Text Processing Unix Commands

Text processing Unix commands are those affecting text and text files. For example, commands like cut, paste, cmp, sort, comm, head, tail, wc, diff, grep are few in the list. While editing files, you may lose track of what changes you have made to which files.

In this article, we will cover commands that compare files.

• cmp command in Unix
• diff command in Unix
• comm command in Unix

cmp command in Unix

• Compare 2 files byte by byte.
• It is mostly useful for scripts as it only reports whether the files are different or not.
• Helps to find out whether the two files are identical or not.

When cmp is used for comparison between two files, it reports the location of the first mismatch to the screen if difference is found and if no difference is found i.e the files compared are identical. cmp displays no message and simply returns the prompt if the files compared are identical.

SYNTAX:
cmp [OPTION] file1 file2

Options can be:

-c :Output differing bytes as characters. -i N :Ignore differences in the first N bytes of input. -l : Write the byte number (decimal) and the differing bytes (octal) for each difference. -s : Write nothing for differing files; return exit statuses only. -v : Output version info

Example 1: lets consider two files :

$ cat file1.txt
My name is Mohan
$ cat file2.txt
My name is Mohit

1. Compare file1 to file2 and outputs results

$ cmp file1.txt file2.txt
OUTPUT:  file1.txt file2.txt differ: byte 15, line 1

2. Skip same number of initial bytes from both input files

$ cmp -i 3 file1.txt file2.txt 
file1.txt file2.txt differ: byte 12, line 1

So we see that the initial 3 bytes were skipped

Example 2:

$ cat file1.txt
My name is Mira
$ cat file2.txt
My name is Mira
$cmp file1.txt file2.txt
$ _

/*indicating that the files are identical*/

diff command in Unix

• • • Reports the differences between files.
    • Compares files line by line.
    • It is useful when record level differences are to be traced.
    • Tells which lines in one file have to be changed to make the two files identical.
    • If the files match, no output is produced.
    • diff uses certain special symbols and instructions that are required to make two files identical

Special symbols are:

• • • a : add
    • c : change
    • d : delete

SYNTAX:
diff [option...] file1 file2

Example:

$cat  test1.txt
sam
sam
Tom
Tom
Tom

$cat test2.txt
Tom
Tom
Tom
Tom
Tom

$diff test1.txt test2.txt
Output:
1,2c1,2
< sam
< sam
—
>
> Tom

The output 1,2c1 means that the lines from 1,2 in the first file need to be changed to match the line number 1 in the second file.  Lines preceded by ‘<‘ are lines from the first file.  Lines preceded by ‘>’ are lines from the second file.

comm command in Unix

• • • • Compares 2 sorted files line by line
      • Produces 3 column output when no Options are passed.
      • Column 1 contains line unique to first file
      • Column 2 contains line unique to second file
      • Column 3 contains line common to both files