Advanced Debugging

Introduction

As Bash scripts increase in complexity—handling background processes, trap signals, and complex mathematical logic—simple execution tracing (set -x) is sometimes not enough. You need advanced debugging techniques to introspect the state of the shell dynamically.

1. Using the caller Built-in

If you have a complex script with dozens of functions calling each other, and a deeply nested function fails, it can be difficult to figure out how execution reached that point.

The caller built-in command returns the context of any active subroutine call (a stack trace).

#!/bin/bash

func_c() {
    echo "An error occurred in func_c!"
    # Print the line number and calling function
    caller 0 
}

func_b() {
    func_c
}

func_a() {
    func_b
}

func_a

When this script runs, caller 0 will output the line number in func_b that called func_c, making it much easier to trace the execution path.

2. PS4 Prompt Customization

When you use set -x for execution tracing, Bash prints a + sign before every executed command. You can customize this prefix by modifying the $PS4 environment variable to include incredibly useful debugging information, such as the script name, function name, and line number!

#!/bin/bash
# Modify PS4 to show the script name, line number, and function name
export PS4='+(${BASH_SOURCE}:${LINENO}): ${FUNCNAME[0]:+${FUNCNAME[0]}(): }'

set -x # Enable tracing

my_func() {
    echo "Inside my_func"
}

echo "Starting script"
my_func

Now, the execution trace will look like:

+(script.sh:11): echo 'Starting script'
+(script.sh:12): my_func(): echo 'Inside my_func'

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