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In this lesson you'll learn how to use the case statement.

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The case statement is the perfect way to make a decision based on the value of a given variable.

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This is especially true when you have a specific set of values that you're looking for.

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So first off I'm going to open up a terminal on my local machine and then go into our class a folder

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of shell class and from there I'm going to go into the local user's directory and now I'm going to start

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the virtual machine with vagrant up.

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Now that the machine is booted we'll connect with vagrant S-sh and from here we can move into our shared

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folder of Ford slash vagrant.

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So far in this course you've been using the if statement to perform tests and to make decisions in your

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scripts today you're going to learn how to make decisions using the case statement.

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Now before we start writing our script let's look at the case command itself.

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First off it's a shell built in.

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And we know this because we can type the type command type Dasch A and specify the command which is

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case and his case is a shell keyword which is a type of shell built in.

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Of course since it's a shell built in you know that we can get some information on how to use it with

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the help built and so will run help case.

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The first line shows us the syntax of how to use the case statement.

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It's all squeezed together on one line so it's a bit hard to read and I'm going to flush it out for

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you in this script I'm about to write so you can see the syntax very clearly.

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Also this output tells us that commands will be executed based on pattern matching.

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So what you do with the case statement is compare a value to a pattern and if the value matches that

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pattern the commands associated with that pattern or below that pattern get executed if the pattern

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isn't matched.

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Nothing gets executed.

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Again this will become pretty obvious with a couple of simple examples.

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So I'm going to name this script l user Dymo 0 9

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will start.

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All scripts like we do with the shebang.

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And then let's add a comment here about what this script is going to do.

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Let's start with something you already know which is the if statement by the way using the if statement

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in the way we're about to use it is not really the ideal way to handle the situation.

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But the reason I'm going to show you this is because one it builds upon what you already know too.

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You're going to see it done this way and other scripts so it's best to know what is going on when you

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see that.

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And 3 it serves as a nice comparison for the better way to handle this situation which is to use a case

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statement which of course will be getting to soon enough.

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So with that little disclaimer out of the way let's say you want to do something if the word start was

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passed in as the first argument to your shellscript you know that the first argument is stored in dollar

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sign one.

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So you can do something like this if

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dollar sign one is equal to start then let's say we just want to echo the word starting then we close

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out our IF statement and with F I that's right and quit or changes here make the script executable

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and then test it out.

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So that looks like it works.

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So if we supply anything other than start as the first argument nothing happens in our script.

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Let's go ahead and try that out.

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We'll try this nothing happens.

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We'll try this nothing happens.

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Of course if we don't supply anything then nothing happens so again that is working as we expect it

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to.

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If we want to account for other cases we need to code for it.

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So let's take a quick look at the help for the IF command will do help if what we need to use to build

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out our IF statement to account for other strings is Elif which is short for else.

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If so let's get back into our script and account for stop as an argument.

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I'm going to go up here and add Elif

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of dollars on one is equal to stop then we'll echo stopping if read our code from the top down.

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If dollar sign one equal start then what we're going to do is echo starting else.

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If dollar sign one equal stop then what we'll do is echo stopping it's pretty pretty intuitive the way

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this is working out.

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So let's say for changes and try it out.

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Sure enough we pass and stop and stopping it's displayed to the screen.

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So let's go ahead jump back in and add another left to handle status.

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So here we'll go.

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Elif

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Alasan one is equal to status then will echo status while we're here let's handle all other cases within

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else.

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Let's also make this an error message by sending the output to standard error and exit with a non zero

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exit status.

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OK let's try this out.

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So we'll do a user test demo nind at SC status display status to our screen which is what we want.

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Let's pass this and restart.

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And it says hey restart is not a valid option.

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Please supply a valid option.

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And so we can check our exit status here and sure enough it's one.

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And of course if we don't display or pass anything into our script we also get the same situation we

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need to supply a valid option so that our script is doing is taking an action based on the value stored

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and dollar sign one in our script.

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Now if you stopped right here that would be absolutely fine.

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You made things work with the tools and skills that you had at the moment which is what we all do anyway

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right.

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But let's add to your skills and recreate this exact same functionality using a case statement.

