Debugging Bash scripts

Debugging Bash Scripts: Techniques and Tools

Bash scripting is a powerful means for automating tasks on Linux systems. However, like any programming endeavor, Bash scripting comes with its fair share of bugs and unusual behaviors. Debugging Bash scripts can sometimes seem more art than science, but there are tools and techniques that can systematically simplify the task. This blog aims to explore some essential debugging techniques for Bash scripts and provide operating instructions for popular package managers to set up debugging environments.

Understanding Bash Script Debugging Options

Before diving into external tools, it's valuable to leverage the built-in Bash options for debugging:

set -x: This command enables a mode of the shell where all executed commands are printed to the terminal. It's incredibly useful for seeing what's happening in your script in real-time.
set -e: This option causes your script to stop immediately if a command exits with a non-zero status (which generally indicates an error).
set -u: This triggers an error message when the script attempts to use unset variables, which is a common source of bugs in scripts.
set -o pipefail: This option sets the exit code of a pipeline to that of the rightmost command to exit with a non-zero status, or to zero if all commands of the pipeline exit successfully.

These options can be enabled by adding the following line at the start of your scripts:

set -euxo pipefail

Debugging Tools

For more complex debugging, you might need more than just Bash’s built-in options. Here are two powerful external tools you can use:

ShellCheck

ShellCheck is a static analysis tool for shell scripts. It can find issues in your scripts that may lead to errors or unexpected behaviors. It checks for syntax errors, deprecated practices, and common semantic issues.

Installation Instructions:

Debian/Ubuntu (apt):

sudo apt update
sudo apt install shellcheck

Fedora (dnf):
```
sudo dnf install shellcheck
```
openSUSE (zypper):
```
sudo zypper install shellcheck
```

To use ShellCheck, simply run:

shellcheck yourscript.sh

Bash Debugger (bashdb)

Bashdb is a debugger for Bash scripts that offers features similar to those found in professional programming languages' debuggers, like setting breakpoints, viewing stack traces, and stepping through code.

Installation Instructions:

Debian/Ubuntu (apt):

sudo apt update
sudo apt install bashdb

Fedora (dnf):
```
sudo dnf install bashdb
```
openSUSE (zypper) does not have bashdb available directly from the official repository. You can still install it by compiling from source or using alternative repositories.

Once installed, begin debugging with:

bashdb yourscript.sh

Practical Tips for Debugging

Start with Simple Tests: Rerun your script with different parts commented out to isolate areas of interest.
Check Assumptions: Use echo statements to print out variable values and the states of different conditions throughout the script.
Unit Testing: Consider writing small "unit tests" for sections of your scripts where you simulate input and check outputs.

Conclusion

Bash script debugging may require a combination of built-in techniques, external tools, and a structured approach to testing. Remember, every bug fixed is an opportunity to better understand scripting and enhance your problem-solving skills. With the above tools and techniques, you're well-equipped to tackle most issues you'll encounter in the world of Bash scripting!

Happy scripting and debugging!