how know if string contain substring by shell code example

Example 1: bash substring test

#!/bin/bash

STR='GNU/Linux is an operating system'
SUB='Linux'

if grep -q "$SUB" <<< "$STR"; then
  echo "It's there"
fi

Example 2: substring if statement variable shell script

if [ $string ?? 'foo' ]; then
  echo "It's there!"
fi