Way to output differences between two files (preferably using command line)
You probably want the Unix comm utility. Windows versions are included in gnuwin32
NAME
comm - compare two sorted files line by line
SYNOPSIS
comm [OPTION]... FILE1 FILE2
DESCRIPTION
Compare sorted files FILE1 and FILE2 line by line. With no options, produce three-column output. Column one contains lines unique to FILE1, column two contains lines unique to FILE2, and column three contains lines common to both files. -1 suppress lines unique to FILE1 -2 suppress lines unique to FILE2 -3 suppress lines that appear in both files
This might work for you (GNU diff):
diff -u file1 file2 | sed -n '1,2d;/^[-+]/p'
+ball
-flan
+elephant