Why starting a KDE application with just su failed?

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Several times users where surprised by the new behavior of KDE 4 applications. Indeed, as the super-user mode for the applications doesn't not work currently, they often have to open a terminal, and use su to start the application as another, notably root. However when just using su instead of su -, this will failed. Why ? The explication is very simple.

First it should be noted that su means Substitute User, and not Super User. Indeed su allow to switch to whatever user you want and not just to the root user. However if no user is specified, then it will used the root user. This is logic because in a Unix/linux system, there's only one user which will normally always exist and with the same name : root. Now that this point is clarified, it should be noted that you have 2 ways to switch to another user : the light way and the heavy way. Now let's try to explain the differences between each ways :

rest here