$ man permissions

Permissions

File-level access rules that control who can read, write, or execute a file.

by Shawn Tenam

When you see "permission denied" in the terminal, permissions are the reason. Files have read, write, and execute flags for the owner, group, and everyone else. sudo runs a command as the administrator. chmod changes permissions. npm install errors, Homebrew ownership conflicts, script execution failures. All permissions issues.

chmod +x script.sh makes a script executable. sudo npm install -g runs the install with admin privileges. If Homebrew gives permission errors, fix ownership with sudo chown -R $(whoami) $(npm config get prefix)/{lib/node_modules,bin,share}. Read the error message. It usually tells you exactly which permission is missing.