Every user and group on a Unix/Linux system has a unique numeric identifier called UID (User ID) and GID (Group ID). These numbers are used internally by the kernel to manage permissions.

Special UIDs

• UID 0 (root) - The superuser has full system access
• UID 1-99 - Reserved for system users (daemon, bin, sys)
• UID 100-999 - Automatically generated system users
• UID 1000+ - Regular users created by the administrator
• UID 65534 (nobody) - Unprivileged user for NFS files

Common Use Cases

• Docker USER: USER 1000:1000
• NFS Permissions: chown -R 1000:1000 /data
• /etc/passwd files: ubuntu:x:1000:1000:Ubuntu:/home/ubuntu:/bin/bash