Hello all.Hope you are doing great! Coming into a point.. I have changed a user's privileges in Rhel 6.6 by making changes in the /etc/passwd file and setting particular user's UID and GID values to "Zero"(as it is for root) so that , that particular user perform some important tasks like .. changing other user's password ..etc . After changing it. When I type su (switch user) command followed by that particular username, it is opening as root not as a that particular user.. Please enlighten me on this topic.Thanks

One would also question your motives behind touching the /etc/passwd file? Also, do you understand what the 'U' in UID stands for, because if you do, you would know that assigning it to another user is highly problematic and violating the constraint.

i already typed: su username

That's not what you said in your first post.
As is also said, messing around with /etc/passwd to achieve this is a BAD THING and you WILL break things.

If you wish to let another user run certain commands as root then make use of

Yes I know dear... But as I am new to Linux and was experimenting it on my own Laptop to learn how it works.. That's why I was asking about it.. The main motive for doing this, just to learn how it works.. Nothing else.

Safety First Kids!

Don't do that. That's not how you "change a user's privileges", all it does is screw up the UID/username mapping as you've seen first hand. Regular users aren't root for a reason. If you want to be root, su to root.

In addition, when you change the UID/GID to zero and when you login for all effective purposes that user is root. As suggest by others if you want to give a regular user a particular privilege you should use sudoers

Sudo: you're doing it wrong - PDF @ 171 pages.
Sudo: you're doing it wrong - YouTubeVid @ 1h:11m

Run, don't walk.

As others have pointed out, this is not walking before running, this is flying at lightspeed straight for the sun.
It would also seem to be a very unusual first thing to want to experiment on.

As suggested, either use su as designed or investigate the sudo option

.. I echo the warnings and advice above, however, in response to your question, the UID (user ID) and GID (group ID) are the >>>real<<< underlying values of the user and group IDs, the way the system recognizes them. So, having made the (inappropriate) changes you described to the /etc/passwd file, you have effectively told your system that when you use the username you are referring to, either directly or through the su command, you are indeed using root.

As suggested by many already, the sudo command is the way to go for most cases requiring alternate user privileges.

Hope this helps ! :-)

Expanding on Rickkkk, usernames are just human friendly labels (see also eg DNS names).
In reality its all done with numbers.... and 1st entry in /etc/passwd with uid/gid = 0 is 'root'.

As mentioned above, don't do that
Use either su or sudo depending on what you want .....

Yes, I understand that it's not good to change uid and gid value. It's rather to choose "Sudoers" file for the same.I have changed privilege of any user by editing this User1 ALL= (ALL: ALL) ALL.. Something like that.. But I want this command for some specific command not for all command.. How to do that?

Maybe if you now tell us what you actually want to do, someone will be able to answer your question?