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Linux and Unix comm command tutorial with examples

Tutorial on using comm, a UNIX and Linux command for comparing two sorted files line by line. Examples of showing specific comparisons and ignoring case sensitivity.

Estimated reading time: 2 minutes

Table of contents

comm man page

What is the comm command in UNIX?

The comm command compares two sorted files line by line and writes three columns to standard output. These columns show lines that are unique to files one, lines that are unique to file two and lines that are shared by both files. It also supports suppressing column outputs and comparing lines without case sensitivity.

How to compare the compare two files line by line

To compare two files line by line using comm it is a prerequisite that files are first sorted. For this example we will be comparing two files with numbers in that are saved as set1.txt and set2.txt. The following is the contents of set1.txt.

1
2
3

The following is the contents of set2.txt.

8
3 
2

As theses files are not sorted sort can be used to accomplish this

comm <(sort set1.txt) <(sort set2.txt)
1
                2
                3
        8

The output is in three columns. The first is lines that are only in file one (set1.txt), the second is lines only in file two (set2.txt), the third is line that are in both files.

How to show specific comparisons

To show specific comparisons using comm pass the option of the column that should be suppressed. In the following example only column three, or the lines that are common to both files are shown.

comm -12 <(sort set1.txt) <(sort set2.txt)
2
3

The following example shows lines that are unique to set1.txt.

comm -23 <(sort set1.txt) <(sort set2.txt)
1

The following example shows lines that are unique to set2.txt.

comm -23 <(sort set1.txt) <(sort set2.txt)
8

How to ignore case sensitivity

To ignore case sensitivity when using comm pass the -i option. This will ignore case sensitivity in the comparison. Consider the two following files that have been sorted.

cat words1.txt
Apple
Banana
Orange

cat words2.txt
apple
banana
orange

If these files are compared using comm there will be nothing common to these two files.

comm words comm words1.txt words2.txt
Apple
Banana
Orange
        apple
        banana
        orange

If case sensitivity is ignore with -i option the words will be shown as common to both files. Note that the uppercase words is shown as it is from the first file.

comm -i words1.txt words2.txt
            Apple
            Banana
            Orange

Further reading

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