Understand output of diff

Abstract Explanation

The normal output format of diff consists of one or more chunks of differences, each hunk shows one area where two files differ. Normal format hunks look like this:

COMMAND
< FROM-FILE-LINE
< FROM-FILE-LINE...
---
> TO-FILE-LINE
> TO-FILE-LINE...

There are three types of COMMAND. Each consists of a line number or comma-separated range of lines followed by a single character indicating the sort of change to make, and a line number or comma-separated range of lines in the second file. The types of change commands are:

1. LaR
   Add the lines in range R of the second file after line L of the first file. For example, 8a12,15 means append lines 12-15 of file 2 after line 8 of file 1; or, if changing file 2 into file 1, delete lines 12-15 of file 2.

2. FcT
   Replace the lines in range F of the first file with lines in range T of the second file. This is like a combined add and delete, but more compact. For example, 5,7c8,10 means change lines 5-7 of file 1 to read as lines 8-10 of file 2; or, if changing file 2 into file 1, change lines 8-10 of file 2 to read as lines 5-7 of file 1.

3. RdL
   Delete the lines in range R from the first file; line L is where they would have appeared in the second file had they not been deleted. For example, 5,7d3 means delete lines 5-7 of file 1; or, if changing file 2 into file 1, append lines 5-7 of file 1 after line 3 of file 2.

Example

We have an original file a.txt, like this:

line1
hello word!
This is line two.
End of file.

And we change file a.txt to b.txt, content of b.txt:

hello word!
This is line two, add sth to line two.
Insert a new line, in line3.
End of file.
Insert another new line after the end of file line.

And we run diff a.txt b.txt to show what’s the differences between two file, we have the following result:

1d0
< line1
3c2,3
< This is line two.
---
> This is line two, add sth to line two.
> Insert a new line, in line3.
4a5
> Insert another new line after the end of file line.

There three change COMMAND: 1d0, 3c2,3, 4a5.

1. 1d0 means delete the 1th line of a.txt to transform it to b.txt, 0 is useless.
2. 3c2,3 means change 3th line of a.txt to
3. 4a5 means append the following lines to transform a.txt to b.txt.