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comm (find lines common to two files)
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$ comm [-123i] file1 file2

will produce, three columns as output: 
	lines only in file1
	lines only in file2
	lines in both file1 and file2
file1 and file2 are EXPECTED TO BE SORTED LEXICALLY.

option:
  -1   suppress printing of column 1
  -2   suppress printing of column 2
  -3   suppress printing of column 3
  -i   case insensitive comparison

You can combine the options as

$ comm -12 file1 file2    #suppress line 1 and line 2

	#You can use subshells to feed comm
$ comm -12 <(sort file1) <(sort file2)

EXAMPLE
$ cat a1
A
B
D
C
$ cat a2
X
Z
Y
A

$  comm <(sort a1) <(sort a2)
		A
B
C
D
	X 
	Y 
	Z