comm compare two sorted files line by line and write to standard output; the lines that are common and the lines that are unique.

Suppose you have two lists of people and you are asked to find out the names available in one and not in the other, or even those common to both. comm is the command that will help you to achieve this. It requires two sorted files which it compares line by line.
Before discussing anything further first let’s check out the syntax of comm command:
Syntax :

$comm [OPTION]... FILE1 FILE2

• As using comm, we are trying to compare two files therefore the syntax of comm command needs two filenames as arguments.
• With no OPTION used, comm produces three-column output where first column contains lines unique to FILE1 ,second column contains lines unique to FILE2 and third and last column contains lines common to both the files.
• comm command only works right if you are comparing two files which are already sorted.

  Example: Let us suppose there are two sorted files file1.txt and file2.txt and now we will use comm command to compare these two.

  // displaying contents of file1 //
  $cat file1.txt
  Apaar 
  Ayush Rajput
  Deepak
  Hemant
  
  // displaying contents of file2 //
  $cat file2.txt
  Apaar
  Hemant
  Lucky
  Pranjal Thakral
  

  Now, run comm command as:

  // using comm command for 
  comparing two files //
  $comm file1.txt file2.txt
                  Apaar
  Ayush Rajput
  Deepak
                  Hemant
          Lucky
          Pranjal Thakral

  The above output contains of three columns where first column is separated by zero tab and contains names only present in file1.txt ,second column contains names only present in file2.txt and separated by one tab and the third column contains names common to both the files and is separated by two tabs from the beginning of the line.
  This is the default pattern of the output produced by comm command when no option is used .

  
  
  
  

  Options for comm command:

  1. -1 :suppress first column(lines unique to first file).
  2. -2 :suppress second column(lines unique to second file).
  3. -3 :suppress third column(lines common to both files).
  4. – -check-order :check that the input is correctly sorted, even if all input lines are pairable.
  5. – -nocheck-order :do not check that the input is correctly sorted.
  6. – -output-delimiter=STR :separate columns with string STR
  7. – -help :display a help message, and exit.
  8. – -version :output version information, and exit.
  
  Note : The options 4 to 8 are rarely used but options 1 to 3 are very useful in terms of the desired output user wants.

  Using comm with options

  1. Using -1 ,-2 and -3 options : The use of these three options can be easily explained with the help of example :

  //suppress first column using -1//
  $comm -1 file1.txt file2.txt
           Apaar
           Hemant
   Lucky
   Pranjal Thakral
  
  //suppress second column using -2//
  $comm -2 file1.txt file2.txt
          Apaar
  Ayush Rajput
  Deepak
          Hemant
  
  //suppress third column using -3//
  $comm -3 file1.txt file2.txt             
  Ayush Rajput
  Deepak       
          Lucky
          Pranjal Thakral
  

  Note that you can also suppress multiple columns using these options together as:

  //...suppressing multiple columns...//
  
  $comm -12 file1.txt file2.txt
  Apaar
  Hemant
  
  /* using -12 together suppressed both first
  and second columns */
  

  2. Using – -check-order option : This option is used to check whether the input files are sorted or not and in case if either of the two files are wrongly ordered then comm command will fail with an error message.

  $comm - -check-order f1.txt f2.txt
  

  The above command produces the normal output if both f1.txt and f2.txt are sorted and it just gives an error message if either of the two files are not sorted.

  3. Using – -nocheck-order option : In case if you don’t want to check whether the input files are sorted or not, use this option. This can be explained with the help of an example.

  //displaying contents of unsorted f1.txt//
  
  $cat f1.txt
  Parnjal 
  Kartik 
  
  //displaying contents of sorted file f2.txt//
  
  $cat f2.txt
  Apaar
  Kartik
  
  //now use - -nocheck-order option with comm//
  
  $comm - -nocheck-order f1.txt f2.txt
  Pranjal
          Apaar
                  Kartik
  
  /*as this option forced comm not to check
   the sorted order that's why the output 
  comm produced is also 
  not in sorted order*/

  4 . – -output-delimiter=STR option : By default, the columns in the comm command output are separated by spaces as explained above. However, if you want, you can change that, and have a string of your choice as separator. This can be done using the –output-delimiter option. This option requires you to specify the string that you want to use as the separator.
  Syntax:

  $comm - -output-delimiter=STR FILE1 FILE2

  EXAMPLE:

  //...comm command with - -output-delimiter=STR option...//
  
  $comm - -output-delimiter=+file1.txt file2.txt
  ++Apaar
  Ayush Rajput
  Deepak
  ++Hemant
  +Lucky
  +Pranjal Thakral
  
  /*+ before content indicates content of 
  second column and ++ before content 
  indicates content of third column*/ 
  

  So,that’s all about comm command and its options.

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