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test -z $username expands to test -z a b which is a syntax error, because test -z only expects one argument. By adding double quotes, like test -z "$username", the expansion becomes test -z "a b"...

The following will fail if there is a space anywhere in the input.

case $string in *\ *) echo try again;; *) break;; esac

If you want to use egrep, you can avoid the Useless Use of Test $?...

You need to quote "$string" inside double quotes if it contains any whitespace. Aside from that, it might be more elegant to use case instead of if, especially for coping with all-whitespace input...