su

Additional Notes

su switches to another user account or runs a command as that user. Without a username, it usually switches to root.

Unlike sudo, su normally asks for the target user's password. On many systems, direct root login may be disabled or discouraged.

Syntax

su [options] [user]
su [options] user -c 'command'

Parameters

• options: Flags that change how su behaves.
• user: User account affected by the command.
• group: Group account affected by the command.
• file: File or directory whose ownership, mode, or access policy is being changed.

Common Options

• -, -l, --login: Start a login shell for the target user.
• -c COMMAND: Run one command as the target user.
• -s SHELL: Use a specific shell.
• -p, --preserve-environment: Preserve environment variables when allowed.

Examples

su -

Switch to a root login shell.

su rani

Switch to user rani.

su - postgres

Start a login shell as postgres.

su - postgres -c 'psql'

Run one command as postgres.

Practical Notes

• su - gives a login-style environment; plain su keeps more of the current environment.
• Prefer sudo for single administrative commands when available.
• Use whoami and pwd after switching users so you know your identity and location.
• Exiting the shell returns to the previous user.