diff command in Linux with examples

16 Sep, 2021
diff stands for difference. This command is used to display the differences in the files by comparing the files line by line. Unlike its fellow members, cmp and comm, it tells us which lines in one file have is to be changed to make the two files identical. 
The important thing to remember is that diff uses certain special symbols and instructions that are required to make two files identical. It tells you the instructions on how to change the first file to make it match the second file. 
Special symbols are: 
a : add
c : change
d : delete
Syntax :  
diff [options] File1 File2 
Lets say we have two files with names a.txt and b.txt containing 5 Indian states.  
Chapters
descriptions off, selected
captions settings, opens captions settings dialog
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English
This is a modal window.
Beginning of dialog window. Escape will cancel and close the window.
End of dialog window.
$ ls
a.txt  b.txt

$ cat a.txt
Gujarat
Uttar Pradesh
Kolkata
Bihar
Jammu and Kashmir

$ cat b.txt
Tamil Nadu
Gujarat
Andhra Pradesh
Bihar
Uttar pradesh
Now, applying diff command without any option we get the following output:  
$ diff a.txt b.txt
0a1
> Tamil Nadu
2,3c3
< Uttar Pradesh
 Andhra Pradesh
5c5
 Uttar pradesh
Let’s take a look at what this output means. The first line of the diff output will contain:  
Line numbers corresponding to the first file,
A special symbol and
Line numbers corresponding to the second file.
Like in our case, 0a1 which means after lines 0(at the very beginning of file) you have to add Tamil Nadu to match the second file line number 1. It then tells us what those lines are in each file preceded by the symbol:  
Lines preceded by a < are lines from the first file.
Lines preceded by > are lines from the second file.
Next line contains 2,3c3 which means from line 2 to line 3 in the first file needs to be changed to match line number 3 in the second file. It then tells us those lines with the above symbols.
The three dashes (“—“) merely separate the lines of file 1 and file 2.
As a summary to make both the files identical, first add Tamil Nadu in the first file at very beginning to match line 1 of second file after that change line 2 and 3 of first file i.e. Uttar Pradesh and Kolkata with line 3 of second file i.e. Andhra Pradesh. After that change line 5 of first file i.e. Jammu and Kashmir with line 5 of second file i.e. Uttar Pradesh. 
Now let’s see what it looks like when diff tells us that we need to delete a line. 
$ cat a.txt
Gujarat
Andhra Pradesh
Telangana
Bihar
Uttar pradesh

$ cat b.txt
Gujarat
Andhra Pradesh
Bihar
Uttar pradesh

$ diff a.txt b.txt
3d2
< Telangana
Here above output 3d2 means delete line 3rd of first file i.e. Telangana so that both the files sync up at line 2. 
Linux system offers two different ways to view the diff command output i.e. context mode and unified mode. 
1. -c (context) : To view differences in context mode, use the -c option. Lets try to understand this with example, we have two files file1.txt and file2.txt: 
$ cat file1.txt                                                                                                                            
cat                                                                                                                                              
mv                                                                                                                                               
comm                                                                                                                                             
cp 

$ cat file2.txt
cat                                                                                                                                              
cp                                                                                                                                               
diff                                                                                                                                             
comm
                                                                                                   
$ diff -c file1.txt file2.txt                                                                                                              
*** file1.txt   Thu Jan 11 08:52:37 2018                                                                                                         
--- file2.txt   Thu Jan 11 08:53:01 2018                                                                                                         
***************                                                                                                                                  
*** 1,4 ****                                                                                                                                     
  cat                                                                                                                                            
- mv                                                                                                                                             
- comm                                                                                                                                           
  cp                                                                                                                                             
--- 1,4 ----                                                                                                                                     
  cat                                                                                                                                            
  cp                                                                                                                                             
+ diff                                                                                                                                           
+ comm 
The first file is indicated by ***, and the second file is indicated by —. The line with *************** is just a separator. 
The first two lines of this output show us information about file 1 and file 2. It lists the file name, modification date, and modification time of each of our files, one per line. The next line has three asterisks *** followed by a line range from the first file (in our case lines 1 through 4, separated by a comma). Then four asterisks ****. After that it shows the contents of the first file with the following indicators: 
(i) If the line needs to be unchanged, it is prefixed by two spaces. (ii) If the line needs to be changed, it is prefixed by an symbol and a space. The symbol means are as follows: 
(a) + : It indicates a line in the second file that needs to be added to the first file to make them identical. (b) – : It indicates a line in the first file that needs to be deleted to make them identical. Like in our case, it is needed to delete mv and comm from first file and add diff and comm to the first file to make both of them identical. 
After that there are three dashes — followed by a line range from the second file (in our case lines 1 through 4, separated by a comma). Then four dashes —-. Then it shows the contents of the second file. 
2. -u (unified) : To view differences in unified mode, use the -u option. It is similar to context mode but it doesn’t display any redundant information or it shows the information in concise form. 
$ cat file1.txt                                                                                                                            
cat                                                                                                                                              
mv                                                                                                                                               
comm                                                                                                                                             
cp 

$ cat file2.txt                                                                                                                            
cat                                                                                                                                              
cp                                                                                                                                               
diff                                                                                                                                             
comm

$ diff -u file1.txt file2.txt                                                                                                            
--- file1.txt   2018-01-11 10:39:38.237464052 +0000                                                                                              
+++ file2.txt   2018-01-11 10:40:00.323423021 +0000                                                                                              
@@ -1,4 +1,4 @@                                                                                                                                  
 cat                                                                                                                                             
-mv                                                                                                                                              
-comm                                                                                                                                            
 cp                                                                                                                                              
+diff                                                                                                                                            
+comm
The first file is indicated by —, and the second file is indicated by +++. The first two lines of this output show us information about file 1 and file 2. It lists the file name, modification date, and modification time of each of our files, one per line. After that the next line has two at sign @ followed by a line range from the first file (in our case lines 1 through 4, separated by a comma) prefixed by – and then space and then again followed by a line range from the second file prefixed by + and at the end two at sign @. Followed by the file content in output tells us which line remain unchanged and which lines needs to added or deleted(indicated by symbols) in the file 1 to make it identical to file 2. 
3. -i : By default this command is case sensitive. To make this command case in-sensitive use -i option with diff. 
$ cat file1.txt
dog
mv
CP
comm

$ cat file2.txt
DOG
cp
diff
comm

Without using this option:
$ diff file1.txt file2.txt
1,3c1,3
< dog
< mv
 DOG
> cp
> diff

Using this option:
$ diff -i file1.txt file2.txt
2d1
 diff
4. –version : This option is used to display the version of diff which is currently running on your system. 
$ diff --version
diff (GNU diffutils) 3.5
Packaged by Cygwin (3.5-2)
Copyright (C) 2016 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later .
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Written by Paul Eggert, Mike Haertel, David Hayes,
Richard Stallman, and Len Tower.
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