Interactive string input and comparison

[scm]

Greetings,

I am trying to write an interactive Elisp program that will take the
input of a string, and depending what the string is, run a shell
command. I also what to have an error printed if the given shell
command does not exist. This is my attempt at the code so far:

(defun ls ()
  (interactive)
  (prompt for input? variable?)
  (if (string-match "ls" ))
    (shell-command "ls -l)

Thanks in advance to anyone who can take the time to answer this
question.