Looping in BASH
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Video Transcription
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>> Hello, cybrarians and
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welcome back to the Linux+ course here at.
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Cybrary. I'm your instructor Rob Gills.
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In today's lesson, we're going to be
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talking about looping in BASH.
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Upon completion of today's lesson,
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you're going to be able to understand
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the purpose of looping in
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BASH and differentiate between
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all types of loops that we could use in BASH.
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Then during our demo at the end of the lesson,
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we'll see how to use loops in our BASH scripts.
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When we're writing scripts, sometimes we
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need to repeat a command multiple times,
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or maybe we need to iterate over file content,
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and this is when the concept of looping comes in handy.
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Now, in BASH,
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there are two loop types,
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there's the for loop and then there's the while loop.
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What we're going to do is take a look at
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both types of loops in this lesson.
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The for loop iterates through
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every single element and
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that could be in a list or series.
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For example, we can look for every file in
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directory and then do
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something upon every file in directory,
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or we could go through every line in
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a text document and then
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perform some action on each line.
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In general, the syntax that we're going to
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use when we're using a for
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loop is for items in
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that series or element in a list or whatever,
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do commands and then done.
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We could also write this as a BASH shell one-liner.
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We can say for items in series;
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do command; done.
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For example, if we wanted to rewrite
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our script that looks for conf files in etc,
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we can write that as one-liner.
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We can say for file in and
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then les etc grep star dot conf.
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What we're doing is we're listing all of
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the contents of etc,
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but we only want to return
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the contents of etc that end dot conf,
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in other words, the dot conf files.
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What we're doing is we're assigning
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that content to the file,
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if the file is the variable name that we're using,
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and then what we do is
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>> we do echo on that file variables.
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>> We're doing variable expansion and then done.
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The end result is it just displays the name
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of every file that ends dot conf,
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just like our scripted previously,
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but we just do it in one line.
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Now, the while loop continues
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as long as a condition that
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specified at the opening of the loop evaluates as true.
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What that basically means is it usually will do
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a while loop like while
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something is less than something else,
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and then at the end of every
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>> iteration through the loop,
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>> we'll increment that by one.
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Eventually we'll reach the point where
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something is no longer less than somebody else.
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We'll say while condition equals whatever,
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do some commands and then done.
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We can say, while one is less than five, do something,
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and then we'll say, add something
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to one until we get to five,
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and it'll loop through that condition
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every time until we get to five.
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This can also be rewritten as a BASH one-liner.
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We could do while condition and
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then do commands and then done.
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Let's take a look at some loops with some demo time.
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Here we are back in our demo environment in Ubuntu
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and let's take the one kind of loop,
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at least in a new script.
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I've already created a file here called
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loops.sh script file and
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so what we're going to do is open this up.
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Now, we can see as per usual,
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we have our interpreter here,
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we have our shebang and then bin BASH,
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indicating that we're using BASH.
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Then we've written little couple of
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comments here indicating what this does.
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Now, what we're going to do is we're
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going to go ahead and loop through
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a folder and we'll ask
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the user which directory do we want to search in,
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and we'll read that information in as search directory.
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Then what we'll do is we'll take
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that variable search directory and
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we'll change directory into that search directory,
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and now we're going to run this forward.
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What we'll do is we'll say for object in ls sort.
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We're going to list all the contents of
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this search directory that
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>> we're already in and sort it,
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>> and then we're going to hand that back to this object.
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This is the variable that we're going to
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use throughout the rest of the script.
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We'll say for object in
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the sorted list of contents in
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the search directory, do something.
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Now, we're going to use this object everywhere
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else is our variable inside of the script down here.
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Now, we're going to do
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some traditional tests and we'll say
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if this object is a file,
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because remember we have this file test,
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the dash f,
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indicates that something is a file.
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We say if the object is a file,
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then echo object is a normal text file.
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Or we could test,
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we can say else if or lif the file is a directory,
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because remember dash d is the file test for directory,
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so we say else if the file is a directory,
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then we say echo object is a directory.
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If we don't know what it is,
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we can say Rob isn't sure what object it is.
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We close this if statement by
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doing the f remember is just if spelled backwards,
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and then we close the for loop by saying done.
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This an example of where we're doing
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looping and we're using
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>> conditionals inside of our loop.
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>> I want it to do that here so we can see
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how both of those could be used with
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each other in order to write
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a really cool script that does something pretty basic.
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Now, let's go ahead and just get out of this file.
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I'll hit "Escape" 'Colon" "W" "Q"
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>> and now we can call it.
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>> We'll call loops.sh. Now,
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we can say, well, what directory you want to search in?
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Well, it's certainly my home directory.
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I know I have permissions to it on
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off becomes pseudo or anything.
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We'll just say searching home
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rob and now what we'll see is
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it'll look through all of the files
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and indicate what they are.
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It'll say Alpha sort is a normal text file,
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Beta is a directory, so on and so forth,
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and we could verify that this is true.
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We could just do an ls on home rob.
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Now, we can see Alpha sort well that is
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a text file apparently
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been Z is a directory sent file is
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a text file and so on and so
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forth all the way down the line.
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With that, we've reached the end of this lesson and in
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this lesson we covered the purpose of looping in BASH.
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We talked about the types
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of loops that are used in BASH,
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and then we covered using loops in
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BASH scripts during our demo at the end of the lesson.
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Thanks so much for being here and I look
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forward to seeing you in the next lesson.
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