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Let me go ahead and comment out all that existing code first and then we'll go ahead and build on it

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from here.

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Now let's build out our case statement.

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The first line consists of the word case followed by the value that you are going to check.

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In our case that value is stored in side dollar sign one.

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And finally we're going to end the line with the word end.

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So we have case our variable and in case dollar sign one in next what we need to do is supply a pattern

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followed by a closing parenthesis.

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For example we want to see if dollar sign one matches the word start.

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So we do this

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this means if dollar sign one is equal to start then execute the code that follows.

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And here's the code that we want to execute.

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If you wanted to execute many commands you would simply supply them on a separate line to end the code

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block use to semi-colons to finish the case statement.

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Spell the word case backwards.

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All right.

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So let's try this out.

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So that works.

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Now let's go ahead.

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Go back and add the other patterns that we're looking for to our case statement.

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So now we're looking for the word stop and then what we're going to do there is Echo stopping to the

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screen and in our code block with a two semi-colons.

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And let's also look for status

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so notice how we're not repeating ourselves multiple times like we did with the if statement we supply

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the value that we're checking once and then all patterns we're looking to match.

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So if we go back up in our script we use if dollar sign one is this LFA dollars on one is that a dollar

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sign one is something else and so on.

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But here in our case statement we say if dollar sign one is this that or the other then we're going

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to do something based on that value.

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Again it's much cleaner in my opinion than the longer if statement that we see above.

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So let's make sure that this little bit of code works

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a stopping argument works let's look at the status argument and that works as well.

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The only thing that is left to account for is anything that does not match the strings start stop and

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status.

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So let's quickly reference the bash man page and get some more information on patterns as they are used

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with the case statement.

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So I'm going to type in man bash and I'm on a search for the case statement here SCO case.

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Here we go.

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It reads a case command first expands word and tries to match it against each pattern in turn using

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the same matching rules as for pathname expansion.

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And then it tells us to see pathname expansion below.

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So that means we have to look at that section of the man page and we do that now pathname Stu XP.

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Here we are really what we're looking for in this section is about pattern matching.

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So let me just page down here until we get it there it is pattern matching.

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So let's see here it says any character that appears in a pattern other than the special pattern characters

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described below matches itself.

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That's the type of pattern we have been using so far.

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For example our first pattern in our case statement is S T A R T which only matches that exact sequence

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of characters the special characters here are asterisk questionmark and brackets.

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The asterisk matches any string including the null string said.

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Another way asterisk matches anything.

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I'm sure you've probably used this before right.

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So for example if you want to get a list of files that end in s t you would run LS space asterisk t

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s t.

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Now we can do the exact same thing.

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In our case statements.

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Now the question mark matches any single character.

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Finally the brackets are used to match any characters contained within those brackets and you can also

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specify a range of characters by separating them with a hyphen.

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This is all good information to know how to use questionmark in the brackets and all that.

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But for our purposes right now we only need the asterisk for our current use case because we just want

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to match anything else that wasn't already specified.

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So we'll get on the main page with typing Q.

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We'll get back to our script by starting our editor again.

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So now let's match anything that is not already specified by using asterisks as our pattern.

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So we'll do this matches anything.

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And then we'll say supply a valid option.

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Send that to standard air.

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Exit our script with the non-zero exit status obviously 1 is not zero so that's going to work for us

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and then we'll close out our statement here with semi colons.

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Remember that your scripts are being executed from the top down.

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So once a match is found the associated code block gets executed if you want to make sure something

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matches Furse will then put it first in your list.

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In this case we're doing the opposite and using the asterisk as a catchall to match anything that hasn't

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already been accounted for and we're putting that last and our list of patterns.

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So let's go ahead and try out our code we'll do Ellies user demo 0 9 start.

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That works let's try stop.

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Whoops I accidentally ended the line with a line continuation character but this is going to work anyway

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I'll just hit enter and sure enough that works let me take that off and hit enter and that's what it

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should really look like Excuse my typing mistake there.

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Anyway let's keep trying.

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Are other options here we want to make sure that status gets matched it does it reports back status.

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Will do restart.

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For example here when we pass and restarted says supply a valid option because it does not match.

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Start stop status but it does match the asterisk and executes that code.

