ephdia - 5:32 am on Oct 10, 2007 (gmt 0)

Hello,
I am using linux(RHEL4) and i am having three basic question.
1)i want to restrict users not to execute some commands.Is it possible?
2)what is the use of pam? can we configure that? how could we do that? please explain PAM function and configuration.
3)Shall we configure two shell for single user.(i.e in /etc/passwd shall i give two shell access )
is the example below correct? /etc/passwd-user:x:045:046:/home/user/:/bin/bash,/usr/bin/scp.
Please help.
Thanks,