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.
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 -13 <(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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See Also
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Tutorial on using join, a UNIX and Linux command to join lines of two files on a common field. Examples of joining two files, sorting before joining, specifying a field separator and specifying the output format. -
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Tutorial on using shuf, a UNIX and Linux command for generating random permutations. Examples of shuffling the lines in a file, picking a random line and shuffling standard input. -
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Tutorial on using fold, a UNIX and Linux command for folding long lines for finite widths. Examples of limiting column width, limiting by bytes and wrapping on spaces.