What do the numbers on a Git Diff header mean? [duplicate] - diff

Question

This question already has answers here:
What does the “##…##” meta line with at signs in svn diff or git diff mean?
(3 answers)
Closed 7 years ago.
Every time I run git diff, for each single changes I made, I get some sort of header with numbers, for example:
## -169,14 +167,12 ## function Browser(window, document, body, XHR, $log) {.....
I wonder what does the four numbers mean? I guess -169 means that this particular line of code that follows was originally in line 169 but now is in 167? And what do 14 and 12 mean?

Answer 1

This header is called set of change, or hunk. Each hunk starts with a line that contains, enclosed in ##, the line or line range from,no-of-lines in the file before (with a -) and after (with a +) the changes. After that come the lines from the file. Lines starting with a - are deleted, lines starting with a + are added. Each line modified by the patch is surrounded with 3 lines of context before and after.
An addition looks like this:
## -75,6 +103,8 ##
foo
bar
baz
+line1
+line2
more context
and more
and still context
That means, in the original file before line 78 (= 75 + 3 lines of context) add two lines. These will be lines 106 (= 103 + 3 lines of context) through 107 after all changes.
Note the difference in from numbers (-75 vs +103), this means that there were other changes in this file before this particular hunk, that added 28 (103 - 75) lines of code.
A deletion looks like this:
## -75,7 +75,6 ##
foo
bar
baz
-line1
more context
and more
and still context
That means, delete line 78 (= 75 + 3 lines of context) in the original file. The unchanged context will be on lines 75 to 80 after all changes.
Note that from numbers in this hunk are equal (-75 and +75), this means that either there were no changes before this hunk, or amount of added and deleted lines in previous changes are the same.
Finally, a change looks like this:
## -70,7 +70,7 ##
foo
bar
baz
-red
+blue
more context
and more
still context
That means, change line 73 (= 70 + 3 lines of context) in the file before all changes, which contains red to blue. The changed line is also line 73 (= 70 + 3 lines of context) in the file after all changes.

Answer 2

I wonder what does the four numbers mean?
Let's analyze a simple example
The format is basically the same the diff -u unified diff.
We start with numbers from 1 to 16 and remove 2, 3, 14 and 15:
diff -u <(seq 16) <(seq 16 | grep -Ev '^(2|3|14|15)$')
Output:
## -1,6 +1,4 ##
1
-2
-3
4
5
6
## -11,6 +9,4 ##
11
12
13
-14
-15
16
## -1,6 +1,4 ## means:
-1,6 means that this piece of the first file starts at line 1 and shows a total of 6 lines. Therefore it shows lines 1 to 6.
1
2
3
4
5
6
- means "old", as we usually invoke it as diff -u old new.
+1,4 means that this piece of the second file starts at line 1 and shows a total of 4 lines. Therefore it shows lines 1 to 4.
+ means "new".
We only have 4 lines instead of 6 because 2 lines were removed! The new hunk is just:
1
4
5
6
## -11,6 +9,4 ## for the second hunk is analogous:
on the old file, we have 6 lines, starting at line 11 of the old file:
11
12
13
14
15
16
on the new file, we have 4 lines, starting at line 9 of the new file:
11
12
13
16
Note that line 11 is the 9th line of the new file because we have already removed 2 lines on the previous hunk: 2 and 3.

Answer 3

Summary:
Assume git diff will output [0-3] lines of context [before/after] [first/last] changes
## -[original file's number of first line displayed],[context lines + removed lines] +[changed file's number of first line displayed],[context lines + added lines] ##