bash-defensive-patterns

Bash Defensive Patterns

Comprehensive guidance for writing production-ready Bash scripts using defensive programming techniques, error handling, and safety best practices to prevent common pitfalls and ensure reliability.

When to Use This Skill

• Writing production automation scripts
• Building CI/CD pipeline scripts
• Creating system administration utilities
• Developing error-resilient deployment automation
• Writing scripts that must handle edge cases safely
• Building maintainable shell script libraries
• Implementing comprehensive logging and monitoring
• Creating scripts that must work across different platforms

Core Defensive Principles

1. Strict Mode

Enable bash strict mode at the start of every script to catch errors early.

#!/bin/bash
set -Eeuo pipefail  # Exit on error, unset variables, pipe failures

Key flags:

• set -E: Inherit ERR trap in functions
• set -e: Exit on any error (command returns non-zero)
• set -u: Exit on undefined variable reference
• set -o pipefail: Pipe fails if any command fails (not just last)

2. Error Trapping and Cleanup

Implement proper cleanup on script exit or error.

#!/bin/bash
set -Eeuo pipefail

trap 'echo "Error on line $LINENO"' ERR
trap 'echo "Cleaning up..."; rm -rf "$TMPDIR"' EXIT

TMPDIR=$(mktemp -d)
# Script code here

3. Variable Safety

Always quote variables to prevent word splitting and globbing issues.

# Wrong - unsafe
cp $source $dest

# Correct - safe
cp "$source" "$dest"

# Required variables - fail with message if unset
: "${REQUIRED_VAR:?REQUIRED_VAR is not set}"

4. Array Handling

Use arrays safely for complex data handling.

# Safe array iteration
declare -a items=("item 1" "item 2" "item 3")

for item in "${items[@]}"; do
    echo "Processing: $item"
done

# Reading output into array safely
mapfile -t lines < <(some_command)
readarray -t numbers < <(seq 1 10)

5. Conditional Safety

Use [[ ]] for Bash-specific features, [ ] for POSIX.