i had made a logical partition using LVM,in that partition i had installed fc7 in one partition,another partition i had left for Linux from scratch.so start working for linux from scratch using the steps mentioned in this site
http://www.linuxfromscratch.org/lfs/view/stable/

when i am setting environment
previous my shell was root@localhost,

after when i used this command ,it has changed to bash

cat > ~/.bash_profile << "EOF"
exec env -i HOME=$HOME TERM=$TERM PS1='\u:\w\$ ' /bin/bash
EOF

cat > ~/.bashrc << "EOF"
set +h
umask 022
LFS=/mnt/lfs
LC_ALL=POSIX
PATH=/tools/bin:/bin:/usr/bin
export LFS LC_ALL PATH
EOF

source ~/.bash_profile

after this command my prompt has changed from root@localhost to bash3.2

so how can i change to my root
can any one suggest me what to do

Just keep going. That's normal because you haven't created a fancy PS1 variable yet. Don't go mucking around and deviate from the book.

Hi,

Looks like you are doing this as user root, which is incorrect. At the end of chapter 4.3 you login as lfs user and the next step (chapter 4.4) is creating the environment.

The above has nothing to do with the prompt (PS1) issue (weibullguy answered that one).
But you have to do chapter 4.4 and most of chapter 5 (up to and including 5.30) as lfs, _not_ as root!