Linux Scripting

0:00No, it may seem like a silly thing to talk about,

0:02but when we're talking about bash scripting

0:04it's really important that we figure out

0:06what text editor we want to use, because bash scripts

0:09are text only.

0:11So you can either use a GUI tool or a command line interface

0:14tool, that's what CLI stands for.

0:16And I really want to talk about what

0:17not to do when it comes to writing

0:20bash scripts with your editor.

0:21So the first thing we need to figure out

0:23is what is the best way to go about editing a text file.

0:27Now you've probably heard the saying,

0:29there are many ways to skin a cat.

0:31Well I think that's an absolutely horrible saying,

0:34we will not talk about skinning cats.

0:36Rather, I want to go right to our command line

0:38so we can look at the various types of editors

0:41we have available to create these bash

0:43scripts that we're going to look at.

0:44Now since we're using a GUI environment or a graphical user

0:48interface on top of Linux, one of the things you're able to do

0:51is use a GUI or a graphical user interface text editor.

0:54Now this one happens to have Pluma text editor installed.

0:58If we open this up, you'll see it's a GUI tool.

1:00And we could open a file, like you'll notice on the desktop

1:03I have this SampleScript that I put on here.

1:06And it'll open, and you can edit it, and then save it.

1:09And it'll work just like every other program you would expect.

1:12Now it does have a few cool features,

1:15for example, syntax highlighting.

1:16Now this is just plain text, but if you look

1:19it actually highlights things based on the code.

1:22So this is bash code, and it knows how to highlight things

1:25differently.

1:26So you know this is green.

1:27This is red.

1:27This is purple.

1:28That's really useful, because if you accidentally do

1:31a typo, like if you misspell a command like while,

1:34it's not going to highlight it, because it's not

1:36going to recognize it.

1:37So it's going to look weird.

1:39It's just a really nice way to keep things organized,

1:42and be able to identify things quickly.

1:44If you forget this close quotation,

1:46for example, it's going to highlight things differently

1:49and that's not what we would expect.

1:51So if we close that it's going to make it look right.

1:53And it's going to be highlighted just

1:55like the rest of the things.

1:56So syntax highlighting is powerful.

1:58You want to use syntax highlighting

2:00if at all possible.

2:01Regardless of whether you're using a GUI or a text

2:04based editor.

2:05Now again this is perfectly acceptable,

2:07if you want to do this.

2:08I'm not going to save any changes here.

2:10You can use that GUI tool if you want.

2:13Now the one thing you don't want to do,

2:14though, is to use a GUI editor that happens

2:17to be like a word processor.

2:19So for example, LibreOffice Writer is here,

2:22you do not want to edit text files with LibreOffice.

2:26Now is it possible?

2:28Sure, we could say file, open, and go to the desktop,

2:32and find our SampleScript.

2:33And it's going to be here, but a couple problems

2:36with this, one you'll notice there's no syntax highlighting.

2:39It just sees this as a text file,

2:41and it wants you to edit things.

2:42Using this we could make it bold,

2:45and things like that, which is not text only.

2:48We only want it to be text, and things like this

2:50just are not compatible with bash scripts.

2:53So do not use something like LibreOffice or Microsoft Office

2:57when creating bash scripts, it's just not ideal.

3:00So I'm gonna say, don't save.

3:02Now if you're using Linux chances are at some point

3:06you're going to find yourself in situation where you only

3:08have access to the terminal.

3:10For example Linux servers are often

3:12called headless, meaning they don't have this graphical user

3:15interface.

3:15So if you're stuck in a position like that where you only

3:18have text, it's really important to understand text editors.

3:22So let's go to the desktop and inside here we'll

3:25see that SampleScript.

3:27Now, if you type nano, nano is the name

3:30of a very powerful text editor.

3:32It's the one I recommend that you

3:33use as we edit files inside a command line window in Linux,

3:39in general.

3:39Nano is just awesome.

3:41It's really easy to use.

3:42You just type nano space the name of the file

3:44you want to edit, press Enter.

3:46You'll notice it does have syntax highlighting which

3:48is awesome.

3:49We would hope that it would have syntax highlighting.

3:52And you can edit files, just simply--

3:55like you would arrow keys and make changes.

3:59And then when you make a change and you want to save it,

4:01there's a little cheat sheet right here at the bottom.

4:03So Control X, for example is how you exit.

4:07So we do that.

4:08It says, do you want to save the modified buffer.

4:10That's basically saying, do you want to save your changes.

4:12You can either do yes, no, or control-c to cancel.

4:15So I'm going to press the Y key.

4:17And then it says what file name do you want to save it as.

4:19Well it's the same file name by default,

4:22and we're going to leave that and press Enter.

4:24It'll save over top of what's there.

4:26But we could actually change the name.

4:27We could say SampleScript 2, and then it

4:29would act like save as, if you're

4:32going to save it as something new we would have two things.

4:35I actually want to save it right over top,

4:37so I'm going to leave the default where it says file name

4:40to write SampleScript, press Enter,

4:42and now this script has been changed.

4:45It's really cool, since we have a GUI environment,

4:47you could do something like this,

4:49if you double click on the script, it's going to say,

4:51all right so this is executable, do you

4:53want to run it in the terminal.

4:55And if we do that, it's going to open a new terminal

4:57and execute the bash script.

4:59Now this is simple little bash script

5:00I made to show you the syntax highlighting and stuff,

5:03however, it does give us instructions.

5:05If you want to stop bashing, press control-c.

5:07So I'll press Control c, and it'll close that window.

5:10So the GUI does give us a couple of nice things,

5:13you can double click on it and say Display,

5:16and it conveniently will then open it in Pluma.

5:19So you can make changes right inside this GUI environment.

5:23So by all means, you can use the GUI tools here.

5:26But knowing how to use it on the command line is important.

5:29I'm going to do everything inside the command line.

5:31And I'm actually going to use an editor called vi.

5:35Now I don't recommend that you use vi,

5:37only because while it does support syntax highlighting,

5:41it's a really complicated editor to get the hang of.

5:43The only reason I use it is because I've been using it

5:46for over 20 years.

5:48Things like nano didn't used to exist.

5:50And so I had to learn to use vi and now

5:52my fingers and brain kind of just

5:53go on autopilot, because in order

5:55to edit you have to do really bizarre things.

5:57Like go to this.

5:59And then you have to go into Insert mode

6:02in order to make changes.

6:04And then you have to press Escape

6:05to get out of Insert mode.

6:06If you want to save it, you have to type colon

6:09wq, which is not intuitive, and there's no cheat sheet.

6:14But this is the editor that I'm going to use

6:17as I show you how to do things.

6:18But again you can follow along and do everything

6:21I'm doing in vi, but use nano.

6:23And it's going to be a lot easier,

6:24and it's really what I recommend you use.

6:27I could use nano but again vi just comes naturally.

6:30If you want to play with vi, by all means, you can do that.

6:33It's going to be installed on every Linux and Unix box

6:35you log into.

6:36Vi is going to be an option.

6:38However nano is going to be there almost as often, so I

6:42really recommend using nano.

6:44Regardless of what editor you use

6:46it's important to remember that it has to be text only.

6:49So do not use a word processor as we're

6:51editing these bash scripts.

6:54So while I don't agree that multiple ways to skin a cat

6:57is a good idea, I do agree that it

6:59is important to understand there are multiple ways that we

7:02can edit text files.

7:04And not one is correct, it's just

7:06that it's going to be personal preference.

7:08Although I do recommend that you do your best

7:10to learn how to use a command line interface text

7:13editor, because with those headless servers

7:16you're not going to have a GUI option.

7:17So if you rely on a graphical user interface

7:20you're going to be struggling when it comes to servers

7:22that don't have a GUI.

7:24So with this lab I urge you to play around

7:27with the different text editors that are here,

7:28get comfortable with one, and whatever one you pick just

7:32use that as we go along and learn how to do bash

7:34scripting in other Nuggets.

7:36Again text is text, it doesn't matter

7:38if you do the exact same thing I do

7:40it's just important that the end result is the same.

7:42I hope this has been informative for you,

7:44and I'd like to thank you for viewing.

0:00You can kind of tell how long somebody has been using Linux

0:02by how much they rely on scripting on their day

0:05to day system administration.

0:07Scripting is awesome.

0:08It's a great way to save time--

0:10if you want to call it being lazy you can-- but basically,

0:13all of those shell commands that you would type in

0:15on the command line one by one, you

0:17can make a script or a file that contains a bunch of them,

0:21and then the shell will allow you

0:23to execute them all at once.

