I don't like this source code-is the quotation mark single or double?

I would like to summarize a better way to write the source code of the program.

Is the quotation mark single or double?

sample.sh

  name="Taro"
  age=19
  country='Japan'
  greeting="I am $name. I am ${age} years old. I live in $country"
  echo $greeting

How can I refactor