the root shell has been changed in the file /etc/passwd, basically pointing to an incorrect directory. So now every time we login as 'su' I get the message
'su: no shell' so we can't login as superuser.

Is there an easy way to rectify this? please use step by step instructions/commands -

I know one way is to boot off a cd on the machine in single mode and then mount the root dir and then change the file...but I'm unsure about the command line. Is there any other way? I've tried linking the path, but permission rights stopped this.

Try if u can change roots shell in /etc/passwd file, if any root currently log in, else try to machine in single user mode and mount the /, then change roots shell.

Awadhesh

If you have permissions, you can copy the shell to the "incorrect location"... what is the full path of the shell in /etc/passwd now?

permissions dont allow me to copy bash to the 'incorrect dir'.
in /etc/passwds it's set to /sbin/bash but should have been /usr/bin/bash.

Have run into this before. The only thing that we could do was reboot.
1. reboot from cdrom
2. mount the '/' filesystem as /a
3. vi /a/etc/passwd and change the shell to the required value
4. save and exit
5. reboot from disk