0:24Now usually it's the BASH shell that we're using.

0:27Any modern Linux distribution is going

0:29to default to using the BASH shell, which

0:31is Born Again Shell is what that stands for.

0:34But that's just the interface that you

0:35have on the command line.

0:37So this is a very simple thing that we can set up.

0:40I want to show you the structure of how

0:41to set up a shell script or a BASH script.

0:44And I want to talk about calling it versus executing it.

0:47And then we're going to learn what a shebang is

0:49and why you want to use a shebang in your shell scripts.

0:52So let's go to the computer and start by making

0:54some simple shell scripts.

0:55Now, I'm here on an Ubuntu computer.

0:57It doesn't matter what Linux you have,

0:59because they're all going to have the Bash script

1:00and we're going to be able to do some things.

1:02I could type ls and it's going to show

1:04us the files in our directory.

1:06I could type ls minus l to get a long listing

1:09so we can see all the files.

1:10We could say something like echo hello,

1:14and it's going to echo hello on the command line.

1:16These are all things we can do on the command line.

1:18I'll say expr 1 plus 1, it's going

1:23to give us the answer of 2.

1:24So these are all things we can do on the command line.

1:26But wouldn't it be nice if we could do this in a script.

1:29And let's actually put that inside a file,

1:32and we'll execute it as a shell script.

1:34Now, I am going to use vi, that's the editor I use.

1:37I just prefer vi, that's what I use.

1:39You can use anything you want-- nano or a graphical editor.

1:42There is a couple.

1:43If you go into this menu, there's

1:44going to be some options.

1:46If you want to use the Pluma Text editor you can do that.

1:49I'm just going to use vi because that's

1:50what I'm comfortable with.

1:51And generally, shell scripts are either

1:54named something like stuff.sh or just any text name.

1:58We don't have to put the extension to make it work.

2:00Some people put the file extension sh

2:03to stand for a shell script.

2:04I generally don't.

2:05I generally just call it like a text file with no extension.

2:08So we're going to call this one stuff,

2:10and here we have an empty file.

2:12So the first thing I want to do is start typing.

2:14I'm going to put a couple of spaces up there.

2:15Spaces don't really matter-- blank lines.

2:17I'm going to type ls, and then I'm

2:20going to want it to do ls minus l, and what is the other one

2:23we did?

2:24We did echo 'Hello Shawn'.

2:27And we'll do and expr 2 plus 2.

2:31All right.

2:31Notice in here, I'm just typing them

2:33just like I would on the command line.

2:35There's no syntax like that doesn't end in a semi-colon--

2:38there's no weird syntax like some programming languages

2:41have.

2:41We just put it in here like we want

2:43it to execute on the screen.

2:45Now, once we get into loops and stuff sure,

2:47there are some structures that we're going to learn.

2:49But as far as the syntax we need to make it function,

2:51we just type it in there like we want it to be in there.

2:54So let's save this file.

2:55And if we type ls now we're going

2:57to see that file 'stuff' is in there.

2:59If we type cat stuff we're going to see it's in there.

3:01We just saved it so it's exactly the file that we just created.

3:04If you want to execute it, what you can type is BASH stuff.

3:09And what it's going to do, it's going to call the BASH shell

3:12and it's going to tell it to execute

3:14the stuff inside the file that we specified.

3:17Type Enter.

3:18And it did all of those things.

3:20Let's see if it really did do all those things.

3:21So I said BASH stuff.

3:22It did an ls.

3:24And then it did an ls minus l.

3:27And then it said 'Hello Shawn'.

3:29And then it did that expr 2 plus 2.

3:32OK.

3:32So it did all of the things that we wanted it

3:34to do one right after another.

3:36Which is awesome.

3:37That's exactly what we would want.

3:39Now, if you don't want to have to type BASH and then

3:42the name of the file, we can actually

3:43make this an executable file.

3:45So we'll say shmod plus x stuff.

3:50Now if we do ls minus l, we're going

3:51to see that stuff has now turned green--

3:53which it may not be green on your distribution,

3:55but if you're using this lab it's going to be green--

3:57and see that it now has the executable flag, which

4:00means this file is executable.

4:02So we could say dot forward slash stuff,

4:06and it's going to execute right on the command line,

4:08because now it thinks it's an application

4:10or an executable file, and it just executes it.

4:14So this is going to work fine.

4:15Now, there is a problem.

4:17If we were to take this program and we

4:20were to take it to another computer that

4:21maybe isn't using BASH as the shell, well then it might fail.

4:26And that's where shebang comes into play.

4:29So let's edit this file again.

4:30It's executable, but we can still edit it.

4:32It's still just a text file.

4:33And a shebang is the very top line of a shell script,

4:37and it's a very specific format.

4:39So I'm going to type insert here.

4:41The pound sign-- or hashtag I guess is the more proper way

4:44to say it nowadays--

4:45exclamation, and then what shell you want it to use.

4:50So for us it's been BASH.

4:52So this combination of characters right here,

4:55the hashtag and the exclamation, is called shebang.

4:59So shebang, and then the full path

5:02of the executable that we want to use

5:04as the shell interpreter.

5:06So this line is telling the computer

5:08what interpreter do you want to use if we

5:11call this as an executable.

5:12So if this doesn't exist--

5:14notice it didn't exist and it just

5:16worked for us-- what it does is it

5:17uses the current running shell if you don't specify what

5:22shell it's supposed to use.

5:23So for us, since we're currently running BASH,

5:25it worked just fine.

5:27However, to be more proper, you always

5:28want to start your shell script with this shebang and then

5:32the path to BASH, which has almost always been BASH.

5:36And what that says is now the computer

5:38is going to know you want to execute

5:39all the stuff in here with the BASH interpreter or the BASH

5:43shell.

5:44So I'm going to save this, and now of course we

5:47can still type stuff and it's going

5:49to do all those things for us.

5:50It did just like we did before.

5:52But the cool thing about this is--

5:53and this is what I really like about making shell scripts

5:56executable--

5:57is we could say sudo move stuff to user local bin.

6:05It's going to ask for a password, which is pa$$w0rd,

6:10and so now if we type ls it's not in there anymore,

6:13because it went into user local bin.

6:16We can see, sure enough, stuff is in there now.

6:18And since this is in our path, anywhere we are in the system

6:22if we type stuff, it's going to do all those things.

6:25So if we make a shortcut for something that we do often,

6:30we can make it a script, put it in user local bin,

6:33and we're going to be able to execute it just by typing it.

6:36Notice, I don't have to type like dot

6:37and like the full path, and I don't

6:39have to type user local bin, I can just

6:41say the name of it, stuff, and it's

6:43going to execute that shell script that we just created.

6:46Now this is a very, very simple shell script, obviously.

6:49All it does is like ls and a couple of echo commands.

6:52But you can see the beauty of being

6:54able to do a bunch of things in a single command

6:57just like that.

6:57So this is like our building block.

6:59And now you understand how shell scripts work.

7:01What I encourage you to do is make

7:03some shell scripts of your own.

7:04See if you can get them to have the results that you expect.

7:08Make a stuff script of your own and have it do other things.

7:11See if you can cd inside of a shell script.

7:14Can you say cd user local bin.

7:17And if you do that inside of a shell script and you type ls,

7:20what's it going to show you?

7:21The stuff in user local bin or the stuff

7:23in your current directory?

7:24Those are the things I want you to experiment

7:26and play with so that you get a feel for how they work.

7:28But don't forget, you want to put that shebang in there.

7:31Not because it won't work without it on our system,

7:34but because that is a best practice so that it always

7:37calls bin BASH instead of whatever shell

7:39we might be running.

7:41Because sometimes when we start doing things

7:43like loops and conditionals, they'll only work in BASH.

7:46They won't work in some other shell like tcsh or sh

7:49or anything like that.

7:50So you always want to put that shebang bin BASH in there.

7:54Truly, shell scripts are awesome.

7:56And they're really fun.

7:57And making them into executables is--

7:59for some reason it's an awesome way

8:02to make programs that are really just BASH scripts,

8:05but they execute just like programs on your system.

8:08They're awesome.

8:09This is just the tip of the iceberg.

8:11Now that you know how to create BASH scripts,

8:14you should be making them all the time.

8:16Experiment.

8:16See what works.

8:17See what doesn't work.

