Compare Two Files Line by Line and Show Only Difference from File 2

Doing line-by-line comparison using awk, you could do:

awk '{ getline x<"file2" } $0!=x{ print x}' file1

• getline x<"file2" reads the entire line from file2 and holds into x variable.
• print x when line from file1 differ with line in file2.

Or same but shorter:

awk '{ getline x<"file1" } $0!=x' file2

You can as well use diff for that task:

diff --old-line-format="" --unchanged-line-format="" 1.txt 2.txt 

Gives the following output:

five
twelve
hundred

You cannot do this without sorting your data. Even if you never explicitly run the sort command, any solution will involving indexing or sorting the data and taking O(n) time or memory to do it. For example, a solution which goes through the files and keeps track of which lines are seen or not seen will take O(n) memory and a solution which sorts the files first will take O(n) time.