the key statement expands day,
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and it says that the day is Monday,
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that's going to echo that, I suggest
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a lot of coffee and so on and so forth.
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It's going to go down and test all of
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these conditions and determine what the day is.
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For example, if you get to Thursday,
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it'll say one day till Friday,
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or if Saturday or Sunday,
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it'll say echo is the weekend, enjoy yourself,
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and then just like we saw with the if statement,
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we close that case statement by spelling it backwards,
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so case spelled backwards is esac,
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and that's how we close a case statement.
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Let's take a look at some
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>> conditional tests we can do in
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>> a script with some demo time.
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Here we are in our demo environment,
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and today we're back here in Ubuntu,
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and what we're going to do is we're going to
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take a look at a new script.
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Now, I've already started
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creating the script a little bit,
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so let's open it up and take a look,
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this script will be called condition state data Sage,
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and you might be saying, wait,
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this script looks familiar, and it is,
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because what we're going to do here
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>> is we're going to use
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>> our existing script and do some work inside of here.
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What we're doing in this script is
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the same thing we were doing before when we
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were taking command line arguments
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and positional parameters,
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but what we're going to do now is we're actually going
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to add some tests here to
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determine whether or not
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>> we've actually gotten value from
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>> the user before we go forward and print anything.
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For example, what we might want to do here
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first and foremost is test and make
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sure that we've gotten content from
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the customer or the user,
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and get that information
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into that name variable that we read in.
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We could do that pretty easily. We could do if,
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and we're going to say -Z $name,
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and then what we'll do is if
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this does not have any content,
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because remember the -Z
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indicates that a string has a length of zero,
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so that means that nothing
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was passed into this variable,
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it's zero length, it has nothing in it,
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then what we'll do is we'll say,
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then echo, and we'll display a message to
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the screen that'll say, no name provided.
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We can do another conditional test.
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We can say elif,
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and we could say same thing.
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Let's test and make sure that the username is provided,
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so we could do dash Z username,
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and this will indicate whether or not we
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have any length to this username, if there's no length,
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then we want to do something else,
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and in this case, what we'll do
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is we'll do another echo,
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and we'll say that no username provided,
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please enter your username,
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and then finally we can close out
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our statement with a final else,
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so we just hit else here,
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and then what we could do is we
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could echo something else,
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because basically we're going to say, hey,
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if this does work,
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if these two tests passed,
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then we can run the same commands we
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did when we did this in our previous script.
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So we can say your full name is dollar sign name,
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and we could say that
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your home directory contains
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the following content,
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and then what we can do is actually use the username
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provided to display that content.
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Then finally, the last thing that we
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always have to do with any if statement
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is use fi to close out the statement.
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Let's go ahead and hit "Escape:wq" and see how we did.
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Now, if we run this command,
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we run condition state data Sage,
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is going to prompt us for the full name,
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is going to prompt us for the username,
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we're going to provide neither,
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and it's going to say no name provided
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because that's the first condition we test.
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Let's run this again,
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and this time we'll provide my name,
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but I'll provide no username, and I'll hit "Enter",
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and it'll say, no username provided,
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please enter your username.
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Let's do both now,
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say my full name is Rob Geelz,
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my user name is Rob,
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and we can see that it evaluates to
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the last condition and
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runs the script like we did previously,
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it prints the user name and it prints
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the user home directory and the content in it.
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That's really all we need to
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know about condition statements,
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so we can see that condition statements here and
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the format a little bit of using an if statement.
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With that, we've reached the end of this lesson.
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In this lesson we covered
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the purpose of conditional statements,
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we differentiated between if statements and case
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statements and determined when
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each one of those might be used,
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and then we used conditional statements
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in scripts during our demo.
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Thanks so much for being here and I look
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forward to seeing you in the next lesson.
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