Linux VPS comm Command Guide with Examples

Linux VPS comm command with its straightforward syntax, comm [options] file1 file2 , is a powerful and efficient tool for comparing and analyzing two sorted files line by line.

After buying Linux VPS learning how to use the comm command is beneficial for using quick and efficient way to compare sorted files on your Linux VPS, especially in scenarios such as handling large datasets, code comparison, analyzing lists of users, and identifying differences in configuration files or log analysis.

The Linux comm command is valuable for a wide range of tasks in data processing and system administration. This article will make you proficient in using the Linux comm command to efficiently manage your Linux VPS.

Prerequisites to use comm command in Linux VPS

• Linux VPS or system running Linux
• Accessing terminal
• Sorted files (in alphabetical order), because comm command works with sorted files.
• Verifying read permissions for the files (using ls -l or stat commands)
• Ensuring the file’s content is in plain text format where each line represents a comparable unit of data.

General comm command syntax in Linux VPS

As we mentioned, the comm command in Linux VPS is used to compare two sorted files. Here is its basic syntax:

comm [options] file1 file2

By default, the comm command outputs the contents of files line by line that are unique to each file, as well as the lines that are common to both. This is valuable for analyzing the contents of files and identifying differences and similarities between two files.

The comm command displays the result in three columns:

• Column 1 (Left Column): Displays lines unique to first file.
• Column 2 (Middle Column): Displays lines unique to the second file.
• Column 3 (Right Column): Displays lines common to both files.

The comm Command Options in Linux VPS

Extensive options are available for the comm command, allowing you to customize the comm command behavior. Here are some of the options for comm command in Linux VPS:

The comm practical examples in Linux

Using the comm command can be more straightforward if you learn how to use it for various tasks and purposes. This section presents some practical examples to make the comm tool easy to use for quick comparisons.

Remember that the comm command is a standard utility available in all Linux distributions such as Ubuntu, Debian, Centos, etc.

To learn how to use the comm command, create two sample files using the touch command, name the file (for example, file1.txt and file2.txt), and write the different data to each file.

Prints the file1.txt and file2.txt content using cat command. for example:

cat file1.txt

Output:

Black
White
Red
Yellow
Blue

cat file2.txt

Output:

Green
Cyan
Purple
white
Black
Blue

let’s get more familiar with the Linux comm command.

Comparing two sorted files using comm

The basic usage of the comm command in Linux involves specifying the two sorted files to compare them line by line. To print the lines that differ between file1.txt and file2.txt along with the common lines, run the following command:

comm file1.txt file2.txt

Output:

                                  Black
                    Green 
White
                    Cyan
Red
                    Purple
Yellow
                                    Blue 
                    white

As you see, this shows White, Red, and Yellow are unique to file1.text, “Green”, “Cyan”, “Purple” and “white” are unique to file2.txt, and “Black” and “Blue” are present in both.

You might wonder why White is not displayed in column 3, this is because the comm command is case sensitive and recognizes White (which starts with uppercase) and white (which starts with lowercase) differently from each other.

Comparing two files while ignoring case sensitivity using comm

As we talked about previously, by default, the comm command is case-sensitive. Therefore, for a case-insensitive comparison, you can use additional tools such as tr to preprocess the files.

So, convert all characters within files to lowercase letters and save the result in new files using the following commands:

tr A-Z a-z < file1.txt > Temp_1
tr A-Z a-z < file2.txt > Temp_2

Therefore, by creating new files, the original files remain unchanged.

Then, sort the converted files using sort command and compare the sorted files using comm:

comm Temp_1 Temp_2

Output:

                                   black
                  green  
                                   white
                  cyan
red
                  purple
yellow
                                   blue   

Suppressing specific columns in comm output

The -1 , -2 , and -3 options are the most used options of comm command for controlling the output columns. Therefore you can hide the specific columns in comm command output using the -1 , -2 , and -3 flags.

