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In this lesson will continue exploring positional parameters and you'll also learn how to create a while

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loop.

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Horses always I'm going to start a terminal on my system and move into the folder for the class and

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then move into the folder for the project that I'm working on and then start the virtual machine.

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And when it comes up we'll connect to it.

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Now that we're connected to our virtual machine will move into the shared folder a Ford slash a vagrant

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and this script I'm going to name all user Dymo 7. SH course we're going to start out our script with

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the shebang and give it a path to the bash interpreter.

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I'm also going to provide a little header here demonstrate the use of shift the shift command and while

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loops and a previous lesson you learned that the first positional parameter of dollar signs 0 contains

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the name of the script itself.

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While dollar sign one stores the value of the first argument passed to the script on the command line

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dollar sign to stores a second argument and so on.

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We never actually used dollar sign one dollar sign to directly in that lesson.

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So let's do that now so you get a chance to see how that works and how they behave in different situations.

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So let's just display the first three parameters.

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Well use an echo statement here.

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And I'm just going to copy and paste

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going to add an echo statement here which is just going to print a blank line.

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Now I'm going to save the changes to the script.

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And then exit.

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Since this is the first time I'm executing this script I need to make sure that it has the proper permissions.

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Now I'm just going to execute the script without any arguments.

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So there are no arguments and that means there are no values to store inside the script as positional

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parameters.

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So of course parameter 1 2 and 3 are blank let's pass in one parameter and see what happens we'll just

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call it with an argument of Apple.

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So now dollar sign one is assigned the value of Apple and that gets printed.

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So let's do this again with bread it's assigned to dollar sign too.

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And let's say Candy.

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So now dollar sign one is Apple dollar sign to his bread dollar sign three is Candy and we can directly

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access those and use those in the script by using dollar sign one and so on.

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If we were to add more command line arguments for example another word here let's say dumplings then

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that is stored in dollar sign for.

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But we didn't account for a user entering dollar sign for.

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So that presents a challenge and so that's why we're going to use things like for loops and while loops

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to handle command line arguments in many cases.

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Next in our script we want to loop through all the positional parameters and the previous lesson we

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used a for loop to do that.

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But this time let's use a while loop.

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So let's look at the while command.

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What type is it.

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Well it's a shell keyword or shell built in.

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So that means we can use help against that command help a while and hit enter to get some information

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about it.

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Now that says while commands do commands done this executes commands as long as the final command in

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the while commands has an exit status of 0.

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You can think of this as being while true do while the condition that is being tested for is true then

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the commands will execute.

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This means that during the while loop you want to change the thing being tested for in some way so that

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the condition eventually evaluates to false or you'll end up in an infinite loop.

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Let's create a while loop here at the command line.

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Let's use a variable called X will assign that the value of one will use.

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While we're going to use our double brackets just like we did in the for loop.

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Now we're going to use a conditional expression we're going to test a half dollar sign.

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X is equal to 1.

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If that is true then what we're going to do is echo.

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This is the value of x and then simply Ecko the value of x.

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Now we're just going to manipulate the variable x in some way and we'll just change it to 7 here and

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then type done.

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And so what happens is while and that conditional expression X equals 1.

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If that's true and it is then it executes the code block between do and done.

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So it echoes what you see on your screen and then X is changed to 7.

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Now the while loop executes again.

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So it says now is X equal to 1.

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Well it is now false because X has 7 so it doesn't execute the commands below and the while loop exits.

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This loop isn't really practical but it demonstrates changing a variable that is being tested against

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in the conditional expression of the While loop.

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By the way if the conditional expression never evaluates to true then the commands in the while loop

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will never get executed.

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For example right now the value of x is 7 so we can do Ekko dollar sign X sure enough at 7.

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And if we execute a while loop again this back up here a couple of commands and hit enter.

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Nothing happens because X dash Q 1 evaluates to false.

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When you're learning about the wild loop and doing some experiments on your own you're going to end

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up creating an infinite loop at some point it's just going to happen.

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So let's do it on purpose and show how easy it is to break out of it before we do then I want to introduce

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you to the true command.

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It's only job is to return an exit status of 0.

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So what type of command is true.

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It is a built in as well as a program.

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Of course we can get help on a built in by using help.

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True and it says it just returns a successful result.

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The exit status is that it always succeeds.

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Now I happen to like the description of the executable better so let's look at the man page for user

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been true all this to man.

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True.

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And it says do nothing successfully.

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I just love that definition.

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So in any case no matter what happens with true it just returns an exit status of 0.

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So we can test this we'll just type the true command and then we can look at Ekko dollar sign questionmark

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sure enough it returns 0.

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If we do true blah it doesn't really matter because again it just always returns zero before we create

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our infinite loop here let's look at just one more command called Sleep sleep happens to be an executable

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program on the system.

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It's not a shell built in.

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So we'll use the man page to learn more about sleep so sleep simply delays execution for a specified

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amount of time.

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You can use sleep followed by a number and a suffix and that suffix can be asked for seconds and for

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minutes for hours or four days.

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So sleep does what it sounds like it just sleeps it doesn't do anything for a specific amount of time.

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So I get to get out of the man page here and then just to sleep for one second.

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Nothing happens and you return to the command line.

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Of course that is the same as sleep one ass and you can even do half a second or portions of a second.

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So let's do this sleep.

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We're just a half a second and hit enter.

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Now let's build our infinite loop so we can do while and we'll test for true true always returns 0 and

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exit status of zero.

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So it's always true.

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So this is going to never change this while loop is going to run for ever.

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So we'll just do while true.

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Do then what we're going to do is just simply echo a random number

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and now we're going to sleep for one second and then we'll close out our while loop here with the syntax

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of done.

