Note that comm takes two sorted files as input. Here are our sample input files:

$ cat A.txt
apple
orange
gold
silver
steel
iron

$ cat B.txt
orange
gold
cookies
carrot

$ sort A.txt -o A.txt ; sort B.txt -o B.txt

1. First, execute comm without any options:

        $ comm A.txt B.txt 
        apple
                carrot
                cookies
                        gold
        iron
                        orange
        silver
        steel

 The first column of the output contains lines that are only in A.txt. The second column contains lines that are only in B.txt. The third column contains the common lines from A.txt and B.txt. Each of the columns are delimited using the tab (\t) character.

2. In order to print the intersection of two files, we need to remove the first and second columns and print the third column. The -1 option removes the first column, and the -2 option removes the second column, leaving the third column:

        $ comm A.txt B.txt -1 -2
        gold
        orange

3. Print only the lines that are uncommon between the two files by removing column 3:

        $ comm A.txt B.txt  -3
        apple
                carrot
                cookies
        iron
        silver
        steel

 This output uses two columns with blanks to show the unique lines in file1 and file2. We can make this more readable as a list of unique lines by merging the two columns into one, like this:

        apple
        carrot
        cookies
        iron
        silver
        steel

4. The lines can be merged by removing the tab characters with tr (discussed in Chapter 2, Have a Good Command)

        $ comm A.txt B.txt  -3 | tr -d '\t'
        apple
        carrot
        cookies
        iron
        silver
        steel

5. By removing the unnecessary columns, we can produce the set difference for A.txt and B.txt, as follows:

                $ comm A.txt B.txt -2 -3

 -2 -3 removes the second and third columns

                $ comm A.txt B.txt -1 -3

 -2 -3 removes the second and third columns