Wildcard Introduction Part II

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Bash Scripting and Shell Programming
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11
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Overview
Difficulty
Intermediate
Duration
20m
Students
7
Description

This course will introduce you to wildcards and show you how to use them on the command line. Since shell scripts are really nothing more than a collection of command lines, this should prove to be really helpful to you. After that, we'll get into specifically how to use wildcards and shell scripts, and you'll watch a demo to give you a practical understanding of wildcard scripting.

Learning Objectives

  • Obtain a foundational understanding of wildcards in Linux
  • Understand how to use wildcards and shell scripts

Intended Audience

This course is intended for anyone looking to learn more about bash scripting and shell programming in Linux.

Prerequisites

To get the most out of this course, you should have some basic knowledge of the command line, but it's not essential.

Transcript

Let's do an ls to get our bearings in and see what files and directories we're working with. Let's use the asterisk or star and match all the files that end in .txt, ls *.txt. What just happened is the shell performed a globbing operation to expand *.txt and it passed those results to the ls command. In this example, running ls *.txt is the same as running ls ab.txt, a.txt, b.txt and songs.txt. Of course you can clearly see it's easier to use a pattern like *.txt than it is to type out all the files by hand. I just wanted to be clear about what is happening when you use wildcards. Now, let's look at all the files that start with the letter a. How would you do that? That's right, type in ls a*. Now, let's find all the files that start with an a and end in dot txt. We can do that by typing in ls a*.txt. Can you see a way that we can make the wildcard even simpler? How about this? Ls a*t and that gives us the same results. Let me clear the screen here and reset our bearings, look at what we're working with. Now, let's use the question mark wildcard. Let's match all the files that are exactly one character in length. We'll do that by running ls ? If you want to display all the files that are two characters in length, then just use two question marks. Each question mark represents a single character. So we'll do ?? and we'll see that all the two character files and directories are returned. In this case they're just files. Let's list all the files that start with an a then contain exactly one character and end in .txt. To do that, we'll run ls a followed by a question mark which represents exactly one character and then .txt. Let's check out the difference between the question mark and star. Let's run ls a*.txt The star matches zero or more characters so running ls a*.txt matches the file named a.txt. Before we continue, I wanna point out that you can use the same flags and options as you normally do with a command while you're using wildcards. Let's get a long listing of all the files that start with the letter a. So, just like we normally would use with ls to get along listing we'll use -l and then we'll use a* to get every file and directory that begin with the letter a. Go ahead and clear the screen again here. Here's an example of using a character class. Let's match all the files that start with the letter c then contain a vowel and end in t. To do this, we'll run ls c[ and all the characters we want to match with a, e, i, o, u and then t. And I wanna point out too that it doesn't matter what order the characters appear in in the bracket. So we can use e, i, o, a, u and we'll get the same results. Let's do another one here, still get the same results. Let's use a range. This range will match all the files that start with an a, b, c, or d. To do that, we'll do ls [a-d] to represent a through d and then star. Here's an example of using a named character class. This command will return a list of files that end in a digit. I've simply been using the ls command to demonstrate how to match files using wildcards but let's do a little bit of actual work with wildcards. Let's clear our screen and run ls again to see what's in our directory. Let's move all of the text files into the directory named notes. To do this, we can use the mv command, move, and then we'll use *.text to match all the text files and we'll end with the directory name, which is notes. And let's see if that worked. Okay, I'm not seeing any of the files that end in .txt, let's look in the notes directory and see if they're there. And indeed they are. Let's move all of the mp3 files into the music directory. We can do this in a couple of different ways. One way would be to run mv*mp3 and specify the songs directory. We can also use *3 since that would match and we can even use a character class like star and then digit. I'll go ahead and just use mv *.mp3 to perform the match. Let's look inside the music directory, and we can see that the two mp3 files that matched our search at mp3 expression were moved into the music directory. Let's use wildcards to delete some files. Here's a good pattern to follow. Test your wildcard pattern with the ls command before you use that pattern with the rm command. This way you can hopefully avoid deleting any files that you do not intend to remove. Let's remove all the files that are two characters in length. We'll check our wildcard pattern here to make sure it returns what we want to do. And it looks like it does, so we'll go ahead and run rm using that same expression. And now the files that matched that pattern are gone. Today, you learned how to use wildcards. You learned that the asterick matches zero or more characters. You also learned that the question mark matches exactly one of any character. You learned about character classes and how you can match any character that is included between the brackets. You were taught about ranges and how you can create your own custom ranges by starting with a character, following it by a hyphen and then ending it with another character. Finally, you learned about the various named character classes and how they can be used to match common sets of characters such as digits, upper case letters and lower case letters.

About the Author
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Jason Cannon
Founder, Linux Training Academy
Students
304
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40
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Jason is the founder of the Linux Training Academy as well as the author of "Linux for Beginners" and "Command Line Kung Fu." He has over 20 years of professional Linux experience, having worked for industry leaders such as Hewlett-Packard, Xerox, UPS, FireEye, and Amazon.com. Nothing gives him more satisfaction than knowing he has helped thousands of IT professionals level up their careers through his many books and courses.

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