Shell Script Debugging

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Debugging Essentials
Overview
Difficulty
Intermediate
Duration
20m
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98
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Description

In this lesson, you will learn about the options built into Bash that will help you find and fix errors in your shell scripts.

Learning Objectives

  • Learn a range of commands used or debugging
  • Use variables to control the behavior of your shell scripts and help you in debugging
  • Learn how to manually debug a script
  • Understanding syntax highlighting and how that can help you avoid common mistakes
  • Learn about a special Bash variable that you can use to give you valuable information about what is happening inside of your scripts
  • Learn about the differences between Windows and Linux file types and the problems you may encounter if you plan to use multiple operating systems to create scripts

Intended Audience

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

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

In this lesson, you will learn about the options built into Bash that will help you find and fix errors in your shell scripts. A software bug is an error in a computer program that causes it to produce an unexpected or incorrect result. Most of the bugs are actually mistakes in the program's code or in its design. If you encounter a bug or an error in one of your scripts, you'll want to see exactly what is happening during the execution of that script. Maybe something isn't working as you initially anticipated and you want to figure out where things are going wrong and how you can update your script so that it performs as expected. Sometimes your script will produce an incorrect result or behave in unintended ways. Sometimes it will simply stop because of a syntax error or a typo. This process of finding errors in your script or fixing unexpected behaviors is called debugging. The Bash shell provides some options that can help you in debugging your scripts. You can use these options by updating the first line in your script to include one or more of these options. The most popular of these options is the -x option. The -x option prints commands and their arguments as they are executed. This means that instead of variables being displayed, the values of those variables are displayed. The same thing goes for expansions. Wild cards aren't displayed but what they expand to is displayed. You'll sometimes hear this type of debugging called print debugging, tracing or an x-trace. If you're using this option in a shell script simply add -x to the end of the shebang line. If you want to do this at the command line, run set, space, -x. Use set, space +x to stop this debugging behavior. You can also use this option for just a portion of your script. Just before you want to start displaying the commands to the screen, add a set -x line. Place set +x on a line after the section of the shell script that you're debugging. Again, set, space, -x will start the x-trace while set, space +x will stop the x-trace. Here's a very simple example that demonstrates the use of the -x option. You can see that -x has been added to the end of the shebang. At the bottom of the screen is the output you'll see when you run this script. You'll notice that there are lines that start with a plus sign. Those are the commands that are being executed from the script. In this example, there are two commands that are executed. The first is setting the value of the TEST_VAR variable. The second command is to echo that value to the screen. In the output, the result of the echo command, which is test is displayed. There is no plus sign in front of it because it is output as result of a command and not a command itself. Here's another example. This shows how you can turn debugging on for just a portion of your script. First, the TEST_VAR variable is set. You'll see in the output that nothing is listed for this command because debugging isn't on at this point. The next line turns onto bugging with a set -x command. Again, nothing is displayed in the output for this action. Now the TEST_VAR is echoed to the screen. The echo command is displayed because the bugging is now on. Of course the output of the echo command is displayed next on the screen. We use set +x to turn off debugging, that said command is displayed on the screen. Finally, the host name command is executed and only its output is displayed. Again, this is to demonstrate how you can encapsulate a block of code with set -x and set +x to debug that particular section of code. You can do the exact same thing with the other options that we'll be covering next. Another useful option that can help you in finding errors in your scripts is the -e option. It causes your script to exit immediately if a command exits with a non-zero exit status. Remember that an exit status of zero indicates the successful completion of a command and any exit status other than zero indicates some sort of error. This can really help you pinpoint exactly where the problem is. You can use this in combination with other options including the -x option. When used as an argument to the Bash command, these options act like any other options for other commands. Options that do not take arguments can be combined and only one hyphen is required followed by the options. Also, it doesn't matter in which order they're used so we can use -ex or -xe. If you want, you can use a hyphen before each option but this is unconventional. Here's an example using the -e option. First of value is assigned to the variable FILE_NAME. Next, the ls command is executed using FILE_NAME as an argument. Finally, the contents of the FILE_NAME variable are displayed to the screen. However, when you execute this script the ls command returns a non-zero exit status since the file doesn't exist. Because the -e option was used the execution of the program halts and the echo command is never attempted. This time, we'll use both the e and x options. The x option causes the commands to be displayed on the screen. First it displays the creation of the FILE_NAME variable. Next it displays the ls command. You can clearly see what ls is doing in this example. It's trying to display information about a file named /not/here. Of course, this causes an error and the script stops because of the -e option. Yet another useful option that can help you in the debugging process is the -v option. It prints the shell commands just like they are read in from the script.

 

- v prints everything before any substitutions and expansions are applied. The -x option performs a variable and wild card expansion, but the -v option does not. You can use them in combination to see what a line looks like before and after substitutions and expansions occur. Here's an example of the -v option. It causes every line of the script to be displayed to the screen before it's executed. You'll notice that the lines are exactly as they are in the shell script. In this script, the only thing that happens is that test gets echoed to the screen. Here's what would happen if we added the x option to the previous script. What's useful about this output is that we can see how a command looks in the shell script and how it actually gets executed. Take the echo command for example. We can see in the script that we run echo $TEST_VAR and that actually causes echo tests to be executed. From the command line using a Bash shell, you can run, help set. This will display information about the options we covered in this lesson plus other options that are available. You might wanna pipe this output to a pager like less so you can easily scroll through all the options and their descriptions. In this lesson, you learned how to use a few different built-in Bash options to aid you in debugging shell scripts. You learned about the -x option which prints commands as they will be executed. You learned how to exit your script if it encounters an error by using the -e option. Finally, you learned how to display the commands in your shell script exactly as they appear by using the -v option.

About the Author
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Jason Cannon
Founder, Linux Training Academy
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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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