"diff" Command

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diff - compare files line by line

When comparing two files, 'diff' finds sequences of lines common to both files, interspersed with groups of differing lines called hunks.

Comparing two identical files yields one sequence of common lines and no hunks, because no lines differ.

Comparing two entirely different files yields no common lines and one large hunk that contains all lines of both files.

In general, there are many ways to match up lines between two given files. `diff' tries to minimize the total hunk size by finding large sequences of common lines interspersed with small hunks of differing lines.

root:~# man diff
root:~# info diff

Following files will be used in examples.

root:~# cat file1.txt
I need flowers.
I need to buy oranges.
I need to run the dryer.
I need to wash the door.
I need to get the car detailed.
I need chocholates.
root:~#
root:~# cat file2.txt
I need to buy oranges.
I need to run the iron.
I need to wash the windows.
I need to get the car detailed.
I need to buy new phone.
root:~#
root:~# cat file3.txt
tiger
lion
hippo
elephant
root:~#
root:~# cat file4.txt
tiger
rhino
elephant
cheetah
root:~#

Example-1

1 - default display diff between two files

root:~#diff file1.txt file2.txt
1d0
< I need flowers.
3,4c2,3
< I need to run the dryer.
< I need to clean the door.
---
> I need to run the iron.
> I need to clean the windows.
6c5
< I need chocholates.
---
> I need to buy new phone.

Important Notes:
It tells how to change the first file to make it match the second file.
Numeric output (1d0,3,4c2,3,6c5) of diff will contain :
     line numbers corresponding to the first file,
     single letter (a for add,c for change or d for delete),
     line numbers corresponding to the second file.
Lines preceded by < are lines from the first file
Lines preceded by > are lines from the second file
Three dashes('---') separates the lines of file 1 and file 2.

Let's Understand the above diff output.

1d0
< I need flowers.

User needs to delete Line-1 of 1^st file so that both files sync up at the begining of line (0).

3,4c2,3
< I need to run the dryer.
< I need to clean the door.
---
> I need to run the iron.
> I need to clean the windows.

"3,4c2,3" means Lines 3 to 4 of 1^st file needs to be changed to match lines 2 to 3 of 2^nd file. After that it tells us what those lines are in each file:
As mentioned,
Lines preceded by < are lines from the first file;
Three dashes('---') separates the lines of file 1 and file 2;
Lines preceded by > are lines from the second file

6c5
< I need chocholates.
---
> I need to buy new phone.

"6c5" means Change Line-6 of 1^st file to Line-5 of 2^nd file.

Example-2

2 - display diff between two files in context mode (-c)

root:# diff -c file3.txt file4.txt
*** file3.txt    Mon Dec 18 14:28:52 2017
--- file4.txt    Mon Dec 18 14:29:01 2017
***************
*** 1,4 ****
  tiger
! lion
! hippo
  elephant
--- 1,4 ----
  tiger
! rhino
  elephant
+ cheetah

Important Notes:
The "from" file is indicated by "***"
The "to" file is indicated by "---"
The "***************" line is just separator
If line is unchanged, it's prefixed by two spaces.
If line is changed, it's prefixed by an indicative character and a space.
    ! Indicates that this line is part of a group of one or more lines that needs to change. There is a corresponding group of lines prefixed with "!" in the other file's context as well.
    + Indicates that a line in the second file that needs to be added to the first file.
    - Indicates that a line in the first file that needs to be deleted.