You probably don't need make

#!/bin/sh

do_task1() {
        echo "hello form task1"
}

do_task2() {
        echo "hello from task2"
}

if ! type do_"$1" >/dev/null 2>&1; then
        echo "task '$1' does not exist" >&2
        exit 1
fi

cmd=$1
shift
do_$cmd "$@"