What is the fastest way to grep -v a file against another one, both line by line.does grep support this ? Or should I use it on two for loops ? 
grep file from file
files;grep
I'm assuming you mean use one of the files as a set of patterns for grep. If this is the case, you seem to be looking for a way to print all lines in one file not found in the other and this is what you want:grep -vFf file_with_patterns other_fileExplanation-F means interpret the pattern(s) literally, giving no special meaning to regex metacharacters (like * and +, for example)-f means read regex patterns from the file named as argument (file_with_patterns in this case).