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In this lesson you're going to be introduced to positional parameters and the for loop here on my local

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system I have a terminal open and I'm going to move into our class working folder.

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And from here into our vagrant project folder this project being local users and then now I'm going

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to start the virtual machine and then connect to it.

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Now we'll move into the shared folder of CD4 slash vagrant and this script I'm going to name all user

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Dymo 0 6 dot sh.

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Of course all scripts we start with the shebang and so will give the path to the bash interpreter.

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And then I'm just going to list the goal of this script or what this script does at the top here as

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our Hetter

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and the first thing we're going to do here is simply display what the user typed on the command line

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and we can do that with the echo command here.

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The variable of dollar zero is actually a positional parameter and positional parameters are variables

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that contain the contents of the command line.

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Before we go any further let's talk about the definition of a parameter versus an argument.

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A parameter is a variable that is being used inside the shell script.

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An argument is the data passed into the shell script.

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So an argument is applied on the command line becomes the value stored in a parameter.

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With that in mind the very first positional parameter is $ zero which contains the name of the script

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itself.

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You can think of this as being what the person typed in the command line to execute the script.

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The next positional parameter is dollar sign one which stores the value of the first argument passed

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to the script on the command line.

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The positional parameter dollar sign to source the second argument dollar sign three stores a third

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argument and so on.

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So now let's look at the output of our script so far.

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Of course we have to add executable permissions before we can execute our script so we'll do chmod

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and then execute the script.

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So notice how dollar signs 0 expanded to how we executed the script.

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This dot board slash in the script name.

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Now let's change how we call the script by providing the full path to the script so we can type forward

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slash vagrant user Dymo 06 and hit enter So again when the script is called in an explicit way like

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this then that is what is stored in dollar signed zero so dollar signs zero appears exactly as we have

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typed on the command line here.

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We're about to take a slight detour for a moment.

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Bear with me here because this will all tie into positional parameters.

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But again it's going to take several minutes to explain this.

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So again stick with me and be patient with me.

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So one thing I've glossed over is the command search path which is stored in the environment variable

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path that's ph th in all uppercase letters.

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Up until this point we've just been supplying either a fool or relative path to the scripts we've created.

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Let's take a quick look at what the bash man page has to say about the PATH variable.

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Before we move forward we'll do a forward search with a forward slash followed by Path.

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We'll just keep hitting in until we get to the PATH variable.

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And here it is.

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It says that path is a colon separated list of directories in which the shell looks for commands.

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Now if you were to look at the command execution section that it references here on your screen you'll

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find that every time you type something in it the command line.

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Bash first tries to find a function with that name and execute it.

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Now we haven't covered functions yet but don't worry we will.

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So if it doesn't find a function with that name it looks for that command and it's a list of built in

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commands.

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If it is a shell builtin command then it executes that.

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Now you've been working with several built in commands such as Echo read type help and so on.

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Now if the command you type at the command prompt or on the command line is not a function or it's not

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a built in then bash searches through the list of directories defined in the PATH variable and executes

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the first match that it finds if no match is found then a command not found error message is displayed.

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So let's get on the main page here by typing queue.

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And let's look at our path Ecko dollar path.

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So in our particular case here if a command is not a function or a shell built in then bash will look

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for that command first and user local bin then end user bin then it will look and user local Esben and

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so on down the list.

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It's important to point out that the path can be changed.

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So it's not something that is guaranteed to be the same for every user across every system.

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So keep that in mind by the way if you want to control the path you can do so in your script by manipulating

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the PATH variable there.

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Let's use a neat little command called which to tell us which command will be executed.

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Let's take the head command that we've used before as an example which Ed.

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So this tells us that when we type and head at the command line the program located at user bin head

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will be executed the first directory in our path variable is actually user local Ben.

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So let's create our own shellscript named head and place it in there we'll need root privileges to do

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that.

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So I'm going to create the file with Sue them user local been had and then here we're just going to

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create a little shell.