For example, run the comm command along with -1 flag to hide column 1 (left column) when comparing file1.txt and file2.txt:

comm -1 file1.txt file2.txt

Output:

                                    Black
                    Green  
                    Cyan
                    Purple
                                    Blue  
                    white

Run the comm command along with -2 flag to suppressing column2 (middle column) when analyzing file1.txt and file2.txt:

comm -2 file1.txt file2.txt

Output:

                                Black
                      
White
                     
Red
                    
Yellow
                                Blue  

To customize the output of the comm command in Linux VPS to display only lines unique to the first file and lines unique to the second file by hiding column 3 (right column), use the -3 flag with the comm command:

comm -3 file1.txt file2.txt

Output:

                    Green  
White
                     Cyan
Red
                     Purple
Yellow  
                     white

You can also customize the output of the comm command by combining -1 and -2 flags to force the comm command to display only lines common for both files:

comm -12 file1.txt file2.txt

Output:

Black
Blue

By combining the options, you can identify the differences quickly, for example,the comm command can print the lines in file1.txt but not file2.txt with -13 flag:

comm -13 sorted_file1.txt sorted_file2.txt

Output:

White
Red
Yellow

Comparing unsorted files using comm

The Linux VPS comm command requires sorted files for accurate comparison and resulting valuable output. When the comm command works with unsorted files, it prints an error message that the files are not in sorted order.

You can verify that the files are sorted using comm --check-order :

comm --check-order unsorted_file1.txt unsorted_file2.txt

If it outputs that files are not in sorted order, you must sort the files before comparing.

Therefore, sort the files before comparing using sort command:

sort -o file1.txt file1_sorted.txt
sort -o file2.txt file2_sorted.txt

After sorting files you can run the cat command to verify that the file is sorted now.

This command creates temporary files, that you can remove them after comparing them using rm command.

Alternatively, you can use the following command to sort and compare files simultaneously without creating temporary files:

comm <(sort file1.txt) <(sort file2.txt)

In this command file1.txt  and file2.txt are unsorted files.

This command directly gives the sorted data to comm for comparison without creating intermediate files.

Note: When sorting files, using sort-u removes duplicate lines within files that allow you to compare only unique lines. ( syntax of this command:sort -u file1.txt -o file1.txt)

There is another way to compare unsorted files using comm which is using --nocheck-order with comm command:

comm --nocheck-order file1.txt file2.txt

The --nocheck-order option forces the comm to display a result and hide the error message. However, you can not trust this command’s output because it can be incorrect or unusable.

Customizing column default separators in comm output

By default, columns in the comm command’s output are separated with spaces. However, you can change the default separator using the -t STR or --output-delimiter=STR option:

comm -t '#' file1.txt file2.txt

Or

comm --output-delimiter=# file1.txt file2.txt

Output:

##Black
#Green
White
#Cyan
Red
#Purple
Yellow
##Blue
#white

This output indicates that file2.txt includes items with one # symbol, file1.txt includes items with no # symbol, and items with two # symbols are common for both files.

Comparing a file with standard terminal input using comm

Using the hyphen as one of the options with the comm command in Linux VPS enables you to directly compare the desired file with the standard terminal input (data entered directly into the terminal). For example, assume we have a file named “file.txt” with the following contents:

Contents of file.txt:

Red
Blue
Black

To compare the file.txt with the terminal input run the following command:

comm file.txt -

Then enter the lines in the terminal that you want to compare with file.tx, then press Enter. For example:

comm file.txt -

White
Orange
Black

Red
                        White
Blue
                        Orange
                                   Black

The column1 provides lines unique to file.txt, the column2 indicates lines you entered in the terminal and the column3 represents lines common for both.

Comparing directories using comm

The comm command is designed for file comparison, not directories directly. However, you can use the comm in combination with ls to compare directories. To do this run the following command:

comm <(ls Directory1) <(ls Directory2)

Sample output:

Differences1
Differences2
                         Differences3
                         Differences4
                                                 The same file

The column 1 represents a list of filenames unique to Directory 1 and column 2 displays a list of filenames unique to Directory 2 and column 3 shows a list of filenames common for both.

Customizing the comm output to display Line Counts

Using the --total option with the comm command in Linux changes the default output of the comm command when comparing two sorted files to show the line counts. to do this run the following command when comparing file1.txt and file2.txt:

comm --total file1.txt file2.txt

Output:

                                    Black
                    Green  
White
                    Cyan
Red
                    Purple
Yellow
                                     Blue 
                       white
3                     4                2

Comparing files by filtering results using comm and grep

The Linux VPS comm command can be piped with other commands for more complex processing. For example, you can filter out lines unique to file1.txt that contain a specific pattern by combining comm and grep in Linux:

comm -23 file1.txt file2.txt | grep 'pattern'

Conclusion

The comm command is an efficient and powerful tool for comparing sorted files. Therefore, becoming proficient in the comm command is essential to perform administration tasks optimally.

This article has provided a comprehensive guide to use Linux VPS comm command, if you need more information about comm command in Linux, refer to the command’s man page or leave a comment below.