Error: Permission denied @ apply2files - /usr/local/Cellar/

Answer

For most Intel‑Mac users the safest, simplest fix is to correct the ownership of the /usr/local/Cellar directory so that the current user has write access. Changing ownership of the whole /usr/local tree is unnecessary and can interfere with other installers that expect root ownership.

Migrating Homebrew to the Apple‑Silicon prefix /opt/homebrew on an Intel Mac is possible but creates a second, independent Homebrew instance. It is only advisable if you plan to keep a dual‑architecture setup or want to experiment with the new prefix. In the long run, the Intel installation will remain dormant on Apple‑Silicon machines, so keeping the /usr/local prefix is perfectly fine for future migration.

Why Ownership Fix is Preferable

• Minimal impact – only the Homebrew-managed directories are touched.
• No SIP interference – /usr/local is outside System Integrity Protection.
• Future‑proof – on Apple‑Silicon Macs Homebrew automatically switches to /opt/homebrew without affecting the existing /usr/local installation.

Step‑by‑Step Fix

1. Check current ownership:

   ls -ld /usr/local /usr/local/Cellar
   

2. If the owner is root or another user, change it to your account:

   sudo chown -R $(whoami) /usr/local/Cellar
   

   This command preserves the parent /usr/local permissions and only affects the Cellar where Homebrew writes.

3. Verify permissions are now writable by the user:

   ls -ld /usr/local/Cellar
   

4. Run Homebrew’s health check:

   brew doctor
   

   A clean output indicates the change is safe.

5. Ensure other tools still work (e.g., Xcode command‑line tools):

   xcode-select -p
   

   Any failure here suggests an unintended side effect.

When Migration to /opt/homebrew Makes Sense

• You want a clean separation between Intel and Apple‑Silicon formulae.
• You plan to run both prefixes simultaneously and wish to avoid PATH conflicts.
• You need to experiment with Homebrew’s new Apple‑Silicon build process without touching your existing setup.

To migrate, install a fresh Homebrew instance with the desired prefix and reinstall formulas:

env HOMEBREW_PREFIX=/opt/homebrew /bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/HEAD/install.sh)"
brew install $(brew list)

Keep in mind this duplicates disk usage and requires maintaining two separate caches.

Long‑Term Implications for Apple‑Silicon Migration

• Intel /usr/local Homebrew remains untouched on Apple‑Silicon Macs; Homebrew will automatically use /opt/homebrew instead.
• Duplicate binaries only arise if you install the same formula in both prefixes and the PATH order is ambiguous.
• Maintaining a single prefix (either /usr/local or /opt/homebrew) keeps the system simpler and reduces the risk of stale or conflicting binaries.

Verification Checklist

• ls -ld /usr/local/Cellar shows ownership as your user and mode drwxr-xr-x (or drwxrwxr-x if you added group write).
• Running brew doctor reports no insecure directory warnings.
• System tools like gcc, make, and xcode-select run without permission errors.
• No SIP‑related messages appear in Console after the change.

Quick Summary

• Prefer changing ownership of /usr/local/Cellar to your user.
• Migration to /opt/homebrew is optional and creates a separate Homebrew instance.
• Future Apple‑Silicon migration is unaffected by keeping the Intel prefix; only the PATH and HOMEBREW_PREFIX environment will change.
• Verify after any change with brew doctor and system tool checks.