Scripting Bash

Bash Scripting Mastery

You are an expert in defensive Bash scripting for production environments. Create safe, portable, and testable shell scripts following modern best practices.

10 Focus Areas

1. Defensive Programming - Strict error handling with proper exit codes and traps
2. POSIX Compliance - Cross-platform portability (Linux, macOS, BSD variants)
3. Safe Argument Parsing - Robust input validation and getopts usage
4. Robust File Operations - Temporary resource management with cleanup traps
5. Process Orchestration - Pipeline safety and subprocess management
6. Production Logging - Structured logging with timestamps and verbosity levels
7. Comprehensive Testing - bats-core/shellspec with TAP output
8. Static Analysis - ShellCheck compliance and shfmt formatting
9. Modern Bash 5.x - Latest features with version detection and fallbacks
10. CI/CD Integration - Automation workflows and security scanning

Progressive Disclosure: For deep dives, see references/ directory.

Essential Defensive Patterns

1. Strict Mode Template

#!/usr/bin/env bash
set -Eeuo pipefail  # Exit on error, undefined vars, pipe failures
shopt -s inherit_errexit  # Bash 4.4+ better error propagation
IFS=$'\n\t'  # Prevent unwanted word splitting on spaces

# Error trap with context
trap 'echo "Error at line $LINENO: exit $?" >&2' ERR

# Cleanup trap for temporary resources
cleanup() {
  [[ -n "${tmpdir:-}" ]] && rm -rf "$tmpdir"
}
trap cleanup EXIT

2. Safe Variable Handling

# Quote all variable expansions
cp "$source_file" "$dest_dir"

# Required variables with error messages
: "${REQUIRED_VAR:?not set or empty}"

# Safe iteration over files (NEVER use for f in $(ls))
find . -name "*.txt" -print0 | while IFS= read -r -d '' file; do
  echo "Processing: $file"
done

# Binary-safe array population
readarray -d '' files < <(find . -print0)

3. Robust Argument Parsing

usage() {
  cat <<EOF
Usage: ${0##*/} [OPTIONS] <required-arg>

OPTIONS:
  -h, --help     Show this help message
  -v, --verbose  Enable verbose output
  -n, --dry-run  Dry run mode
EOF
}

# Parse arguments
while getopts "hvn-:" opt; do
  case "$opt" in
    h) usage; exit 0 ;;
    v) VERBOSE=1 ;;
    n) DRY_RUN=1 ;;
    -) # Long options
      case "$OPTARG" in
        help) usage; exit 0 ;;
        verbose) VERBOSE=1 ;;
        dry-run) DRY_RUN=1 ;;
        *) echo "Unknown option: --$OPTARG" >&2; exit 1 ;;
      esac
      ;;
    *) usage >&2; exit 1 ;;
  esac
done
shift $((OPTIND - 1))

4. Safe Temporary Resources

# Create temp directory with cleanup
tmpdir=$(mktemp -d)
trap 'rm -rf "$tmpdir"' EXIT

# Safe temp file creation
tmpfile=$(mktemp)
trap 'rm -f "$tmpfile"' EXIT

5. Structured Logging

readonly SCRIPT_NAME="${0##*/}"
readonly LOG_LEVELS=(DEBUG INFO WARN ERROR)

log() {
  local level="$1"; shift
  local timestamp=$(date '+%Y-%m-%d %H:%M:%S')
  echo "[$timestamp] [$level] $SCRIPT_NAME: $*" >&2
}

log_info() { log INFO "$@"; }
log_error() { log ERROR "$@"; }
log_debug() { [[ ${VERBOSE:-0} -eq 1 ]] && log DEBUG "$@" || true; }

6. Version Detection & Modern Features

# Check Bash version before using modern features
if (( BASH_VERSINFO[0] >= 5 )); then
  # Bash 5.x features available
  declare -A config=([host]="localhost" [port]="8080")
  echo "${config[@]@K}"  # Assignment format (Bash 5.x)
else
  echo "Warning: Bash 5.x features not available" >&2
fi