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Hello from my head and then we'll go ahead and save it.

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We'll add the executable bit to it here.

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Now let's see which head will be executed so I'll type in which head.

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And it says user local bin head will be executed.

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Let's use the dash option to which which forces it to print all matching executables that are in our

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path.

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So there are the two commands named the exact same thing.

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Both are named head.

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But the first one that is found is executed.

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So let's go ahead and just execute this command type and head and hit enter.

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Sure enough our version of the head command is executed.

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Now if you want to override the default then just supply the full path so we can do that with user been

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had let's say if we would just want the first line of the ETSI password file so execute that.

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So this is what we've been doing all along which is explicitly defining a path to a command.

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Actually to our scripts.

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Let's get rid of the version of our head so we'll just you say to our M User local then head here is

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one little quirk with rapidly adding and removing programs from your path.

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BASH uses a hash table to remember the full path names of executable files.

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It looks up the location to the executable once and then remembers it.

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If you look at the type head command which will do here type read it says that head is cached to user

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local bin head.

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That simply means that bash will execute that command at the path without first searching for that command.

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So here's the quirk.

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BASH thinks that head is still located at user local been had held for we just deleted it just a few

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seconds ago.

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So let's execute the command and see what happens.

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So if we type in head and hit enter and says user local bin had no such file or directory.

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So if you run into this type of issue you can make bash forget all these hashed locations by using the

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hash built in with hash dash are.

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Now if we do type head we can see that it searches the path and it finds the one that really is there

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and user bin.

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Of course you can read about the hash a shell built in with the command help hash and there you can

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see the dash r option that we use says to forget all remembered locations.

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Now typically hashing is not a problem because commands don't usually just randomly appear and disappear.

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During this same shell session and probably not during the execution of a shell script.

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In any case it's just something to be aware of.

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OK so I know that was a rather long aside but let's bring this back around to positional parameters

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and specifically about dollar signs zero.

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First let's prove that our script is not in our current path.

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We can do that with a which command.

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Which user.

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Demo 0 6.

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And sure enough there is no user demo 6 in our path and it lists our path there.

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Use your local bin user bin and so on.

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Now let's take a copy of our script and place it into our path.

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By putting it into user local bin and we can do that was soooo sue for copy and then provide the path

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to the directory want to copy it to and hit enter.

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Now our script is in our path so we can execute it without supplying a path to the script.

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Now notice what's stored in dollar sign 0.

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It's the full path to the command which is user local bin l user Dasch demo 06 dot sh.

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Even though we only typed in l user dasht him 06 not SH at the command line bash.

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Found it in our path and executed it at that path.

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And that's why the path and the file is stored in dollar signs zero.

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So let's review what gets stored in dollar signs 0 in different situations so we'll do that forward

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slash l user Dymo 6 so dot slashes what is executed and that's what's in dollar signs 0.

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If we're explicit again and provide a full path such as the Ford site a vagrant user Dymo 6 that will

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be reported as well.

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However if a command is our path and we execute it like so then it reports the full path to that command.

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Hey before we move on let's remove the script out of our path before we continue so will do Sea-Doo

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r.m user local BNL user demos 6. sh.

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There's a command called basename that removes the directory component and just returns the file name.

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So let's read up about that command man base name here at the top of your screen you can see that basename

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strips out the directory and suffix from filenames.

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So let's see what happens when we execute this.

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We'll just hit Q to get all the help page there and we'll type base name vagrant user demo 6. SH and

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then it just returns just the file name without the directory portion of the file.

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By the way basename isn't doing any smart checking to see if the file exists.

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It just strips off the directory portion of a string that is passed to it so we can give the base name

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something that clearly doesn't exist.

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But it will still do its job so we can do.

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Base name port slash not forward slash here.

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And of course it reports here because that's the file name portion of that path to that file even though

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the file doesn't exist and we can prove that with ls not here.

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Sure enough there's no such file or directory counterpart.

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The basename command is D-I our name so let's look at the man page for that.

