How to debug bash scripts with two tools

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Bash scripts have you ever in a funk? Learn how to begin the method of debugging these scripts, with the assistance of instruments that work seamlessly along with your code.

Learn how to debug bash scripts with two straightforward instruments
Bash scripts have you ever in a funk? Learn how to begin the method of debugging these scripts, with the assistance of instruments that work seamlessly along with your code.

For a lot of Linux directors, bash scripts are a must-use. These helpful scripts assist make Linux probably the most versatile working programs on the planet. There’s little or no you can not do with a well-written script. However what occurs once you’ve rigorously constructed that script, solely to search out it does not run as deliberate? One thing is amiss. If the script is prolonged and sophisticated, you may need a heck of a time monitoring down the problem(s).

Thankfully, there are many instruments accessible that can assist you debug your bash scripts. One such instrument is ready x. This explicit debugging instrument means that you can mark in a script the place you need to focus your debugging efforts. With set x you’ll be able to both debug a whole script, a piece of a script, or a single line. A second helpful instrument means that you can log output of script to a file. This is available in fairly helpful, particularly when your script is longer than regular (otherwise you need to debut the output later).

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Let’s check out how these two instruments are utilized in a easy bash script.

What you want

The one factor you want for this can be a Linux distribution. Most each distribution will include the required instruments, nevertheless, the distribution should embrace bash model 5.zero or higher. Yow will discover out which model of bash your distribution consists of with the command bash —model. As a result of we’re working with the command line, it does not matter in the event you use a desktop or a server distribution.

The script

The essential script we’ll use for this take a look at is:

#!/bin/bash

var="Howdy World"

# print it
echo "$var"

# One other means of printing it
printf "%sn"http://www.techrepublic.com/"$var"

echo "Your property is: $HOME"

That is a reasonably simple script. Create a brand new file with the command nano hiya.world with the above contents. Save and shut the file. Give the brand new file executable permissions with the command:

chmod u+x hiya.world

Now execute the script with the command:

sh ./hiya.world

You need to see precisely the output you count on (Determine A).

Figure A

Determine A: Our bash script output.

After all, there’s nothing mistaken with this script, however it makes for a superb instance.

The modified script

Now we will modify the script, to incorporate our debugging instruments. The primary is the logging entry. Beneath #!/bin/bash, add the next:

exec 5> debug_output.txt
BASH_XTRACEFD="5"
PS4='$LINENO: '

The primary line names our debugging file, debug_output.txt. The second line tells bash the place to write down its xtrace info (which can be declared with set x). Lastly, the third line will print out line numbers, to make our debugging even simpler.

Subsequent, we have to use set x to point which code we need to debug. To illustrate the echo $HOME part of the script is working as anticipated, whereas the echo $var is not. Let’s encase that part with set x so we are able to solely debug that portion of the script. We declare the start of the part to be debugged with set -x and the tip with set +x. So the brand new part of code will appear to be:

set -x

var="Howdy World"

# print it
echo "$var"

# One other means of printing it
printf "%sn"http://www.techrepublic.com/"$var"

set +x

Save and shut that file.

Now run the sh ./hiya.world command. This time round, the output is sort of completely different (Determine B).

Figure BFigure B

Determine B: Our debugged output.

As you see, each line of command begins with a : character, and the output generated by the traces of code instantly follows and doesn’t embrace the : character. This makes it far simpler to debug what’s going on.

In case you do not discover any errors within the code that you just marked with set -x and set +x, transfer these traces to a different part and run the command once more.

Debugging through log file

In case you’re coping with a bash script that’s too lengthy for normal output, you may need to check out that debugging log file we declared. Nonetheless, you may discover that file empty. Why? As a result of we have run the script with the assistance of the sh shell, and the sh shell is not appropriate with the BASH_XTRACEFD command. As an alternative of working the script with sh ./hiya.world, merely problem the command ./hiya.world. It will use bash, as a substitute of sh, and our debugging output can then be seen (Determine C) with the command:

much less debug_output.txt
Figure CFigure C

Determine C: Our debugging output learn from the file.

As a result of we have written our script in such a means, we are able to:

  • Run the script with sh ./hiya.world to see our debugging info despatched to plain output (which implies it’s printed within the terminal window after you run the command).
  • Run the script with ./hiya.world to have our debugging info written to the debug_output.textual content file.

You may additionally discover, in the event you run the script with the command ./hello_world, the debugging info will not be despatched to plain output, and can look precisely because it did in Determine A.

Child steps

And this can be a quite simple method to start the method of debugging your bash scripts. To many, this can seem to be child steps, however when you get the dangle of utilizing these instruments, you may discover them a useful means to debugging these scripts.

Additionally See

http://www.techrepublic.com/http://www.techrepublic.com/

Picture: Jack Wallen

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