How can I read a whole file into a string variable

Use ioutil.ReadFile:

func ReadFile(filename string) ([]byte, error)

ReadFile reads the file named by filename and returns the contents. A successful call
returns err == nil, not err == EOF. Because ReadFile reads the whole file, it does not treat
an EOF from Read as an error to be reported.

You will get a []byte instead of a string. It can be converted if really necessary:

s := string(buf)