Shell Programming and Scripting

I am creating a menu driven system and i want to show the last login times of different users, instead of using the 'last' command i wanted to know if there is anyway i could make a brief search tool where the user can input which user they are looking for and then the login times for that specific user will be displayed? I was thinking a grep command may be used but how would i go about indicating via code i want the users input to be substituted into the search? I hope this doesnt sound too confusing, if so let me know and i'll elaborate.

you mean you want to use the user i/p in grep??

Code:

echo "enter user name : \c"
read user_name
grep "$user_name" filename

yeah if possible i would want the username inputted to be entered into the grep command, but what would the code to implement this look like thats my only problem.