A fully automated Bash script to migrate from ESLint + Prettier to Biome. It initializes Biome, migrates ESLint and Prettier configurations, removes old config files, and updates package.json scripts. Designed for Bun environments with full Next.js ESLint compatibility.

migrate.sh

#!/usr/bin/env bash
#
# migrate-to-biome.sh
# ---------------------------------------------------------------------------------------------------
# 🔧 Purpose:
#   Fully automate the migration from ESLint + Prettier → Biome using Bun.
#
# 🧠 What it does:
#   1. Initializes Biome (if missing)
#   2. Migrates ESLint & Prettier configurations safely (handles Next.js)
#   3. Removes all ESLint/Prettier config files (including eslint.config.mjs)
#   4. Detects and removes old lint/format scripts
#   5. Removes all ESLint & Prettier dependencies
#   6. Installs Biome as devDependency
#   7. Adds Biome lint & format scripts with --write flag
#
# 🧩 Requirements:
#   - Bun installed and accessible (`bunx`, `bun add`, `bun remove`)
#   - jq installed (for JSON editing)
#
# ---------------------------------------------------------------------------------------------------
# © 2025 Samet • MIT License
# ---------------------------------------------------------------------------------------------------

set -euo pipefail
IFS=$'\n\t'

# ----------------------------- #
# 🌈 Helper functions
# ----------------------------- #
log() { echo -e "\033[36m[INFO]\033[0m $*"; }
warn() { echo -e "\033[33m[WARN]\033[0m $*"; }
error() { echo -e "\033[31m[ERROR]\033[0m $*" >&2; }
success() { echo -e "\033[32m[SUCCESS]\033[0m $*"; }

require() {
  if ! command -v "$1" &>/dev/null; then
    error "Missing required command: $1"
    exit 1
  fi
}

# ----------------------------- #
# 🧭 Pre-flight checks
# ----------------------------- #
require bunx
require jq

if [[ ! -f package.json ]]; then
  error "package.json not found in current directory."
  exit 1
fi

# ----------------------------- #
# ⚙️  Run Biome migrations
# ----------------------------- #
log "Running Biome migration commands..."

# 1. Initialize Biome if not found
if [[ ! -f biome.json && ! -f biome.config.json ]]; then
  log "No Biome config found. Initializing Biome..."
  bunx @biomejs/biome init || true
  success "Biome initialized."
fi

# 2. Handle Next.js ESLint configs safely
TEMP_ESLINT_DIR=".tmp_eslint_migrate"
mkdir -p "$TEMP_ESLINT_DIR"
RESTORE_ESLINT=false

if [[ -f "eslint.config.mjs" ]]; then
  log "Temporarily patching Next.js ESLint config to avoid @rushstack crashes..."
  cp eslint.config.mjs "$TEMP_ESLINT_DIR/eslint.config.mjs"
  echo "export default {};" > eslint.config.mjs
  RESTORE_ESLINT=true
elif [[ -f "eslint.config.js" ]]; then
  log "Temporarily patching eslint.config.js..."
  cp eslint.config.js "$TEMP_ESLINT_DIR/eslint.config.js"
  echo "module.exports = {};" > eslint.config.js
  RESTORE_ESLINT=true
fi

# 3. Run migration commands
bunx @biomejs/biome migrate eslint --write || warn "ESLint migration encountered non-critical issues."
bunx @biomejs/biome migrate prettier --write || warn "Prettier migration encountered non-critical issues."
success "Biome migration steps completed."

# 4. Restore ESLint config temporarily, then remove permanently
if [[ "$RESTORE_ESLINT" == true ]]; then
  log "Restoring original eslint.config.* for cleanup..."
  mv "$TEMP_ESLINT_DIR"/* . || true
  rm -rf "$TEMP_ESLINT_DIR"
fi

# ----------------------------- #
# 🧹 Clean up old config files
# ----------------------------- #
log "Cleaning up old ESLint/Prettier config files..."
FILES_TO_DELETE=(
  "eslint.config.js" "eslint.config.mjs"
  ".eslintrc.js" ".eslintrc.cjs" ".eslintrc.json" ".eslintrc.yaml" ".eslintrc.yml"
  ".eslintignore"
  ".prettierrc" ".prettierrc.js" ".prettierrc.json" ".prettierrc.yaml" ".prettierrc.yml"
  "prettier.config.js" "prettier.config.cjs" "prettier.config.mjs"
  ".prettierignore"
)

for file in "${FILES_TO_DELETE[@]}"; do
  if [[ -f "$file" ]]; then
    rm -f "$file"
    echo "🗑️  Deleted $file"
  fi
done
success "Old config files cleaned (including eslint.config.*)."

# ----------------------------- #
# 📦 Remove ESLint & Prettier dependencies
# ----------------------------- #
log "Scanning for ESLint/Prettier dependencies to remove..."

REMOVE_PACKAGES=$(jq -r '
  (.devDependencies // {} + .dependencies // {})
  | keys
  | map(select(test("eslint|prettier"; "i")))
  | join(" ")
' package.json)

if [[ -n "$REMOVE_PACKAGES" ]]; then
  log "Removing old linting packages: $REMOVE_PACKAGES"
  bun remove $REMOVE_PACKAGES || warn "Some packages could not be removed."
else
  log "No ESLint/Prettier dependencies found."
fi

# ----------------------------- #
# 📥 Install Biome
# ----------------------------- #
log "Installing Biome as devDependency..."
bun add -d @biomejs/biome
success "Biome installed successfully."

# ----------------------------- #
# 🧩 Update package.json scripts
# ----------------------------- #
log "Scanning package.json for ESLint / Prettier scripts..."
mapfile -t MATCHES < <(jq -r '.scripts | to_entries[] | select(.value | test("eslint|prettier"; "i")) | "\(.key): \(.value)"' package.json)

if [[ ${#MATCHES[@]} -eq 0 ]]; then
  log "No ESLint/Prettier scripts found."
else
  echo -e "\n🧹 Possible ESLint/Prettier scripts found:\n"
  for i in "${!MATCHES[@]}"; do
    printf "  [%d] %s\n" $((i + 1)) "${MATCHES[$i]}"
  done

  echo -e "\nEnter numbers (comma-separated) of scripts to delete, or press Enter to skip:"
  read -r -p "> " SELECTION

  if [[ -n "$SELECTION" ]]; then
    for index in $(echo "$SELECTION" | tr ',' ' '); do
      if [[ "$index" =~ ^[0-9]+$ ]] && (( index >= 1 && index <= ${#MATCHES[@]} )); then
        NAME=$(echo "${MATCHES[$((index-1))]}" | cut -d: -f1 | xargs)
        jq "del(.scripts.\"$NAME\")" package.json > package.tmp.json && mv package.tmp.json package.json
        echo "🗑️  Removed script: $NAME"
      fi
    done
  else
    warn "Skipped script removal."
  fi
fi

# 5. Add Biome scripts with --write flag
log "Adding Biome lint & format scripts..."
jq '.scripts.lint = "biome lint --write" | .scripts.format = "biome format --write"' package.json > package.tmp.json && mv package.tmp.json package.json
success "Updated package.json with biome scripts (with --write)."

# ----------------------------- #
# 🎯 Final Notes
# ----------------------------- #
echo -e "\n🎉 \033[1mMigration complete!\033[0m"
echo "✅ ESLint & Prettier fully removed (including eslint.config.* files)."
echo "✅ Biome installed and configured with auto-fix scripts."
echo "➡️  Review biome.json for final rule adjustments."
echo "➡️  You can now run:"
echo "   bun run lint"
echo "   bun run format"