8:18And in the other Nuggets, we're going

8:19to get more and more complicated to make those BASH scripts even

8:23more useful.

8:24I hope this has been informative for you

8:25and I'd like to thank you for viewing.

0:00Now, batch scripts are incredibly convenient when

0:02it comes to time saving, for putting

0:04a bunch of different commands into a single statement.

0:07However, where they really get powerful

0:08is when you add the concept of variables.

0:10Now, I'm going to try to cover a whole bunch of things regarding

0:13variables, but we're going to do it on the command line.

0:15I actually want to show you variable stuff in action,

0:18but this is just a list of what we're going to cover.

0:21I realized when I wrote it down that back ticks sounds

0:23a little scary, but I assure you,

0:25I'm not talking about bloodsucking critters

0:27on your back.

0:28This is just how we can do a command inside of a script.

0:30So, let's go right to the command line

0:32so I can show you how to use variables inside your scripts.

0:36Now back on our command line here,

0:37I don't have anything in there.

0:38Let's create a script.

0:39So v i.

0:41Actually, let's do touch script, chmod plus x script.

0:47That way it's going to be executable.

0:49So let's edit this file.

0:50First, let's do our shebang then bash on the top.

0:53So we know that it's a bash script,

0:55and then I'm actually going to assign a variable.

0:58Now, a variable is just like you learned in high school algebra.

1:01It stores a value.

1:03So let's say test--

1:05we'll just put that as the name of our variable,

1:07equals-- now this is important, there

1:08can't be a space when you're assigning a variable in bash.

1:11The equal has to be right next to the variable name.

1:15So, test equals-- we'll put in quotes, "some string"

1:21and then down here we'll say echo,

1:24and then how we refer to it is putting a dollar sign in front

1:27of it.

1:28All right, so what this is going to do

1:29is it's going to assign this string to the test

1:32variable and then down here, echo test,

1:34it's going to just echo it to the screen.

1:36Let's actually see how that works.

1:38You can say script and boom.

1:40It just echoed some string.

1:42If we edit it again--

1:44of course now we get really nice syntax

1:45highlighting because it recognized

1:47using the shebang line.

1:48It knows that this is a bash script.

1:50So now it's going to do coloring, and syntax

1:52highlighting, and stuff for us, which makes it easier to read.

1:55Anyway, so we've assigned test.

1:57Now what if we wanted to read something

1:59from the command line?

2:00We don't know what we want it to echo, all right.

2:02So let's say instead of assigning this,

2:05we're going to read from the command line.

2:09And we tell it, what variable do we want it to read?

2:11Well, we'll say test again, that's fine.

2:13So we want you to read the test variable,

2:16but we need to give our command line some sort of hint

2:20as to what it's waiting for.

2:22We'll see what this does when we do it, but echo,

2:25give me some text.

2:28All right, so we say, give me some text

2:30and then it'll wait for us to input text and we press Enter

2:33and it'll assign the text that we enter to this variable.

2:37Let's look at this really quick.

2:39So, script-- it says give me some text.

2:42This is my text, press Enter, and it repeated it,

2:46because remember, the script just

2:48echoed out whatever variable we assigned.

2:50So what we've done is we've allowed some interaction.

2:53So the script will do different things now,

2:55depending on what we enter.

2:56See, it's going to respond whatever text we enter,

2:58it's going to give us.

2:59Now there's another really cool way

3:01to give a script information, so we don't have

3:04to make it part of the code.

3:06Instead of this, we can use a special variable,

3:10and that special variable is going to be dollar sign 1.

3:13Now we can do dollar sign 1, dollar sign 2, dollar sign 3,

3:16and what that ends up being is--

3:19I'll show you, an argument at the end.

3:21So if we do script and we press Enter,

3:24nothing's going to be printed because there are no arguments.

3:27But if we say script, and this is dollar sign 1,

3:34we press Enter, what it does--

3:36this first argument after the script

3:40becomes dollar sign 1 inside the script.

3:43So it echoed dollar sign 1, and we can do the same thing

3:45with multiple ones.

3:47So we could say echo dollar sign 1, dollar sign 2.

3:52Now if we say, script thingone, thingtwo,

3:57it's going to repeat that.

3:59What it did is it assigned dollar sign 1 to this,

4:02and dollar sign 2 to that, and it

4:03used those inside the script.

4:05What if we wanted to do something, not just

4:08with strings?

4:09Now bash often treats things as strings,

4:12but what if we wanted to say "give me two numbers."

4:18Read NUM 1, read NUM 2.

4:22Then we can do something like, expr dollar sign NUM 1,

4:27plus dollar sign NUM 2.

4:30And what this should do is add those numbers

4:32and print them to the screen.

4:33Let's see if that actually is what happens.

4:36Script-- it says give me two numbers.

4:37I'm going to say number 1--

4:39oh, my VM got away from me there.

4:41Let's clear the screen.

4:42All right, try this again.

4:43Script-- give me two numbers.

4:44I'm going to give you number 12 and number 65,

4:48and the sum is 77.

4:50We can make that a little more user friendly.

4:53We could say echo, "the sum is" and so now give me

4:58two numbers--

4:5856 and 222.

5:02The sum is 278.

5:03So that's really cool.

5:05One of the things--

5:05remember I said back ticks?

5:07I wanted to show you how back ticks work because,

5:09what if we wanted to assign the sum to another variable?

5:14What we can do is say, sum equals, and then

5:19if we put this command in back ticks--

5:23now the back tick is the key to the left of the 1 key,

5:28if you're on a US keyboard.

5:30But a back tick means basically, I

5:32want you to execute whatever is inside this back tick,

5:35and that is what I want you to assign to this.

5:38Or I want you to just do this, replace

5:41whatever is between the back ticks with the results

5:43of what's in here.

5:45All right, so what we've done is we've assigned sum--

5:48or we've assigned this to the variable sum.

5:52And then we can say echo, dollar sign, sum is the answer.

5:59So what we should see now is the same thing, except it will say,

6:02"the sum" and then "is the answer."

6:05Let's see how that works.

6:07Script-- give me two numbers--

6:085 and 6.

6:11The sum is, 11 is the answer.

6:12Now that's because I left this one in,

6:14where it says the sum is, but it did--

6:16it actually assigned it to that variable sum

6:19and then it said 11 is the answer.

6:21Does that make sense, what it's doing there?

6:23We could also, instead of saying read variables,

6:25we could change these and instead of read NUM 1,

6:29read NUM 2, we could say expr dollar sign 1, dollar sign 2

6:34and have them be the arguments that we've given

6:36the script on the command line.

6:38So there's lots of things that you can do in order

6:41to manipulate variables inside a script,

6:44but it's really important to understand how to assign them.

6:47And that's one of the things that got me,

6:49there can be no space between the variable name

6:52and the equal, otherwise it doesn't assign it properly.

6:55And these back ticks can be really useful

6:57because you can put anything in here.

6:59If you wanted to put the command LS in there,

7:02then it will assign the entire string of the output of LS

7:05to a variable, and you can do something

7:07with that variable later.

7:08So I encourage you to play around with variables

7:11as much as you can, and we've learned to assign them

7:13in multiple ways.

7:14We can assign them inside the script,

7:17we can assign them by reading from the command line,

7:19and we can actually use the dollar sign 1, dollar sign 2,

7:22dollar sign 3, to get them from command line arguments,

7:25as well.

7:26And depending on the script you're doing,

7:28all of those various things can be extremely

7:30useful on your scripts.

7:33And I've been making shell scripts

7:34for a very, very long time.

7:36And I have to say, the most common way that variables are

7:39assigned-- usually for a sysadmin purpose--

7:42are when you use them as arguments,

7:44like that dollar sign 1, dollar sign 2.

7:45It makes it very flexible.

7:46You can say whatever your script is, and then

7:49say you want to ping an address, or you

7:51want to SSH to it, you can actually just

7:53give it arguments on the command line,

7:56rather than having it read in.

7:58If it's waiting for you to prompt,

7:59that's another step that's kind of a pain.

8:02And you can't use it in a Cron job or something,

8:04to automate things, if it's waiting on the command

8:07line for you to read.

8:08So it's important to know that you can read things

8:10in from the command line, but these arguments-- command line

8:13arguments-- are very powerful.

8:14And that's usually how you get data into a shell script.

8:17Also back ticks are important.

8:19Hopefully you understand what was going on there.

8:21And assigning variables inside a script is also useful.

8:25But remember that dollar sign 1, dollar sign 2 thing,

8:27because that's going to be extremely, extremely useful

8:30as you create your own files.