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This says that Dhia are named strip's the last component from the file name.

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Let's get you to exit the main page and execute the command Symbol type Diyar name.

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They grant all user demo 6 and hit enter and forward slash vagrant is returned because that is the directory

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name or the directory portion of that full path like basename D-I our name doesn't do any smart checking.

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We can give it a non-existent path and it will simply return the portion that looks like the directory

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so we can do this idea or name.

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This is not here.

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Sure enough it returns.

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This is not because it looks like the here file is in the directory.

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Ford slash this forward slash is for Slash not.

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But of course that file and directory that whole tree doesn't exist but Deeya our name doesn't check

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it doesn't care.

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It just returns the path or the directory of the file again.

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Obviously this works on things that do exist but it doesn't matter to the Diyar are named command or

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base named command.

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Now let's use these two commands in our script.

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So what we're going to do is display the path and filename of the script

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in earlier lessons you use a dollar sign followed by a command that was wrapped in parentheses to assign

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the output of that command to a variable that is called command substitution.

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In this case we are only going to use the output of each of these commands once so we just included

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them right in the echo command.

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Again with this dollar sign parentheses syntax does or this command substitution syntax does is return

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the output of the command.

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You can assign the output to a variable or use that output without assigning it to a variable.

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Either way is fine.

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Perhaps assigning the output of the commands to a variable first makes for a bit cleaner code.

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But I just wanted to show you that you don't have to use command substitution and variable assignment

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only.

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You can use it in line as well.

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So let's save an execute our script and see what we get here.

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The first line says you executed this command which is dollar signs 0.14 Lachelle user dasht him 0 6

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that sh.

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You use dot as the path to the user Dasch demo 6. sh script.

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So Dot was the output of the Dhia our name command and l use your dash Dymo 6 that SH was the output

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of the basename command.

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So let's try executing this script in another way will provide the full path forward slash vagrant Elisir

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Dymo 6. Sh.

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So this time it lists Ford slash vagrant as the path and then of course the base name of the script

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is l user Dasht demo 06 dot sh.

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OK let's look at the bash man page real quick.

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And we're looking for the special parameter section of the page here and then we'll keep hitting and

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to continue to our next match and then the thing I want to point out here is the pound sign and the

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pound sign it says expands to the number of positional parameters in decimal.

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So a dollar sign pound sign tells us how many arguments were supplied on the command line.

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Let's continue with our script here.

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Menus being v to execute the most recent command that starts with the V which is a VM so I can get back

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into my editor really quickly here.

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So let's tell the user how many arguments they passed in.

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Remember that the curly braces are optional so some people might actually just use dollar sign hash

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sign.

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So just be aware that if you're reading other people's scripts but I'm willing to use the curly braces

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here to be very clear.

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Of course here you could have simply used the dollar sign hash sign directly in the echo command similar

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to what we did with the command substitution in the previous echo command.

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It's really a style choice.

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I just wanted to see how you can do it both ways.

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So the first time I didn't use a variable this time I used a variable and then used the value of that

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variable in the echo command.

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So let's save our changes and then do some testing here.

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So we'll just execute our script says you supplied zero arguments on the command line.

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So let's just type something in after the command line or just type hello and it says we supplied one

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argument on the command line and will type in another word.

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You supply two arguments and so on.

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So this one says you supplied three and that is giving us the proper information so it looks like Dollar

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Sign hash sign works and we're all good there.

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So let's continue editing our script.

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Because the real goal of this little example shell script that we're working on is to generate a random

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password for each user that's supplied on the command line.

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We want to make sure that at least one user is supplied on the command line.

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So let's do a check with an if statement.

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So use if space double opening brackets space when we use a variable and between brackets here we're

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going to enclose it in quotation marks and the variable we're going to look at is number of parameters.

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And then we want to make sure that that's Dasch L T which stands for less than one.

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So if the number of parameters is less than 1 then what we're going to do is echo a usage statement

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much like you would see in a man page

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instead of manually typing in the name of the script.

