Let’s clearly compare the difference between:

🔸 su root

vs

🔸 su - (which is shorthand for su - root)

📌 su root

  • Switches user: to root
  • Keeps your current shell environment
    • Your $PATH, $HOME, $USER, etc., are mostly unchanged
  • Does not simulate a full login
  • You remain in the same working directory

🔍 Example:

$ whoami
user

$ su root
Password:
# whoami
root
# echo $HOME
/home/user

💡 You’re root, but using your regular user’s environment.

📌 su - (or su - root)

  • Switches user: to root
  • Starts a login shell for root
    • Loads root’s .bash_profile, .profile, etc.
    • Changes $HOME to /root, $USER to root, etc.
    • Updates $PATH to root’s path
  • Changes directory to /root

🔍 Example:

$ whoami
user

$ su -
Password:
# whoami
root
# echo $HOME
/root

💡 You’re root, and using root’s proper environment.

✅ Summary Table

Feature su root su - / su - root
Switches to root? ✅ Yes ✅ Yes
Loads root’s environment? ❌ No ✅ Yes
Changes $HOME to /root? ❌ No ✅ Yes
Changes $PATH? ❌ No (may miss /sbin) ✅ Yes (includes /sbin, etc.)
Starts login shell? ❌ No ✅ Yes

🧠 When to Use Which?

  • Use su root: when you just need root temporarily without changing your environment.
  • Use su -: when running root tasks that depend on environment variables, root’s .bashrc, or access to /sbin, etc.

🧪 su - vs sudo -i

Command Meaning Key Differences
su - Switch to root login shell Requires root password
sudo -i Run an interactive login shell as root Uses your password (with sudo rights)

🔍 1. su - (Substitute User)

  • Starts a new login shell as root (like logging in directly).

  • Loads root’s environment, .bashrc, .profile, etc.

  • You must know root’s password.

  • Syntax:

      su -            # become root
  su - otheruser  # become another user

🔍 2. sudo -i (Sudo Interactive Shell)

  • Simulates a login shell as root using your user’s sudo privileges.

  • Loads root’s environment like su -.

  • Uses your password, not root’s.

  • Requires you to be in sudoers.

✅ Summary

Feature su - sudo -i
Requires root password ✅ Yes ❌ No
Uses sudo ❌ No ✅ Yes
Loads root env ✅ Yes ✅ Yes
Safer audit/logging ❌ Minimal ✅ Logged
Recommended for sudo-based systems (Ubuntu, etc.) ❌ Not really ✅ Yes

🛡️ Which Should You Use?

  • On Ubuntu and other sudo-based systems: prefer **sudo -i**

  • On older systems or when root password is known: **su -** is fine

  • For audit and logging: sudo -i is better (it logs who switched to root)