8:31And I encourage you, play around with assigning variables,

8:34and echoing, and all that stuff, so you

8:35get a feel for how to assign variables and manipulate them

8:39inside your bash scripts.

8:40I hope this has been informative for you

8:42and I'd like to thank you for viewing.

0:00One of the problems with using BASH for automating tasks

0:03is that when a human is doing tasks,

0:06you can determine what needs to be

0:07done based on different conditions

0:09or different circumstances.

0:11Thankfully with BASH, we can program some logic

0:14so that things happen based on other things.

0:17So we can say, if this, then do this, else do that,

0:20things like that.

0:21And we can use various conditionals.

0:23We can use file tests, like is a file there?

0:26Is it not there?

0:27Compare is value 1 bigger than value 2?

0:30Does this string equal that string?

0:32There's lots of things we can test in order

0:34to figure out if this certain conditional circumstance

0:38exists, then do this.

0:39And conceptually, it's logical, right?

0:42I mean, it's the way that we think.

0:43If we happen to be this young adventurer

0:46and we're going along on our path, we get to a point

0:49where there's a monster off in the distance.

0:51And we can say, if we have a sword,

0:55then go battle the monster.

0:57If we don't have a sword, then we just

0:58keep going straight through.

1:00So this is a conditional.

1:01We're going along the path.

1:02Hey, do I have a sword?

1:03Yes.

1:04Go battle the monster.

1:05Do I have a sword?

1:06No.

1:06All right.

1:07Then just keep going through, otherwise the monster

1:09will eat you.

1:10So if-- we'll just say yes or no.

1:12You can do it or not do it.

1:14But there's another step we can take.

1:16We could be traveling along this same path

1:18and we get to this point.

1:19And it says, if you have a sword,

1:22then go fight the monster, else go play LEGOs.

1:26And then when you're done playing LEGOs,

1:28then you continue on your path.

1:29So this gives you a choice where you

1:31have to do something at this point in your adventure.

1:35If you have a sword, you go battle the monster.

1:37If you don't have a sword, then you go play LEGOs.

1:40But the point is, this conditional statement

1:43can't just be bypassed.

1:45There's a condition where if this is true, then you do this.

1:48If it's false, then you do this.

1:50And there is no circumstance where you don't do anything.

1:53So you can have an if statement without an else.

1:55But if you add an else, all of a sudden you're

1:58doing one or the other.

2:00So this would be the "then" part of it,

2:02and then this would be the "else" part of it.

2:04And this works really easily in BASH.

2:06But it can be a little confusing as

2:08to how to frame these test conditions,

2:11like what the syntax in BASH actually is.

2:13So we're going to look at that on the command line.

2:15But I wanted you to understand conceptually what's

2:17going on when we make these choices in a program

2:20or in our adventure.

2:22Now, I learn best by doing, so let's create a script

2:25and learn how to create it along the way.

2:26First, I'm going to show you a little trick.

2:28If I type ls, you'll see we have no scripts in here.

2:31A lot of times what I'll do is I'll say echo.

2:33And then in single quotes, I'll put my shebang.

2:37And then I'll redirect that into a file called namegame.

2:42And then I'm going to chmod plus x, namegame.

2:47And now if I vi namegame, what I get here--

2:50the reason I did it on the command line

2:52is all of a sudden, since I've done

2:53that, now I get syntax highlighting, which

2:56is convenient.

2:57Otherwise, the first time it opens,

2:59if it doesn't see the shebang statement,

3:01we don't get syntax highlighting.

3:02So anyway, that's just one of the tricks that I do.

3:04But first of all, let's make a script that

3:08compares two different names.

3:10Now, I'm going to type this out and speed it

3:12up, but feel free to follow along and type it out as well.

3:15I didn't include it in here, because I

3:17wanted you to type it out.

3:19Something about physically typing out a command

3:21makes it stick into your head easier.

3:23So first of all--

3:32OK.

3:32So I typed this out.

3:33And basically, the logic goes like this.

3:35"My name is Shawn"--

3:36I set it to the string Shawn.

3:38And then it says "My name is Shawn.

3:39What's your name?"

3:40And then it reads the variable YOURNAME from the command line,

3:43so we'll have to type that in.

3:44And then it does this.

3:45It says if and then open brackets--

3:48inside here is the test condition.

3:50So when we say if, we say if this stuff

3:52inside these brackets is true--

3:55and the next line says, then.

3:57And it does everything after then, until it gets to fi.

4:01So fi, why fi?

4:02Well, it's just if backwards.

4:04I don't know why they've done it that way.

4:06So if and fi.

4:07That's how you can tell this is an entire if statement.

4:11So it's going to compare those names.

4:12Let's save this and execute it and see how it works.

4:16namegame.

4:17"My name is Shawn, what is your name?

4:19"Bill."

4:20Nothing happens.

4:21Let's run it again.

4:23namegame.

4:23"My name is Shawn, what is your name?"

4:25"Shawn."

4:26"We have the same name."

4:28It worked.

4:28Yay!

4:29Now unfortunately, at first when I said my name was Bill,

4:32we didn't get any response.

4:33And that's not any fun, really.

4:34So why don't we go back and edit this file?

4:37And we can add an else statement.

4:39Remember we talked about with the monster and the LEGOs?

4:41Let's add the LEGOs part of this.

4:43So we can say then echo that, else echo

4:49"we do not have the same name.

4:53Sad day."

4:54And so now, no matter what we type, we should get a response.

4:58Now I want to point out this indentation.

5:01BASH scripting doesn't care about indentation at all.

5:04I only indent here to make it more readable.

5:07So everything inside then-- because we

5:08can have more than one.

5:10"That is so cool."

5:14So everything in between then and else or then

5:18and fi if we don't have an else statement is executed.

5:20And I indent them just to make them more readable.

5:22But we could mash them all on the front.

5:24It doesn't really matter at all.

5:26All right.

5:26So let's save that.

5:28And then we'll play the name game again.

5:29namegame.

5:31"My name is Shawn, what is your name?

5:33"Bill."

5:33"We do not have the same name.

5:35Sad day."

5:36But what if my name is Shawn?

5:38"We have the same name!

5:39That is so cool."

5:40All right.

5:41So you can see logically what goes on.

5:44And we don't have to have it prompt us.

5:46Just like with our variable Nugget

5:48where we learn how to assign variables,

5:49this could be anything inside of our script

5:52that uses for comparative's sake.

5:54And speaking of comparative, that is where this came from.

5:58So the conditional tests.txt, I included it in this lab

6:02so we can look at it.

6:03And you'll see there are all sorts of things you

6:05can put inside the brackets.

6:08So if we're doing like comparison operators

6:11for numbers, we can use things like dash eq.

6:13And we would do it like this.

6:15We say this variable is equal to this variable.

6:20Now remember, BASH treats all variables as strings.

6:23However, if you use a numerical comparison

6:26like dash eq, dash ne for these numerical things,

6:29it will use numbers as integers.

6:32So you can assign numbers to these

6:34and it will actually compare them as integers

6:37if you use these types of comparisons.

6:40Now, we already did this.

6:41We already did a string comparison,

6:43where we said if it equals, then they are the same thing.

6:46But there are other things you can do.

6:47We can use equal.

6:48We can use not equal.

6:50We can use dash z.

6:51If the string is null, like if it doesn't contain anything--

6:55if we do a typo over here and like we

6:58use a variable that doesn't have anything assigned,

7:00then it would be null.

7:01And this test would do that.

7:03If we would do that, we would just say in brackets,

7:06dash z string1.

7:07Is string1 one null?

7:09And if it is, then it would be true.

7:10Is the string not null, using -n.

7:13And then, this is what I want to finish the Nugget

7:15with are these file tests.

7:17How this works is you can say dash a does the file exists?

7:21Dash e, does the file exist?

7:23These are the same, a and e.

7:25d.

7:26Does it exist and is it a directory?

7:29File exists and it's a regular file.

7:31It's not a directory or a symbolic link.

7:33The file exists and it is a symbolic link.

7:35So these tests are inside brackets

7:39and it's based on a file.

7:40Now, this is cool too.

7:41What we can test can be a file path,

7:43like the full path to the file, or it

7:45can be a variable that has the path assigned.

7:48Now, what do I mean by that?

7:49Let's make another script and we can see it in action.

7:52Now, I'm going to do the same thing.

7:53I'm going to say echo in single quotes bin/bash.