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Now when we hit enter Echo gets executed it executes and displays a random number.

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Sleep rest for a second if you will.

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And then the loop repeats because while true always evaluates to true.

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So this will just go on forever.

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Now to break out of this loop all you have to do is hold down the control key and type C so Control

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C sends an interrupt signal to the command which makes it stop.

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Now this isn't something you need to the while command you can use this to stop any command running

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in the foreground.

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For example if you execute sleep and want to interrupt it and return to the command line sooner you

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can use control.

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See there as well.

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So let's say we give sleep the instruction to sleep for 10 minutes and then we decide we don't want

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to wait that long so we'll just go ahead and type Control-C and it breaks out of the command here on

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the screen you can see the care.

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See that shows that indeed these commands were interrupted with control.

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See there's one more small piece of the puzzle left to cover before we use a while loop in our script

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to loop over all the positional parameters.

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This last piece of the puzzle is the shift the built in the type of a shift.

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It's a shell built in.

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Let's get some help on this here.

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It says it renames the positional parameters of N plus 1 and plus 2 dollars on wandless onto and so

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on.

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Now what the shift command really does is that it removes positional parameters from the list starting

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with the dollar sign when it just chops off dollar sign one.

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So when you execute the shift command the value stored in dollar sign one is removed.

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It's just gone.

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What also happens is that the value stored in dollar sign to shift down to dollar sign one dollar sign

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three replaces the value in dollar sign to four replaces the value in three and so on and so forth.

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Also the special parameter of a dollar sign pound sign is reduced to buy one.

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Now as you remember a dollar sign Palestine contains the number of arguments that were supplied on the

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command line by the fall.

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Shift shifts everything.

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One place executing shift by itself is the same thing as executing shift space one if you want to shift

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everything by two places use shift to.

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In that case dollar sign pound sign will be decreased by two.

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If you want to shift everything by three positions you shift three and so on.

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Now if this sounds confusing Don't worry it's actually harder to explain than it is to demonstrate So

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when you see it being used in a script it will make total sense.

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So let's do that right now.

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Let's use a shift in our script.

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So we'll loop through all the positional parameters.

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We'll use a while loop while double brackets here again make sure there are spaces in between while

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under double brackets and a space after the double brackets.

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We're going to test for a dollar sign pound sign here.

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The number of parameters and we'll say if it's greater than zero.

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That means if there were any arguments on the command line then this will evaluate to true and that

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point what we want to do is echo the number of parameters.

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Is a dollar sign pound sign and then will echo the first parameter here.

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And we're just going to repeat this a couple more times here.

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So will echo a dollar sign too and a dollar sign three then will echo a blank line and then what we're

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going to do is use the Shift the built in command.

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So just like we said shift is actually going to modify dollar sign pound sign because when it's executed

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it removes one parameter from the beginning dollar sign one is gone it shifts everything down.

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So that means at that point the number of positional parameters has changed.

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And Dollar Sign pound sign has changed to reflect that.

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So we are modifying the condition that we are testing for through this loop so we shouldn't end up in

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an infinite loop here or some going to end this with done.

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Save my changes and then execute the script.

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So if we execute the script without any arguments then there are no positional parameters to loop through.

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So that's why we only get the top portion of the script displaying some output but we don't get any

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of the commands in the while loop being executed.

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Now let's apply just one argument here.

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That caused the while loop to execute one time.

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So dollar sign one is assigned to Apple then shift occurs and so that removes dollar sign one to shift

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everything down.

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Well there's nothing left to shift down.

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So dollar sign pound sign evaluates to 0 0 0.

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Greater than zero is false.

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So the while loop stops executing because it's false.

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So let's go ahead and add a nother argument on the command line and see what happens.

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So we'll add bread.

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So the first time the loop gets executed dollar sign one contains an apple dollar sign to contains bread.

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The shift command is executed.

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Apple blows away the original dollar sign one goes away gets lopped off the beginning of the list.

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Their bread which is stored in dollar sign to then becomes a dollar sign one.

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And there are no positional parameters.

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Two and Three.

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So let's keep doing this and you can clearly see what's going to happen.

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We'll do candy at the outset of the script dollar sign one is Apple dollar sign too is bread dollar

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sign three is Candy the first time the while loop is executed and shift gets executed.

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The number of parameters gets decreased by 1.

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So then there are only two parameters bread and candy bread being dollar sign one candy being two.

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While Luke gets executed again bread gets chopped off the beginning there and you're only left with

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candy which then becomes dollar sign one the shift command is executed again.

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Candy goes away and there are no more positional parameters so zero greater than zero is false in the

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while loop exits.

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So let's do this again with one more argument.

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So as you can clearly see every time we go through the while loop and the shift command is executed

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everything shifts down by one to the while loop executes four times and then exits.

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It might not be obvious when you would want to use the shift command beyond our while loop that we've

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demonstrated here.

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So here are a couple of thoughts to keep in mind.

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One time shift comes in handy is when you want to have a user supply information as arguments to your

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script and there is one argument or piece of data that can contain multiple words.

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For instance data you would want to use in the comment field when creating a user.

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In this case you would tell the user to supply a username which would be dollar sign one then you would

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shift and then what is left over becomes the comments field.

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Now assign 1 2 and 3 and so on.

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Now that's one example.

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Also think about processing command line arguments.

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For example something like this dash dash dash for Beauce.

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So just keep these ideas in mind and will be coming back to them in future lessons and exercises.

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To quickly recap in this lesson you used a positional parameter starting with dollar sign one explicitly.

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You review the special variable of dollar sign pound sign.

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You are also introduced to the true sleep and shift commands.

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Finally you put all this new information together in order to create a while loop.