how to take config backup through cli - guest account

When the config is loaded into memory all the files are merged into the "running config". This is what tmsh is displaying when using the list command so there is no command to view the individual files (without having bash shell access which a guest account does not have).

However, you can list config by the tmsh modules. Bigip.conf and bigip_base.config contain config from these tmsh modules: cli, cm, ltm, net, sys. Try these tmsh commands they will probably give you what you need:

list ltm  
list net  
list sys
list cm
list cli