bash if contains code example

Example 1: checking if a substring exists in a string bash

string='Haystack';

if [[ $string =~ "Needle" ]]
then
   echo "It's there!"
fi

Example 2: bash if file contains string

if grep -q SomeString "$File"; then
  Some Actions # SomeString was found
fi

Example 3: bash substring test

#!/bin/bash

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