comm2.md

comm2

Purpose

This was inspired by the standard comm Unix utility. I was using it one day to compare a couple of files but grew dissatisfied because it requires that the input files are sorted. This script does not. It simply compares the files as-is without assuming that they're sorted.

Syntax

Syntax: comm2 [-123] [--json] file1 file2

Options and arguments

Option	Description	Default
-1	Eliminate lines unique to file1	The default is to print all lines
-2	Eliminate lines unique to file2	The default is to print all lines
-3	Eliminate lines unique to both files	The default is to print all lines
--json	Print results in JSON format	The default is to lines with minimal formatting, only separating the three categories by a tab character.

Example

$ comm2 <(echo -e one\\ntwo) <(echo -e two\\nthree)
	one
two
		three
$ comm2 <(echo -e one\\ntwo) <(echo -e two\\nthree) | cat -A
^Ione$
two$
^I^Ithree$
$ comm2 -12 <(echo -e one\\ntwo) <(echo -e two\\nthree)
two
$ comm2 -12 --json <(echo -e one\\ntwo) <(echo -e two\\nthree)
[
  {
    "line": "two",
    "pos1": 2,
    "pos2": 1,
    "status": "common"
  }
]
$

Notes

• The minimal formatting output makes use of tab characters to separate lines belonging to different categories:

  • Unless eliminated, a line appearing in the first tab-delimited field was unique to file1
  • Unless eliminated, a line appearing in the second tab-delimited field was unique to file2
  • Unless eliminated, a line appearing in the third tab-delimited field was common to both files.
  • If -1, -2, or -3 is specified, there will be fewer fields in the output. For instance if -1 is specified but -2 is not, lines unique to file2 will appear as the first field in the output, not the second.

• Two file names must be specified. Note the command substitution technique used in the example:

  $ foo ... <(bar ...)  ...
  

  If your input comes from a command or pipeline, you could use the same technique where the shell writes the data to a temporary file and passes the filename as an argument to comm2. All comm2 sees is a filename - it has no idea that command substitution was performed.

• Using the regular comm with one of these above examples:

  $ comm <(echo -e one\\ntwo) <(echo -e two\\nthree)
  one
    	two
  comm: file 2 is not in sorted order
    three
  $
  
  

  The behavior is is pretty much as expected and the error message can easily be disposed of but I'm not convinced that more complicated examples will work as well. If I come up with a more complicated example that demostrates the benefit of my script, I'll share it.