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Let's say we want to modify the script a bit and allow a user to provide the word status or state and

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still execute the same code.

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So let's look at help case and I glossed over this earlier but in the help output here for the case

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statement it says that you can use a pipe to separate multiple patterns.

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So I'll go back here and then we'll change status here to status pipe.

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What you can think of as an OR.

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And then we'll type in the word state that we want to match.

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Now if dollar sign one equals a status or it equal state the code echo status will be executed by the

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way instead of using two pattern separated by a pipe symbol we could have used as th asterisk as the

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Panner.

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Now that would match the words status and state.

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However it's not as exact because it would match other things like statu stateless static and many others.

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I just wanted to share another way you could approach this but I like them more exacting option.

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We went with here so let's try it out.

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So will do l user Dymo 9 state and that reports a status.

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And of course we can do status here as well and that works just so you know you can use multiple pipes

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in your pattern.

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So let's expand our script to accept dash dash status and dash dash state so we can do this dashed status

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or dash dash state save our changes and try our code.

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So do dash dash state first and that works dash dash status that works and if we supply something that

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doesn't match like this then we fall back into the asterisk pattern and get that code block there.

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Let's wrap up this lesson by talking about spacing and style

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just so you know a case statement like this is really the basis of most init scripts.

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For example on older Linux distributions you would run ETSI a net the S-sh D status and you would be

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provided with the status of the S-sh d service.

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Likewise you would stop the service by executing at CNET daddy s s h d stop and so on and so forth just

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kind of like we have laid out here all the systems services were handled by a simple shell script that

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used a case statement.

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Anyway let's quickly talk about spacing what I chose to do when creating my case statement was to indent

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each section by two spaces.

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For example the patterns were indented two spaces from the beginning of the case statement.

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Likewise the commands underneath each pattern were also indented two spaces from the beginning of the

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pattern.

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If you are for spaces type of person you can do that if you like using tabs instead of spaces.

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That would be fine as well.

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You can even forego the spaces altogether and have everything at the same level of indentation.

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But in my opinion it's harder to read a case statement or an if statement or really anything else without

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indentation so I don't recommend that approach.

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If you're only executing one command following a pattern it's a common practice to keep that command

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on the same line as the pattern match and to also include the closing double semi-colons on that same

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line as well.

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Let me create a copy of the case statement to manipulate and I'll clean it out as well.

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So just

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copy that below and then I'll go ahead and comment it out.

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There we go while I'm here I'm going to clean up that status pattern so I'll just do this.

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So let me go ahead and put these lines together here.

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Notice that I'm leaving a space after the parentheses and before the semi-colon.

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That's not technically required by syntax but it's a little bit cleaner in my opinion.

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Now I'm not going to change the code block for our catch all pattern which includes the asterisk pattern.

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It's multiple commands and it could get confusing running all those commands together plus by the way

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we haven't talked about the command separator you have to use if you want to execute multiple commands

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on a single line and we'll be getting to that later.

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Now if you look at this case statement it's a fairly compact yet it's pretty clear.

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It's pretty easy to read down the list of pattern start stop status and then the asterisk for everything

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else.

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So let's make sure that this newly formatted case statement still works.

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OK it still works with start.

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Does it work with Stop it does.

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Let's give it nothing which should match the asterisk pattern and sure enough it tells us to supply

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a valid option so that still works even though we formatted it slightly differently before we wrap things

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up.

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I wanted to point out that you can use the case statement in conjunction with values other than dollar

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sign 1.

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For example you could prompt the user for input and use a case statement to make a decision based on

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that input.

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You could create a simple menu system that way for example.

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Well that brings this lesson to a close in this lesson you learned how to use the case statement.

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If you find yourself writing a large if elif statement comparing different values against the exact

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same variable.

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Consider using a case statement in its place.

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You can check the contents of a variable against the list of patterns that you specify.

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You can do exact matches by specifying the exact string that you're looking for.

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You can also perform matches with patterns that include asterisk question marks and brackets to perform

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the same set of actions against multiple patterns separate each pattern with a pipe.

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Remember to end each pattern with a closing parenthesis and the code block for a given pattern executes

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until it encounters two semi-colons.