How to read line by line and match text with next line?

Solution 1:

You can accomplish this with a simple for-loop

#!/bin/bash

mapfile -t < text.txt
for ((a=0,b=1; $b<${#MAPFILE[@]}; a++,b++)); do
     [[ ${MAPFILE[$a]} = ${MAPFILE[$b]} ]] && echo ${MAPFILE[$a]}
done > unique.txt