When to use 'su'?

You would use the su command in the following scenarios:

Switching Users: When you need to switch to another user account without changing the environment. This is useful for performing tasks as a different user while retaining your current environment settings.
Running Commands as Another User: If you want to execute a specific command as another user without fully switching to that user's shell, you can use su followed by the command. For example:
```
su username -c 'command'
```
Testing Permissions: When you want to test permissions or access files as another user without logging out and back in.
Temporary Access: If you need temporary access to another user's account for administrative tasks or troubleshooting, and you don't need the full environment of that user.

Remember that when using su, you will need to provide the target user's password unless you are switching to the root user (if you have the necessary permissions).