To run commands as another user, use su -c or sudo.

Problematic code:

whoami
su
whoami

Correct code:

whoami
sudo whoami

Rationale:

It's commonly believed that su makes a session run as another user. In reality, it starts an entirely new shell, independent of the one currently running your script.

su; whoami will start a root shell and wait for it to exit before running whoami. It will not start a root shell and then proceed to run whoami in it.

To run commands as another user, use sudo some command or su -c 'some command'. sudo is preferred when available, as it doesn't require additional quoting and can be configured to run passwordless if desired.

Exceptions

If you're aware of the above and want to e.g. start an interactive shell for a user, feel free to ignore this message.