What do the numbers on a Git Diff header mean?

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] @@

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.

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.