7:57We're going to call this file toggle.

7:59chmod plus x toggle.

8:02vi toggle.

8:03All right.

8:04So here we have this.

8:06Now what I want to do is I want to do some file tests.

8:09So let's assign a variable, MYFILE equals current directory

8:15forward slash thefile.txt.

8:19We don't have to say dot txt, but I did.

8:21So the variable MYFILE is assigned this.

8:25It's the path, the complete path to a file,

8:28current directory, thefile.txt.

8:30So our if statement now is going to be--

8:36So can you figure out what's going on here?

8:38We have we're going to set the name again of MYFILE variable

8:41to this path.

8:42We're going to say, if it exists, if MYFILE exists,

8:46then remove the file.

8:48Else, if it doesn't exist, touch my file.

8:51In other words, it's going to toggle creating and deleting

8:54depending on if it's there.

8:55If it's there, it's going to delete it.

8:57If it's not there, it's going to create it.

8:58So let's see how this works.

9:00We'll type ls.

9:01We do not have MYFILE or whatever,

9:03so we're going to say toggle.

9:05I'll type ls.

9:06Look, thefile.txt.

9:08It's right there.

9:09Let's run it again.

9:12Ha!

9:13It's gone.

9:14It deleted thefile.txt.

9:16We run it again, it's going to toggle.

9:18And sure enough, it's there.

9:20Run it again, it's gone.

9:22So what we've done is we've created a conditional statement

9:25that will toggle this file's existence on and off.

9:29And here is where it gets fun.

9:30You are in this lab.

9:32I want you to play with all of these things.

9:34See if you can figure out how to use a symbolic link.

9:37See if you can determine if a file is executable or not.

9:41So we could say, is this script executable?

9:43If not, then chmod it so it is executable,

9:46et cetera, et cetera, et cetera.

9:47There's all sorts of things you can do and play

9:49with these conditionals and comparisons

9:51and create your own scripts to do all sorts of things.

9:54Combine this knowledge with our variable usage,

9:57so we can do things like that name game that we just did.

9:59Anyway, this is a really great way to learn,

10:02and that is just to get in here and write your own scripts.

10:05And I gave you this cheat sheet, so you

10:07have a whole bunch of information

10:08on how to form these comparison or these conditional

10:12statements, so that you can get some awesome results out

10:15of your scripts.

10:16So your assignment is easy.

10:18Using the things that we've learned here,

10:20if, then, else, file tests, comparisons, et cetera,

10:23go play.

10:24I want you to play, play, play with BASH scripting in your lab

10:28file.

10:28In the lab, just play and do all those things

10:30so you get the hang of how to make those if statements.

10:33You don't have to have else, but you can have else.

10:36Play with having them or not having them.

10:37How does it work?

10:38How does it not work?

10:39Does something not work?

10:40And if it doesn't work, can you figure it out?

10:42That's the fun of learning and that's

10:43what I want you to do from here on out.

10:45I hope this has been informative for you,

10:46and I'd like to thank you for viewing.

0:00Now, sometimes when you're dealing

0:01with conditional statements in BASH,

0:03there's more than just a yes or no option.

0:06You have multiple conditions.

0:08In that case, there are several ways--

0:09there's multiple ways-- you can deal with it.

0:11You can either nest your if/then/else statements,

0:14or you could use the elif, which is like else if,

0:17or you can use a case statement.

0:19There's lots of ways to handle situations like that.

0:21We're going to quickly look at all of them.

0:23Because every situation, one, is going to make more sense,

0:26or you're just going to have personal preferences when

0:29it comes to scripting as to what makes the most sense.

0:31But first, let's get our scenario down.

0:34Here we have Dance Party 2017.

0:37And there were three people at this awesome dance party.

0:40There was Bob, Tom, and Martha.

0:43So our script is going to try to determine

0:46if the user of the script happened

0:48to be at Dance Party 2017.

0:51So just keep this in mind.

0:53There were three people at Dance Party 2017.

0:56And we're going to see if the people running the script

0:59happen to be one of the people at the party.

1:01So let's go to the command line.

1:03For this Nugget, I did leave the conditionaltests.txt in there

1:06for your reference while you're making your own script.

1:08But let's start out by using my trick--

1:10echo in single quotes, do the shaband.

1:16And then let's edit this party file.

1:19And we'll start out-- now, first of all,

1:21let's say what is this person's name.

1:23Because we want to figure out if they're at the party.

1:25So we're going to say echo, what is your name.

1:29And then we're going to read in the variable name.

1:32So we have that variable.

1:33Now we're going to say if name equals Bob,

1:40then you might have been at the party, Bob.

1:49Else echo, you were probably not at the party, name,

1:59unless you lied about your name.

2:04And then FI to close out that statement.

2:06So here's the deal.

2:07It says what's your name.

2:08And if your name is Bob, it's going

2:09to say you might have been at the party, Bob.

2:11But what if Tom or Martha put their name in there?

2:14Well, with what we know now, we're

2:16in trouble, because we can't say if name is Bob, Tom, Martha.

2:21So what we can do is add an or thing to this.

2:24So if we do space pipe pipe-- so the pipe symbol is above

2:28the Enter key there--

2:29we can add another condition that it has to meet

2:32or that it can meet.

2:33So or means this or that.

2:36We could actually do and and.

2:39And that means that these two conditions

2:41both have to be true.

2:42But I'm going to stay with or.

2:44And then we do another conditional statement.

2:47So name equals Tom space close bracket.

2:53This is something I want to mention.

2:54Notice I always put a space after here

2:57so there is a space between Tom and the bracket.

2:59The reason I do that is because BASH

3:01can be really picky about thinking that you were trying

3:04to compare it to Tom bracket.

3:07So it needs that space in there in order

3:10to really make sure that it knows

3:12that you're closing it off.

3:13So it's best to put spaces inside the brackets

3:16so that BASH doesn't get confused.

3:18Now, I'm also going to add another or.

3:20And I'm going to say or name equals

3:25Martha space, close bracket.

3:27So now the logic is if your name is Bob, Tom, or Martha,

3:33you may have been at the party, so and so.

3:35Otherwise, you were probably not at the party,

3:37so and so, unless you lied about your name.

3:39So let's see if that or statement works.

3:41We'll say party.

3:42What is your name?

3:43Mike.

3:44You were probably not at the party, Mike,

3:46unless you lied about your name.

3:48Well, Martha.

3:50You might have been at the party, Martha.

3:52How about Tom?

3:53You might have been at the party, Tom.

3:55How about Tommy?

3:57You were probably not at the party, Tommy,

3:59unless you lied about your name.

4:01So again, the computer doesn't know that Tommy and Tom

4:04might be the same person.

4:05But you can see how the logic works

4:07using the or statement for multiple conditions.

4:10Now, there is a different way we could have gone about this.

4:13So let's edit party, and let's say we didn't want to do these

4:16or conditionals.

4:21So here's our if your name is Bob,

4:23then you might have been at the party, Bob.

4:25But what we could do is we could nest a statement.

4:28So we could say else if name equals

4:34Tom, then echo you might have been at the party.

4:40Actually, here we don't even have to use the variable name.

4:43We could just say Tom.

4:44Because we know that this is only checking for the name Tom.

4:46So let's say you might have been at the party, Tom.

4:49So this is now a nested if statement

4:53inside the already-existing one.

4:56So we could say, if the name is Bob,

5:00then you might have been at the party, Bob.

5:02Else if your name is Tom, then, echo,

5:04you might have been at the party, Tom.

5:06But what about Martha?

5:08Well, then we need to actually do this.

5:09We need to add another line and say else

5:13if name equals Martha, then--

5:20you notice it's starting to get really, really complicated--

5:23echo, you might have been at the party, M Dog.

5:31OK.

5:31Does this make sense now?

5:33We say if your name is Bob, you might have been at the party.

5:35Else, if your name is Tom, then you

5:36might have been at the party.

5:37Else, from this if statement, if your name is Martha,

5:41then you might have been at the party, M Dog.

5:43And then we have to close off each one of those

5:45nested if statements with FI.

5:47Now, this should functionally do something very similar.

5:50And we'll test it very quickly.

5:51What's your name?

5:52Bob.

5:53You might have been at the party, Bob.

5:54What's your name?

5:56Tom.

5:56You might have been at the party, Tom.

5:58What's your name?

5:59Martha.

5:59You might have been at the party, M Dog.

6:02And what's your name?

6:03Mike.