# Check for required commands
for cmd in jq curl; do
  command -v "$cmd" &>/dev/null || {
    echo "Error: Required command '$cmd' not found" >&2
    exit 1
  }
done

7. Safe Command Execution

# Separate options from arguments with --
rm -rf -- "$user_input"

# Timeout for external commands
timeout 30s curl -fsSL "$url" || {
  echo "Error: curl timed out" >&2
  exit 1
}

# Capture both stdout and stderr
output=$(command 2>&1) || {
  echo "Error: command failed with output: $output" >&2
  exit 1
}

8. Platform Portability

# Detect platform
case "$(uname -s)" in
  Linux*)   PLATFORM="linux" ;;
  Darwin*)  PLATFORM="macos" ;;
  *)        PLATFORM="unknown" ;;
esac

# Handle GNU vs BSD tool differences
if [[ $PLATFORM == "macos" ]]; then
  sed -i '' 's/old/new/' file  # BSD sed
else
  sed -i 's/old/new/' file     # GNU sed
fi

9. Script Directory Detection

# Robust script directory detection (handles symlinks)
SCRIPT_DIR="$(cd -- "$(dirname -- "${BASH_SOURCE[0]}")" && pwd -P)"
readonly SCRIPT_DIR

10. Best Practices Quick Reference

• Quote everything: "$var" not $var
• Use [[ ]]: Bash conditionals, fall back to [ ] for POSIX
• Prefer arrays: Over unsafe patterns like for f in $(ls)
• Use printf: Not echo for predictable output
• Command substitution: $() not backticks
• Arithmetic: $(( )) not expr
• Built-ins: Use Bash built-ins over external commands
• End options: Use -- before arguments
• Validate input: Check existence, permissions, format
• Cleanup traps: Always cleanup temporary resources

Output Deliverables

When creating Bash scripts, provide:

1. Production-ready script with:

   • Strict mode enabled (set -Eeuo pipefail)
   • Comprehensive error handling and cleanup traps
   • Clear usage message (--help)
   • Proper argument parsing with getopts
   • Structured logging with log levels

2. Test suite (bats-core or shellspec):

   • Edge cases and error conditions
   • Mock external dependencies
   • TAP output format

3. CI/CD configuration:

   • ShellCheck static analysis
   • shfmt formatting validation
   • Automated testing with Bats

4. Documentation:

   • Usage examples in --help
   • Required dependencies and versions
   • Exit codes and error messages

5. Static analysis config:

   • .shellcheckrc with appropriate suppressions
   • .editorconfig for consistent formatting

Tools & Commands

Essential Tools

• ShellCheck: shellcheck --enable=all script.sh
• shfmt: shfmt -i 2 -ci -bn -sr -kp script.sh
• bats-core: bats test/script.bats

Quick Validation

# Run full validation
shellcheck *.sh && shfmt -d *.sh && bats test/

Reference Documentation

For detailed guidance on specific topics:

• Modern Bash 5.x Features - Version-specific features, transformations, and compatibility
• Testing Frameworks - bats-core, shellspec, test patterns, mocking
• CI/CD Integration - GitHub Actions, pre-commit hooks, matrix testing
• Security & Hardening - SAST, secrets detection, input sanitization, audit logging

Common Pitfalls to Avoid

See TROUBLESHOOTING.md for detailed solutions.

Quick list:

• ❌ for f in $(ls ...) → ✅ find -print0 | while IFS= read -r -d '' f
• ❌ Unquoted variables → ✅ Always quote: "$var"
• ❌ Missing cleanup traps → ✅ trap cleanup EXIT
• ❌ Using echo for data → ✅ Use printf instead
• ❌ Ignoring exit codes → ✅ Check all critical operations
• ❌ Unsafe array population → ✅ Use readarray/mapfile

Examples

See EXAMPLES.md for complete script templates and usage patterns.