su identification

Basil Fowler b.j.fowler at
Thu Dec 24 13:11:45 GMT 2009

You have confused the two commands.

su means "switch user" Let us assume that you have three users alpha, bravo 
and root.  Let us assume for the moment that you are logged in as user alpha 
and alpha's password is ahpla. Let us also assume that the password for bravo 
is ovarb, and the password for root is master. I know that these passwords are 
ridiculous, but this is for example.

I know that you are using a Debian based distro (Ubuntu) because you are 
invoking the apt-get command. In such distros the first user alpha is set up 
as a master user account and often the root account is disabled. This is good 
for single user machines, but is a PITA for multiuser setups.

When you use su, if it has no arguments, it assumes that you wish to invoke 
the root account and will demand Root account's password (in this case MASTER) 
If you follow su with an account name (such as bravo) will will be prompted 
for  bravo's password (in this case ovarb). When the correct password for the 
TARGET account is entered, you will then be running as the target account (to 
leave, type 'exit' or control-d)

Now, with distros such as Ubuntu, the root account is disabled, so su does not 
work. A correct password (equivalent to MASTER) does not exist. You have to 
type in sudo su. You then will have a root shell, and you can run "make 

sudo is a program that is designed to solve a difficult administrative 
problem. How do you give an ordinary user power to perform an action (such as 
managing printers) that normally requires root privileges without giving him 
/her complete control over the machine?

Sudo looks up a special file - readable only by root - /etc/sudoers, which 
lists who can do what under which conditions, and allows or denies the desired 
action accordingly.

In the case of Ubuntu, the /etc/sudoers file states that the first user 
(alpha) is given permission to do anything provided that ALPHA's password is 
entered, and everyone else (bravo) nothing.

To modify the /etc/sudoers file a special editor (visudo) is used, because any 
error in the format of the file will block sudo - this is intentional.

To activate the root account - not to be done unless you know what you are 
doing, the /etc/passwd file has to be modified. A root password will have to 
be created, and the shell command (usually /bin/false) changed to /bin/bash.

RedHat based distros such as Mandriva have a separate root password at 
installation, and each user has a separate password. They use kdesu and su. 
Sudo has to specially downloaded, because it is only needed in exceptional 

Hope this helps.

Basil Fowler


On Thursday 24 Dec 2009 08:35:24 spir wrote:
> Hello,
> I'm aware this question is ~ OT, not really kde related, but have no idea
>  where else to post it, and surely many people here know the answer. To
>  install new software (actually, the language Io, for which there seems to
>  be no package), I had to type:
> su -c "sudo make aptget"
> This will not work. I'm asked for a password, then get "su: Authentication
>  failure". Well, this simply cannot be, for I have only one admin password
>  AFAIK and it works fine for sudo and kdesudo. I tried an old password I
>  had long time ago (changed to make it more difficult), in case the one for
>  'su' may have sticked to old times while the one for sudo has changed, but
>  it is not accepted neither.
> I take the opportunity to ask about the use of 'su' in a normal user
>  session. Also, is there another way to perform the above action?
> Denis
> ________________________________
> la vita e estrany
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