6:04Nothing.

6:05Oh.

6:05Why did it do nothing?

6:07What is your name, Mike?

6:08Well, let's look at our script.

6:10We didn't do any sort of error or any other else

6:15with this one.

6:16So if it gets all the way to it, it says your name is Bob, no.

6:20Then if your name is Tom, no.

6:22Then if your name is Martha, no.

6:24But notice I didn't put an else say anything about you

6:26weren't at the party.

6:27So this incredibly complicated script isn't as powerful

6:31as that original one was.

6:32But logically, you can follow what's going on.

6:35And that's exactly what I wanted to get at.

6:37Sometimes, you can nest these if then statements just like this.

6:41Now, there is another way that you

6:42can nest the same information.

6:44Instead of saying if then else, if then else, if then else,

6:48what you can do is say if name equals Bob, then echo this.

6:52And here's how we can change it.

6:54We can say elif--

6:56just kind of combine this on one line--

7:00else if name equals Tom, then they

7:03might have been at the party.

7:04Elif name is Martha, then your name

7:08might have been at the party.

7:10And notice how these are highlighted in red.

7:12Else if, or elif, becomes part of that same initial

7:15if statement.

7:16So we don't need to close off all of these other

7:18if statements with FIs.

7:20So it's the same exact thing, but hopefully this

7:23is a little bit clearer.

7:24What's your name, name?

7:25If name equals Bob, then echo you might

7:27have been at the party, Bob.

7:28Else if your name is Tom, then echo

7:30you might have been at the party, Tom.

7:32Else if your name is Martha, then

7:34echo you might have been at the party, M Dog.

7:37And now we could add else echo didn't see you there.

7:44So we can say if, elif, elif, else.

7:47So you can have as many elifs as you want, and it just

7:50keeps going down the line, and then we close with else.

7:52So if none of those are true, then we still

7:54get didn't see you there.

7:56So let's save this--

7:58party, what's your name, Mike.

8:01Didn't see you there.

8:02What's your name?

8:03Tom.

8:03Might have been at the party, Tom.

8:05So it works just like the other one,

8:07and it's a little bit clearer than all that nested stuff.

8:11In fact, it's a lot clearer than that if then else,

8:13if then else, if then else, 5, 5, 5, 5, 5, 5, 5 at the end.

8:16So elif is a great way to do that.

8:18But there is one more way I want to cover--

8:20and then we'll be done-- and that is a case statement.

8:23And case means you take the results of something,

8:26and rather than if then else or if else elif, you say here's

8:31my variable.

8:32Match it up to one of the results below.

8:33So let's look at this again, VI Party

8:36So we're going to start out the same.

8:38We're going to say what's your name, read your name.

8:40And then I'm just going to get rid of all this,

8:42because it's going to look very different with our k statement.

8:46Now, I'm just going to type this entire section out,

8:49and then we'll cover it after the fact.

8:51Because there's a lot of text going on,

8:53and I'd rather type it out and then

8:54go through it by hand afterwards.

8:58This looks more complicated, but it actually

9:00makes it easier if you have a long list of options

9:02that you want to cover.

9:03So here we get the name assigned,

9:05the name of the variable is assigned.

9:06And then we say case name in.

9:09And then we say what we want this condition to be--

9:12Tom.

9:14And then we put a close parentheses.

9:16So this is the option close parentheses.

9:18What we want it to do-- it can be multiple lines.

9:21And then we stop it with semicolon, semicolon And then

9:23the next option--

9:24Martha with a close parentheses, the stuff

9:27we wanted to do there closed with the double semicolons,

9:30Bob, semicolons.

9:31And then at the end, rather than an else statement,

9:34this is called the default option, just asterisk.

9:37So if it's anything, then it does this.

9:40Because once it does--

9:42if Martha is it and it does this,

9:44it only executes this one, and then it goes to the end.

9:46It doesn't keep going down the list.

9:48As soon as it meets one of the criteria,

9:50it executes the thing in that section and then stops.

9:52So if it gets to the very end, it

9:54will execute this one, because this matches everything.

9:57Asterisk matches everything that might be left,

9:59and it says you weren't there, dude.

10:01And then just like the if and FI, we

10:03end a case statement with esac.

10:06Because esac is case backwards.

10:08So let's test this really quickly.

10:11What's your name?

10:12Tom.

10:13You weren't there, dude.

10:14Why does it say that?

10:15Because I put a capital O. Whoops.

10:17Party.

10:18Tom.

10:19Hi, Tom.

10:19You might have been there.

10:21Party.

10:22Bob.

10:23Glad you could make it, buddy.

10:24Martha.

10:25Hey, Martha.

10:26You can really dance.

10:27Oh, I actually meant you can really dance.

10:29But here, it said can you really dance.

10:31Anyway, it recognized Martha.

10:33And of course, Mike wasn't there.

10:35You weren't there, dude.

10:36So you can see that there are multiple ways

10:38to accomplish the same thing.

10:41But depending on your circumstance,

10:42it might be easier.

10:43So what I encourage you to do-- look at the script

10:45that we've done, the different scripts we've done in this,

10:48and make yourself a very complicated

10:50multiple conditional argument or statement,

10:53and see if you can make it work the way you expect it to work.

10:57Does that make sense?

10:58I want to see if you can make it-- not just

11:00our Bob, Tom, Martha scenario, but whatever scenario you have.

11:03Maybe it includes arithmetic.

11:05Maybe it includes $1, $2, $3 variables

11:09that you do right on the command line.

11:12Mix the things that you've learned

11:13and experiment so that you get the hang of how

11:16conditional statements work, even if they're

11:18nested or really long, multiple conditionals, et cetera,

11:22et cetera.

11:22And really, all there is left to do is play.

11:25Just go in and try to do all of the things

11:27that we demonstrated.

11:28Do multiple conditionals.

11:29Do nested if then elses.

11:31Do elif statements.

11:32See how many you can string together.

11:34Do a case statement if that makes more sense to you.

11:36A lot of these things are different ways

11:38of doing the exact same thing.

11:40However, when you understand how they all work,

11:42you can make the best decision for whatever script

11:45you happen to be writing.

11:46I hope this has been informative for you,

11:48and I'd like to thank you for viewing.

0:00Now, the whole idea of using bash scripts on your system

0:03is to make repetitive tasks easier

0:05to automate the process of doing the same thing over and over.

0:09And that's where loops really come into play,

0:11if you need to do something more than one time.

0:13Now loops are really cool.

0:14In this Nugget we're going to talk about while loops

0:16and until loops.

0:17They are very, very similar, except that they're completely

0:20the polar opposite.

0:21I'll show you what I mean when you get to the command line.

0:23But the cool thing is loops like while and until work

0:26on conditional statements.

0:27So it'll keep looping until conditions are met.

0:30You can also increment things.

0:31It's possible to make endless loops which you may want to do

0:34but sometimes-- actually usually--

0:36endless loops are an error, so it's

0:38important to know how to get out of those.

0:40But let's go right to the command line

0:42and make some loops so that we can see how beneficial they

0:44can be to our scripting world.

0:47Now first I want to demonstrate an incremental loop.

0:49So let's create a loop or let's create our file.

0:53We'll call it loopy.

0:54And let's edit that.

0:56All right so inside here we have our file.

0:58Now what I want to do is have a script

1:00that will count from 1 to whatever we input.

1:02So I'm going to start out by saying

1:05x equals 1 all right so that's our variable to start with, x.

1:10Now I want to say echo "Give me number."

1:15Read "Num."

1:16Echo "Cool, I can count to Num, check it out."

1:23Now we need to create our while loop.

1:25So how we do that it was to say while and then open brackets.

1:28Just like an if conditional statement,

1:30this is going to be a conditional statement.

1:32So while this stuff in the brackets is true.

1:35So while Num is g.t.

1:38greater than $x, "do".

1:42So it's going to repeat this loop as long as number

1:46is greater than x.

1:48All right so first let's start out.

1:49What are we going to do?

1:50First, we're going to say echo x.

1:53Next, we have a command that we haven't used yet.

1:56It's called "let".

1:57Now let is a way to assign a variable using mathematics, OK.

2:00And I'm going to show you what I mean.

2:02We can say let x equal x plus 1.

2:07All right so let x equal what it is plus 1.

2:11Now if we were to just assign--

2:13if we were to just say x equals x

2:15plus without the let statement, it

2:17would equal the string x plus 1.

2:19And that's not what we want.

