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

Question

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.

Credit goes to Markus Bertheau.