test string content specific charactr in shell code example

Example 1: bash substring test

#!/bin/bash

STR='GNU/Linux is an operating system'
SUB='Linux'
if [[ "$STR" == *"$SUB"* ]]; then
  echo "It's there."
fi

Example 2: substring if statement variable shell script

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