2:20We want it to do the math.

2:22So we have to use this let statement.

2:24So let x equal x plus 1, done.

2:27Now I don't think this is going to work quite right, let's see.

2:30So we'll save this file.

2:32Let's run loopy.

2:33And it says give me a number, we're going to say 10.

2:36Cool I can count to 10, check it out.

2:381,2,3,4,5,6,7,8,9.

2:42What went wrong?

2:42Well this is where troubleshooting comes into play

2:45and we can see what happens.

2:46It starts with x equaling to 1.

2:49OK, and then it says x equals x plus 1.

2:53When it gets to 9--

2:54when it says echo 9, it says let x equal x plus 1.

2:57So then x equals 10, it goes back up to here

3:00and it says while 10 is greater than 10.

3:04See it's not anymore, so it never prints out 10.

3:07Now we could solve that.

3:09We could say all right, you know what?

3:11x equals 0 and let's do this.

3:17Let's say let x equal x plus 1 before we echo.

3:24And now let's see if that makes sense logically.

3:26So it starts out as zero.

3:27It says cool, I can count to 10--

3:29if we put in 10-- check it out.

3:31And it says while 10 is greater than zero, OK,

3:34so it's going to go through.

3:35It's going to add 1 to it so it's going to say 1.

3:38And then when it gets to 9--

3:40So echo 9-- it goes through the top.

3:43It says while 10 is greater than 9 sure enough.

3:46So then it's going to add 1.

3:47It's going to say echo 10.

3:48So by switching this around we should

3:51have corrected our error.

3:52Let's find out.

3:53Loopy, give me a number 10.

3:551,2,3,4,5,6,7,8,9,10.

3:57Sure enough it counted all the way to 10 just

3:59by switching around our logic.

4:01And that's really important when you're

4:03doing these conditional statements

4:04to think through your logic.

4:06Now the other kind of conditional statement

4:08I want to show you.

4:10We'll use the same script, we'll just edit it out.

4:12I didn't mean to do that, I can count to two.

4:15Vi loopy.

4:16All right, so the next thing I want to do

4:18is instead of using arithmetic--

4:20Let's get rid of all the stuff in here--

4:23I want to use a text comparison.

4:25So let's play a game and we'll set some variables here.

4:30We'll say best equals pizza.

4:34And then we'll say echo "Can you guess the best food?"

4:40Read "Guess".

4:43And now let's do a loop.

4:45So it's going to enter in this variable guess

4:47and we're going to say maybe somebody puts in chicken.

4:50Well now we're going to say I want

4:51you to run the following loop until they guess correctly.

4:55So until-- open a bracket, this is a conditional statement

4:58just like an if statement.

5:00So until guess is equal to--

5:03Now you can use one equal or two.

5:05When I'm comparing things I like to use two

5:07equals because other programming languages generally do that,

5:10but you can use one or two equals in this case--

5:13best.

5:14So until guess equals best, I want you to do the following.

5:18I want you to say echo "Guess again"

5:21and then read "Guess" done.

5:24And then once they do finally guess it, echo "Yay!

5:28You guessed the right answer!"

5:31So let's see if we can follow this logic.

5:33Well first of all we said that pizza is the best food.

5:36Then it says "Can you guess the best food?"

5:37It asks for a guess and that initial guess is given a value.

5:44And then it says it's going to run the loop until guess

5:49is equal to best.

5:50Now if they guess on the first one.

5:53If they type in pizza here it's never

5:55going to run this loop at all because it's

5:58going to say I want you to run this until these two equal

6:01each other.

6:01Well if they equal each other initially

6:03it's never going to execute this loop at all.

6:06It's just going to go right down to here and say "Yay!

6:08You guessed the right answer!"

6:09However, if they guessed wrong it's going to start this loop

6:12and it's going to keep going through these two things

6:15until they match.

6:17Does that make sense?

6:18Hopefully it does.

6:19Let's see how it works.

6:20Save this file.

6:21Say loopy "Can you guess the best food?"

6:24Chicken.

6:25Guess again.

6:25Fish.

6:27Turkey.

6:27No.

6:28Pizza.

6:29"Yay!

6:29You guessed the right answer!"

6:31So you can see it just keeps doing the loop where

6:33it says guess again, guess again, guess again, until they

6:37enter the exact same string that we

6:40set at the beginning of that script.

6:42Hopefully that makes sense.

6:44Now you may notice I showed you how to do

6:45while loops and until loops.

6:47We learned about how conditionals

6:48work just like they do in the if statement

6:50and we learned how to increment using the let command.

6:53I did not mention endless loops or I didn't go over that.

6:57That's what you need to do.

6:58Go in and see if you can make a loop that never ends.

7:02It just keeps going, and going, and going, and going.

7:05And what you need to do when that happens

7:06is hit Control C on your keyboard.

7:09That's the magic.

7:11Control and then the C key will break any endless loop

7:15that's going on.

7:16You may have to hit it a couple times if it's

7:18like echoing over, and over, and over it might not

7:20realize you hit control-c.

7:21So just keep doing control-c, control-c, control-c.

7:24Eventually it will stop the script and you'll get back

7:27and then you can edit it and see what you can fix.

7:29Again, you are assigned to this as your task.

7:33Try to create an endless loop but play

7:35with all these conditionals and incrementing

7:37and all that sort of stuff.

7:38It's fun to play with loops.

7:39While loops and until loops are pretty much

7:42the exact same thing, except one functions while it's true

7:46and one functions until it's true.

7:48And hopefully that makes sense.

7:49I hope this has been informative for you

7:51and I'd like to thank you for viewing.

0:00The for loop is a really cool loop

0:02when it comes to using bash loops

0:03because it doesn't use conditional based testing.

0:07So it's not like it's as if this is true,

0:09then stop, or if this is true, then keep going.

0:11You can do a bunch of things with a for loop.

0:13You can use sequences of numbers.

0:15So you can say, do a loop 10 times,

0:18and it'll go 1 through 10.

0:19And you can also do-- and this is really what makes it

0:22special--

0:22an unrelated set of data, you can turn into a loop.

0:26So what a for loop does, is it takes a set of data--

0:29and again, it could be a sequence,

0:30or just a random chunk of data, file name, stuff like that--

0:33and it loops through every single possibility

0:36in your set of data.

0:38So what do I mean by that?

0:39Well let's say that we have two different types of things.

0:41We have a filing cabinet here filled

0:43with files that are numbered from 1 to 1,000.

0:46With a for loop, you can say, all right.

0:48I have 1 through 1,000 files, and I

0:52want you to then go one, two, three, all the way to 1,000,

0:56and then do something to every single one of those.

0:58And that we could accomplish with a while or until loop.

1:01The cool thing is, when you take a situation like this and say,

1:04OK.

1:05On my wall, I have a pair of pliers, a crescent wrench,

1:07a hammer, and a saw, and I want you to do something

1:10with everything on the wall.

1:11Well even if you don't want to go through and do it one by one

1:15by one, you can say, take this whole set of data--

1:17whatever it happens to be--

1:19and then go through every item on the list

1:21and do something to that.

1:23So it means that you're no longer restricted

1:25to just numbers or a conditional test

1:27for text-- you can actually take any set of data,

1:30and do some task to every item in that set.

1:33So let's go to a command line and look

1:35because it's a lot easier once you get the feel for what's

1:38going on, and it's so powerful.

1:41It's just awesome.

1:42All right-- first thing, let's create our file.

1:44So like I normally do-- echo, single quotes, the shebang.

1:48Then bash, into "for loop," we'll call it.

1:51And then we're going to chmod that, plus x, so that we

1:54can execute it directly.

1:55All right, now let's open this up and create our loop.

1:59Now the format for a for loop is "for x in,"

2:04and then whatever the data set is.

2:06The data set can be anything-- it

2:08can be a sequence of numbers, it can be files.

2:10So let's do it simply-- say 1, 2, 3, 7, 8, 9, do, echo,

2:18number, x, done.

2:21So this is the form, and let's look

2:22quick before we execute it.

2:24So "for x in," and what it does is

2:26it takes whatever data set you specify after this,

2:29and it can just be a data set typed out,

2:31or it could be a range, which we'll do next,

2:33but we'll say "for x in," and then what it does

2:35is it assigns every data item to the variable,

2:39and it goes through and does it like that.

2:41So what we should see is, number 1, number 2, number 3,

2:44number 7, number 8, number 9 as it goes through.

2:47And notice how it starts and stops each loop.

