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As I want to be to the point on shell scripting, I'm going to organize this entire lesson around

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a couple of shell scripting examples.

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Here is example number one.

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For your convenience, I've put it in the slide.

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I'm going to write it for you.

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And while I write, I will explain what it is doing and what these different components

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are all about.

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OK, let's give it a very original name, script1.

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Notice that in a shell script name, you don't need an extension.

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And that is because the Linux operating system is very capable of detecting the contents

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of any file using the file command.

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Now, every shell script should start with what you call the shebang.

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That's calling the interpreter for the subshell that is running your script.

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The reason is that not always bash is used.

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Bash is often installed, but you might be running a script from a different shell.

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And this will make sure that the code in the script will be interpreted by bash.

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Now, particularly when you are using more advanced features, this is important because

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the advanced features will not always be available in all the other shells.

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As good practice in a shell script, you should put comments as well behind the hash.

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And that is as a documentation so that later on you can still understand what you are doing.

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And other people have some documentation in your script to find out what you are trying

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to do.

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Next, I'm going to use construction that is known as a here document.

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Now, in this case, it's a very simple here document.

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You might be wondering, why don't we just use echo?

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Because the here document is printing a message on screen.

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So the construction is cat, smaller than, smaller than, keyword.

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Use any keyword that you want.

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I like to write these keywords in uppercase, it's not mandatory.

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So feel free to use lowercase if you're better with that.

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And then we can see the text that I am going to send to cat.

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And that will be text that is displayed on screen.

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Now, echo is typically for one-liners.

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And this here document can be used if you want to print more information on screen.

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But the nice thing about the here document is that you don't define lines.

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You define a block of text that should be shown.

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And regardless of current screen resolution, it will always look good.

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Then the script continues with a read.

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A read is how a script is going to wait for user input.

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So the script will stop and it will put the user input in a variable.

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And the name of this variable is there.

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And next, we are going to work with that variable using cd$dir.

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If you define a variable, you just define the name of the variable.

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If you use the value of the variable, you need to put a dollar in front of it.

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And that's why I'm doing cd$dir followed by pwd followed by ls.

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And then I'm using exit0.

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Now, the exit0 is not needed at all.

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But the only reason I've got it here is because I want to tell you about exit codes.

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The exit code is telling the parent shell about the success or failure of the script.

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And exit0 is implicit.

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If all went all right, then the script will exit with the exit code exit0.

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And you can use exit$? from the parent shell to figure out that everything was doing all right.

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So now I'm done with my script.

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And I need to make it executable, chmod plus x on script 1.

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And now I can run it using script 1.

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And there we can see that script 1 is asking, what directory?

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Well, I want to go to slash home.

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And there we can see that it works on slash home.

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And it shows the contents of the slash home directory.

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And I'm back in slash root.

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You might be surprised about that.

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What is the reason behind it?

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Well, the reason behind it is that when you run a script, the script is running a subshell.

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And all of the commands are happening in the subshell.

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And that is why if you cd to a directory in a script, that cd, change directory,

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is valid within the context of the script, but it doesn't affect the parent shell.

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There's a way to do that.

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And if you want to run the commands in the script in the parent shell, you need to source it.

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Let me show you how you can source a script as well.

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In order to source it, I need a couple of modifications.

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I need to remove the shebang for the simple reason that the script is included in the current shell.

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So you are bound to the current shell.

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I also don't want the exit 0, because that would close the current shell.

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Exit 0 in a subshell is OK, because you need to close the subshell anyway.

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Exit 0 in a script that is included in the current shell would close the current shell.

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And that is not OK.

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So now I'm going to use the source command.

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And source script 1 is asking what directory?

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Well, slash home, and you can see it has permanently changed my directory.

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So I need to manually get back to my original directory.

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I need to show you another way to source a script, and that is the dot command, dot script 1.

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First time you see that, you might be surprised,

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because you might be used to dot slash run from the current directory.

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Now here we don't do dot slash, because the dot does not have the meaning of the current directory.

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The dot is the command that is equivalent to source.

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As you can see, it is doing the exact same thing.

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And that was script 1. I hope you liked it.