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We can just simply use dollar sign 0 to echo back more or less what the user typed in and the command

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line and then we'll tell them what they need to do.

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They need to supply a user name and optionally they can supply another user name and actually optimally

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they can supply even more usernames so we'll use the ellipsis there.

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Now if they get to this point that means that the script did not successfully execute because they didn't

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give us any people to create passwords for.

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So what we're going to do is exit with a non-zero exit status and I'm just going to use one here.

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Those are if statement with f I.

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Now by the way let's look at the first line here again.

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If the number of parameters is less than 1 then bubble block.

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So here what I'm actually doing is just demonstrating something that we haven't used before just a slight

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variation of Dash l t.

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And where did I get that from a hopefully remember that you can get help on these tests by looking at

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help test at the command line.

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Of course you'll find in there that Dasch LTE stands for less than of course what is the only thing

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that's less than one in our particular situation working with whole numbers here on the command line.

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Negative numbers don't make sense because you can't supply negative arguments and this sorts of thing.

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So obviously the the only thing that's less than 1 is zero.

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So you could have said if the number of parameters equals zero then give them the usage statement and

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exit.

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So again either way is right.

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It's just your personal preference how you think what makes sense to you.

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It's style again both ways work just whatever comes to mind just use that.

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Again I'm using this here so you can see some slight variations in different ways to do this.

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Now let's see if our changes and start passing in varying number of arguments to our script.

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We'll hit enter.

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We did not use any arguments on the command line and we get our usage statement and if we check the

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return code with dollar sign question mark or you can actually do this dollar sign question mark with

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the curly braces then sure enough we get an exit status of one from our script.

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Since it didn't successfully execute all the way.

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So let's get back to our script and let's just give it one thing sure enough you supplied one argument

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to let's do a b c d ok that's for arguments and I want to point out something here that we used actually

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with the user add command in an earlier script which is enclosing things in quotation marks so a b c

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d.

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So this says that you supplied three arguments on the command line but maybe at first glance you think

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that B and C are two different things but in reality because they're enclosed in quotation marks they're

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treated as one argument.

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Let's do this again let's see this one.

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This whole thing is two.

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And here is three.

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So again in this instance we supplied three arguments on the command line.

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Another thing you can do is simply enclose everything in quotation marks and that's fine as well.

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I'm just going to use singlehood marks because that works to Dujuan.

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This is to and this is theory.

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So again three arguments on the command line which interprets to three positional parameters inside

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the script.

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There are three because they are enclosed in quotes so it doesn't split at spaces when the data passed

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in is enclosed in quotation marks.

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Now again the idea behind this script is that we're going to perform the same action over a list of

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items.

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Now this is the perfect time to use a for loop.

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Let's take a second and look at the four shell built in the type dash.

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A command says for is a shell keyword.

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So that's a shell built in.

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And so it can do is get help for.

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I know there's going to be a good bit of information that pipes up to the screen so I'm just going to

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pipe this.

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The head command to get just the first 10 lines of output.

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Hey the first line of the help says for name and words do commands.

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Done the name you see in the help here is actually a variable name for each item in the list of words.

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The variable name is set to that item.

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Then the commands are executed and the process is repeated for each item in the list.

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Let's create a loop right here at the command prompt will do for x x is the name or our variable name.

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And and we're going to provide a list to do this.

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Frank Lear and Doug The next thing we want to do is type in do.

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Hit enter and then it's stylistically appropriate to indent so I'll just put a couple of spaces here.

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Echo type hi and then refer to the variable name here.

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X and yet inner and close are for loop with done.

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In this example x is first assigned to Frank then the command of Echo gets executed using that variable

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dollar sign X. Then the next time the loop repeats player is assigned to x.

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The commands execute.

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And finally Doug is assigned to x and the commands execute.

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If I hit the up arrow here to pull up the command you can see the semi-colons which represent the command

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separators.

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So if you want to you can place this command all on one line but it's easier to read if you do not do

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that.

