Bash

🐞 Bash Debugging & trap Command

Debugging is crucial when your script isn’t behaving as expected. Bash provides built-in tools like set -x, trap, and line tracing to help identify bugs.

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🛠️ Enable Debugging with set

set -x

Print each command and its arguments as they execute.

#!/bin/bash
set -x

name="Kashif"
echo "Hello, $name"

Output:

+ name=Kashif
+ echo 'Hello, Kashif'
Hello, Kashif

set +x

Turn off debugging.

set -x
echo "Debugging"
set +x
echo "No debug"

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⚙️ trap – Capture Script Events

Syntax:

trap 'commands' SIGNAL

Common Signals:

Signal	Meaning
EXIT	When script exits
ERR	On any error (use with set -e)
INT	Ctrl+C interrupt
DEBUG	Before every command

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🧹 Cleanup Example (trap EXIT)

#!/bin/bash
trap 'rm -f /tmp/tempfile' EXIT

touch /tmp/tempfile
echo "Working..."
sleep 3
exit 0

Even if the script fails or is killed, /tmp/tempfile will be removed.

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🧨 Catching Errors (trap ERR)

#!/bin/bash
set -e
trap 'echo "❌ Error on line $LINENO"; exit 1' ERR

cp file.txt /nonexistent/dir

Output:

❌ Error on line 5

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🔎 Line-by-Line Trace (trap DEBUG)

#!/bin/bash
trap 'echo "Running line: $BASH_COMMAND"' DEBUG

echo "Line 1"
ls /invalid/path
echo "Line 3"

Useful for deep debugging.

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🧰 Combine All: Debugging Template

#!/bin/bash
set -euo pipefail
trap 'echo "Error on line $LINENO: $BASH_COMMAND"; exit 1' ERR
trap 'echo "Finished script"; exit 0' EXIT
trap 'echo "Interrupted by user"' INT

echo "Starting work..."
# Simulated error
cp file.txt /root/

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📌 Best Practices

Practice	Benefit
Use set -euo pipefail	Safer scripts
Trap ERR and EXIT	Cleanup and error logs
Debug with set -x	Trace issues easily
Use logging functions	Record errors in a log file

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📚 More Resources

• trap command explained
• set command options
• Bash Debugging Guide

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📁 File: advance/debug-trap.md
Author: Kashif Alam

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