Hi,

I want to know the importance of /etc/pam.d file in Linux ?

It restricts unauthorised users, but the same we can do in visudo file also..

then what is the importance of PAM module ?

Regards,
Appu.

it's NOT a file for a start, it's a directory, hence the ".d" suffix

Sudo uses PAM underneath to actually do the user authentication. on a PAM depedenent system, which is most (I think slackware doesn't / didn't use it) PAM modules are used for the very significant majority of all authentication actions on the system. Please don't think for a second that only sudo relates to authentication. How do you authenticate users in ssh? PAM. How do you authentication users logging into Gnome? PAM. It does plenty of other things besides the authentication, and does them in a highly flexible and configurable way, but just think that no PAM = No authentication. I'd suggest you didn't remove it!

also not "visudo" is just a noddy wrapper around a text editor. Don't confuse it with sudo itself.