I have tried this on Fedora
and on Ubuntu
.
kshitiz@kshitiz:~$su
su: Authentication failure
kshitiz@kshitiz:~$sudo su
root@kshitiz:/home/kshitiz#
I am entering the same password in both the cases.
I have tried this on Fedora
and on Ubuntu
.
kshitiz@kshitiz:~$su
su: Authentication failure
kshitiz@kshitiz:~$sudo su
root@kshitiz:/home/kshitiz#
I am entering the same password in both the cases.
su
requires the password of the account whose privileges you are trying to assume (apparently root
in this case).
sudo
requires the password of the current user - that is, the password for user kshitiz
.
By running sudo su
, you are effectively becoming root
, then running su
to get a root shell - that is, your privileges are already elevated to root
before the call to su
is executed, which is why you don't get prompted for the root password again.
su
uses the root password, while sudo
uses the current user password. At least in Ubuntu the root password is scrambled during installation.
In fact, you can specify in the /etc/sudoers
file (use visudo
) which password the user who issues sudo
has to enter. If these lines
# Defaults targetpw
# ALL ALL = (ALL) ALL
were uncommented, you would have to enter the root-password to run passwords with root-permissions via sudo
.
su
with no arguments is because it stands for "switch user" (not "super user") but, if you don't specify anything, the default is root. You can, however, specify anyone, eg. "su kshitiz" -- in which case you would need kshitiz's password, not root's. Make sense? – goldilocks Mar 13 '13 at 11:52sudo
is to give selected users permissions to run some programs as root (or other users). This way those users can run any program as root. It is a bit safer than letting Joe Random switch to root to do something (and stay there!), but not much better. As configuration for personal machines it is fine, elsewhere a much more careful configuration is required. – vonbrand Mar 13 '13 at 14:01