Question

Imagine file 1:

#include "first.h"
#include "second.h"
#include "third.h"

// more code here
...

Imagine file 2:

#include "fifth.h"
#include "second.h"
#include "eigth.h"

// more code here
...

I want to get the headers that are included in file 2, but not in file 1, only those lines.
So, when ran, a diff of file 1 and file 2 will produce:

#include "fifth.h"
#include "eigth.h"

I know how to do it in Perl/Python/Ruby, but I’d like to accomplish this without using a different programming language.

Answer 1

If it’s ok to use a temp file, try this:

grep include file1.h > /tmp/x && grep -f /tmp/x -v file2.h | grep include

This

• extracts all includes from file1.h and writes them to the file /tmp/x
• uses this file to get all lines from file2.h that are not contained in this list
• extracts all includes from the remainder of file2.h

It probably doesn’t handle differences in whitespace correctly etc, though.

EDIT:
to prevent false positives, use a different pattern for the last grep (thanks to jw013 for mentioning this):

grep include file1.h > /tmp/x && grep -f /tmp/x -v file2.h | grep "^#include"