Linux comm command

On Unix-like operating systems, the comm command compares two sorted files line-by-line.

This page covers the GNU/Linux version of comm.

Description

With no options, comm produces 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. Each of these columns can be suppressed individually with options.

Syntax

comm [OPTION]... FILE1 FILE2

Options

Examples

Let's say you have two text files, recipe.txt and shopping-list.txt.

recipe.txt contains these lines:

All-Purpose Flour
Baking Soda
Bread
Brown Sugar
Chocolate Chips
Eggs
Milk
Salt
Vanilla Extract
White Sugar

And shopping-list.txt contains these lines:

All-Purpose Flour
Bread
Brown Sugar
Chicken Salad
Chocolate Chips
Eggs
Milk
Onions
Pickles
Potato Chips
Soda Pop
Tomatoes
White Sugar

As you can see, the two files are different, but many of the lines are the same. Not all of the recipe ingredients are on the shopping list, and not everything on the shopping list is part of the recipe.

If we run the comm command on the two files, it will read both files and give us three columns of output:

comm recipe.txt shopping-list.txt

        All-Purpose Flour
Baking Soda
        Bread
        Brown Sugar
    Chicken Salad
        Chocolate Chips
        Eggs
        Milk
    Onions
    Pickles
    Potato Chips
Salt
    Soda Pop
    Tomatoes
Vanilla Extract
        White Sugar

Here, each line of output has either zero, one, or two tabs at the beginning, separating the output into three columns:

1. The first column (zero tabs) is lines that only appear in the first file.
2. The second column (one tab) is lines that only appear in the second file.
3. The third column (two tabs) is lines that appear in both files.

(The columns overlap visually because our terminal prints a tab as eight spaces. It might look different on your screen.)

Next, let's look at how we can bring our separated data into a spreadsheet.

Creating a CSV file for spreadsheets

One useful way to use comm is to output to a CSV file, which can then be read by a spreadsheet program. CSV files are text files that use a certain character, usually a comma, tab, or semicolon, to delimit data in a way that can be read as a spreadsheet. By convention, CSV file names have the extension .csv.

For instance, let's run the same command, but this time let's redirect the output to a file called output.csv using the > operator:

comm recipe.txt shopping-list.txt > output.csv

This time there is no output on the screen. Instead, output is sent to a file called output.csv. To check that it worked correctly, we can cat the contents of output.csv:

cat output.csv

All-Purpose Flour
Baking Soda
                Bread
                Brown Sugar
        Chicken Salad
                Chocolate Chips
                Eggs
                Milk
        Onions
        Pickles
        Potato Chips
Salt
        Soda Pop
        Tomatoes
Vanilla Extract
                White Sugar

To bring this data into a spreadsheet, we can open it in LibreOffice Calc:

Before it opens the file, LibreOffice asks us how to interpret the file data.

We want the column delimiter to be tab characters, which is already checked by default. (There are no commas or semicolons in our data, so we don't have to worry about the other checkboxes.) It also gives us a preview of how the data looks, given the options we selected.

Everything looks good, so we can click OK, and LibreOffice will import our data into a spreadsheet.

Now if we wanted to, we could save the spreadsheet in another format such as a Microsoft Excel file, or an XML file, or even HTML.

Suppressing columns

If you only want to output specific columns, you can specify the column numbers to suppress in the command, preceded by a dash. For instance, this command will suppress columns 1 and 2, displaying only column 3 — lines shared by both files. This isolates the items on the shopping list that are also part of the recipe:

comm -12 recipe.txt shopping-list.txt

All-Purpose Flour
Bread
Brown Sugar
Chocolate Chips
Eggs
Milk
White Sugar

The next command will suppress columns 2 and 3, displaying only column 1 — lines in the recipe that are not in the shopping list. This shows us what ingredients we already have in our cupboard:

comm -23 recipe.txt shopping-list.txt

Baking Soda
Salt
Vanilla Extract

And the next command will suppress column 3, displaying only columns 1 and 2 — the items in the recipe that are not on the shopping list, and the items on the shopping list that are not in the recipe, each in their column.

comm -3 recipe.txt shopping-list.txt

Baking Soda
        Chicken Salad
        Onions
        Pickles
        Potato Chips
Salt
        Soda Pop
        Tomatoes
Vanilla Extract

Related commands

cmp — Compare two files byte by byte.
diff — Identify the differences between two files.
join — Join the lines of two files which share a common field of data.
sort — Sort the lines in a text file.
uniq — Identify, and optionally filter out, repeated lines in a file.