how to check if file exists in entire directory unix code example

Example 1: .sh script: check if file exist

#!/bin/bash
if [ -e x.txt ]
then
    echo "ok"
else
    echo "nok"
fi

Example 2: shell logical and for file existence

if [[ -f $FILE1 ]] && [[ -f $FILE2 ]]; then
    echo "OK"
fi