Which file should I edit to customize the login information?
How to custom the system startup information?
In most distributions you can simply edit /etc/issue
If Arch is the case, I can say that it works.
look here for a special character guide.
It's not a shell script though, it's only a description file, if you want to put other info into it, you should edit a service script or an init script to append the info you want to /etc/issue.