Compare two files line by line
I have two files (not sorted) and need to compare line by line (i.e. first line of file1 to be compared to all the lines of file2 and so as for the rest of file1). Output will be an array of length of file2. Any suggestion in BASH other than a grep inside two read line loops ( which is time consuming for files ~1000s of lines) is much appreciated.
Thanks in advance.