2:50We can put anything in between do and done,

2:52and those are the things it's going

2:54to do every single iteration in the dataset.

2:58So let's save this file, and execute it, and we should see--

3:02sure enough--

3:04number 1, 2, 3, 7, 8, 9.

3:06Now we don't have to spell each one out.

3:08We can use a range, and how we would do that is--

3:13get rid of this-- and now we're going

3:14to say, open the curly braces.

3:17Say 1 dot dot 10, for example, close the curly braces,

3:22and now this is going to do this for a set range.

3:26So it's going to, instead of typing 1 space, 2 space, 3

3:29space, 4 space, 5 space--

3:31this is going to do the same thing,

3:32but it's going to give us a range of 1 through 10.

3:35And quickly look at this, we'll save the file,

3:37we'll execute for loop, and we should see number 1 through 10.

3:40Sure enough, number 1, number 2, number 3,

3:42all the way through number 10.

3:44And if you have version four or later,

3:46which you almost certainly do of bash,

3:49there's this other cool thing.

3:50We can do a step counter of something here.

3:54And now this looks a little confusing,

3:55but it's doing the same thing.

3:57It's saying a range of 1 to 10, but I

4:00want you to count by twos.

4:03All right, so the range is still the first part,

4:05but then another dot dot, and then the step counter

4:08means what you count by.

4:10By default, it'll count by one, but we're

4:12going to say count by two.

4:15Now if we do this and save it, it goes 1, 3, 5, 7, 9.

4:20Now why didn't we see 10?

4:21Well because it counts by two, so it says it starts with 1,

4:25and it says adds 2, 3, adds 2, blah, blah, blah.

4:27So we get to 7, it adds 2, gets to 9, it adds 2,

4:31and it's 11, which is no longer in our range from 1 to 10.

4:36So even though we said go from 1 to 10, when we count by twos,

4:39we never get to 10, so we don't see number 10 here.

4:43We just see 1, 3, 5, 7, 9.

4:45And that's how the range variable works.

4:47But we don't just have to use numbers,

4:50which is really, really cool.

4:52All right, let's open this up again.

4:54And now we can go here, and instead

4:57of typing numbers or a range so it calculates that,

5:01we can say fish, chicken, puppy, tuna.

5:05So this is just a list of things--

5:08again, it's a dataset--

5:10a number of things in our dataset.

5:12But it's not a range, it's not a number-- this just

5:14happens to be four strings, and it's

5:17going to loop through this set of data for us.

5:20Now it's no longer going to be a number.

5:21Let's say, "look it's a," and hopefully, it

5:25should say, look it's a fish, look it's a chicken,

5:27look its a puppy, and look it's a tuna.

5:29Let's see if that's what happens.

5:31Sure enough, look it's a fish, look it's a chicken,

5:33look it's a puppy, and look it's a tuna.

5:35So you don't have to use a range,

5:38and it's not incrementing.

5:39Also remember it's not based on a conditional-- it's not

5:42saying, keep doing this until we're out, or keep doing this

5:46while we still have things.

5:48It just takes the entire set, and goes

5:51through every item in that set, and will

5:53do whatever is inside the loop.

5:56Now here's the cool part.

5:57We don't have to just type it in.

6:00Remember there was a range?

6:01Well just like we can set numbers with a range,

6:04we can also get our set of data from an external command.

6:09What do I mean by that?

6:10Well, there's two different ways we can do this.

6:12The one that we've talked about in the past--

6:14remember the back tick?

6:15So if we did back tick ls, and put that-- and back tick again,

6:19so ls is inside back ticks.

6:21What it's going to do inside of our bash script is replace ls--

6:26anything inside the back tick--

6:27with the results of the command in there.

6:30Now you could also--

6:31just so you know, this is kind of important-- sometimes

6:33with weird characters, you have to do it another way.

6:36You could also do dollar sign and then

6:39the command in parentheses.

6:41This is the same thing as putting it in a back tick.

6:43I personally like the back tick-- for some reason

6:46it's easier for me to remember.

6:48So I'm going to say ls in the back tick.

6:50And now, instead of say "look it's

6:53a," we could say "I see in our current directory,"

6:59and it should go through every item in our ls command

7:02and print it to the screen.

7:03It should use the results of ls as our data set.

7:07Well first of all, let's type ls and see what's in here.

7:09We have all of the folders, and then the file, forloop.

7:13So let's see what happens when we execute this.

7:16It says, I see in our current directory desktop,

7:18I see in our current directory documents.

7:20I went through all of them, including our file itself

7:23because it's in our current directory.

7:25And it says, I see in our current directory,

7:27all of these things.

7:28So you can actually get your data set from any command

7:32out there.

7:33And whether you're doing iteration,

7:35or whether you're doing a range, or you just

7:37have a bunch of things on there, the for loop

7:39is extremely powerful, and an awesome way

7:42to go about doing things.

7:43Now a real world example that I might do

7:46is use a for loop to get a bunch of IP addresses.

7:49So first of all, let's do if config,

7:51see what range we're in.

7:52172.20.0.94, so the subnet mask is this--

7:58so we have the range of 172.20.0.1 through 255.

8:03Does that make sense?

8:04That's the network that we're currently in.

8:06So let's see if there's any other hosts on our network.

8:09Now we can do that--

8:10well there's a bunch of ways we could do that,

8:11but what I want to do is use a for loop to do it.

8:13So I'm going to say, for x in, and then I'm

8:17going to say the range of 1 dot dot 255.

8:22So that's the range that it's going to go through.

8:25And now, let's say, echo pinging 172.20.0.x.

8:35And then I don't want it to hang when it can't find something,

8:38so I'm going to use the timeout command-- this

8:40is just something else you can play with.

8:42I'm going to say timeout 0.5.

8:44So what this is going to do is, it's

8:46going to attempt to ping for 0.5 seconds.

8:50One ping, 172.20.0.$x and we should see it try to ping every

8:59item in this range.

9:01172.20.0.1, 172.20.0.2, all the way to 255 by just writing

9:08these couple of lines of code.

9:10Now this is just something you can

9:11play with-- you don't have to do a ping,

9:12you can do whatever you want in here.

9:13But if I wouldn't have put the timeout part here,

9:16it would have waited until it timed out on every single IP

9:19address, and I didn't want that.

9:21So the timeout command basically just

9:23runs whatever you put after it for however

9:25long you tell it to.

9:26So it's going to attempt for 0.5 seconds

9:28to send one ping packet to this address.

9:32Let's see how it works.

9:33I haven't tested this, so hopefully my memory

9:35has served me properly.

9:36So we'll say for loop--

9:38and sure enough, it's going pretty quickly.

9:40OK, it's trying to ping everything-- it's saying,

9:42I'm pinging this, and then it shows

9:43us the results of the ping command.

9:45So far, I haven't seen any ping response.

9:48Hopefully 0.5 seconds isn't too fast for the timeout command.

9:52Hopefully, when we get to something,

9:54it'll actually show us.

9:56OK, I snipped off a couple seconds there

9:58so we can see-- we're getting close to our own IP,

10:00hopefully we'll get a response.

10:01Ah!

10:02There we go, we got one.

10:03So it found our address, and we got that ping reply.

10:06So far, that's the only computer on our network,

10:08and I guess that makes sense.

10:10It's going to be the only computer on our network

10:12apart from the repo server over here.

10:14So you can let it go through the entire range

10:16and see if there are any other computers on your network.

10:19And again, this isn't the most efficient way

10:21to ping on your network, but it's a good example

10:23of what a for loop can do.

10:25It's done a lot of typing for us by creating these IP addresses

10:29and feeding it into the ping command.

10:32So conceptually, they're a little bit different

10:34because, again, it's not based on conditional statements,

10:37right?

10:38We don't test to see if the loop should continue.

10:40It actually just takes a complete set of data,

10:44whether it's a sequence of numbers

10:45or unrelated set of data, like a list of files,

10:49or whatever the results from a command that you put in,

10:52and it just does a loop that many times.

10:54It goes through the entire set of data

10:56and executes, between the do and the done statements,

10:59everything that you've done.

11:00And it's amazingly powerful, and something

11:03that I use a surprising amount of time

11:06while I'm being a system administrator.

11:08So the for loop.

11:09Awesomely powerful, I urge you to play with it

11:11in the lab environment-- come up with some new scenarios for how

11:15you would use a for loop in real life.

11:17I hope this has been informative for you,

11:19and I'd like to thank you for viewing.