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.

```bash
#!/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.

```bash
#!/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.

```bash
# 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.

```bash
# 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-s