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Again it's valid syntax here.

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Like you see at the Help for on your screen.

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But again it's just easier to read if you put these on multiple lines when I hit enter again to execute

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that again.

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If you look near the top of your screen in The Help output for the four key word it says if in words

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is not present then dollar sign as sign is assumed.

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So let's find out what dollar sign at sign is by looking at the bash man page.

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I'm going to look for the special parameters here and the main page.

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Here's the section.

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Now let's start with the at sign special parameter.

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It expands the positional parameters starting from 1 when the expansion occurs within double quotes.

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Each parameter expands to a separate word that is dollar sign is equivalent to dollar sign one dollar

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sign too and so on.

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Now this is nearly the same as Dollar Sign asterisk.

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It also expands to the positional parameters starting from 1.

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However if it's enclosed in double quotes it expands to a single word.

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We don't want to treat all the parameters as a single word what we want is a list of words as they were

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supplied on the command line.

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That's why we're going to use dollar sign at sign and not dollar sign asterisk in our script.

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By the way the use of dollar sign asterisk and dollar sign at sign are often confused.

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OK let's get out of the main page here with Q and get back to editing our script.

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So we're going to use a for loop will do for will supply a variable name we'll call these usernames

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user underscore name and use quotations here.

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Dollar Sign at sign do.

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So we're going to use the command we use to generate passwords in the previous script.

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So we'll do that with pass word equals well use command substitution with dollar sign followed by an

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opening parentheses

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and then we're going to display that user name and password.

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Now obviously we can't know exactly how many positional parameters a user will attempt to supply at

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the command line.

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We can check for them but we just don't know in advance.

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So instead of you know generating a password for dollar sign one dollar sign two dollar sign three and

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so on.

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Even though they may only supply two arguments for example then we're going to do is use dollar sign

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at sign.

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So no matter how many arguments they pass into the script it is covered by dollar sign and sign so we

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don't have to guess what the user is going to do.

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They can supply one to three or ten or more.

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And this script is going to work just fine.

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Well just supply one thing on the command line and see what happens.

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So sure enough the for loop executed one time generated a password and displayed it for the user.

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Let's add another user on the command line.

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Let's said some more actually.

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So by using the for loop in conjunction with dollar sign outside and we were able to account for everything

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passed in on the command line just for demonstration purposes I want to show you what the differences

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between dollar sign and sign and dollar sign.

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Asterisk.

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So let's just go back into our script and make that change.

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So here I'm going to change the at sign to an asterisk save our changes and then execute our script

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again.

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So as you can see it tweeted everything on the command line as one argument.

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So the for loop only ran one time and generated a password for that one argument.

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Again that's the difference between dollar sign asterisk and dollar sign at sign.

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Let's go back and fix our script here and change this asterisk to the at symbol here.

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Save our changes and check our script.

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And sure enough it runs like we want it to.

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Let's take just a minute to recap this lesson and by the way we covered a lot here.

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First you learned about positional parameters and how.

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Dollar Signs zero expansive the name of the shelf skip and dollar sign.

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One expands to the first argument supplied on the command line dollar sign to expands to the second

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argument and so on.

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You also learned a few different special positional parameters such as Dollar Sign pound sign which

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represents the number of parameters.

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Dollar Sign asterisk which expands to the list of positional parameters starting at dollar sign one

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and dollar sign at sign which also expands to the list of positional parameters starting a dollar sign

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one.

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However the difference between an asterisk and dollar sign and sign is only noticed when enclosing them

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in quotation marks.

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That difference is that dollar sign Asterix treats all the positional parameters as a one string and

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dollar sign and sign treats them as separate words.

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We also took a slight detour in the middle of the lesson to talk about how the path shell variable is

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used to store a list of Colon's separated directories that make up the command search path.

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You also saw how the path variable can affect what commands get executed and why that is along the way

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you were introduced to the which command as well as the base name and or name commands.

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Finally you learn how to create a for loop both at the command line as well as in a script.