Level 29 - there is no way to get the line number

[ray-mints]

After using git diff or gitk to see changes and running githug command once again there is a question:

What is the number of the line which has changed?

According to this stackoverflow answer there's no way for git diff to show a line number.
It is not clear that user should use another software (like vim) to get the answer.

[duianto]

I solved it by reading up on how the diff chunks (hunks?) header works.

For example with a committed file (test.txt) containing these lines:

one
two
three
four
five
six
seven
eight
nine
ten

After changing line five to changed, saving the file, and calling git diff, it looks like this:

diff --git a/test.txt b/test.txt
index c9e9e05..796305d 100644
--- a/test.txt
+++ b/test.txt
@@ -2,7 +2,7 @@ one
 two
 three
 four
-five
+changed
 six
 seven
 eight

The lines:

--- a/test.txt
+++ b/test.txt

means that the changes in file a/test.txt (the committed changes) are marked with a - (dash) sign,
and the changes in file b/test.txt (the not yet committed changes) are marked with a + sign.

The diff chunks header @@ -2,7 +2,7 @@ says that:

• @@ are just chunk header symbols, probably to make the chunk headers more visible
• -2 means that the first line that's shown from a/test.txt is two
• 7 means that 7 lines are shown from file a/test.txt
• +2 means that the first line from from b/test.txt also is two
• 7 lines are also shown from file b/test.txt

That means that the first line that's changed is line 5 because the visible chunk starts on line 2 two and if we count down we get to -five which is the first changed line in file a/test.txt. Which is the fifth line.

The context lines before and after the changed lines can be removed. A comment https://stackoverflow.com/a/50049834 on the SO (Stack Overflow) page that you linked to, says:

A quick way is to use git diff -U0. That will set the lines of context to 0, which will make the @@ values match the actual changed lines. By default, the @@ values include 3 lines of before/after context, which is not convenient for humans.

git diff -U0 outputs:

diff --git a/test.txt b/test.txt
index c9e9e05..796305d 100644
--- a/test.txt
+++ b/test.txt
@@ -5 +5 @@ four
-five
+changed

No context (extra lines) are shown above and below, and the chunk header lists the changed line number
-5 +5 (line 5 in both a/test.txt and b/test.txt).

These two pages (they were among the first hits when searching for git diff) laid out the basics:
https://www.git-tower.com/learn/git/ebook/en/command-line/advanced-topics/diffs
https://www.atlassian.com/git/tutorials/saving-changes/git-diff
but it was most helpful to do some experiments (what happens if I do this...).

This might also be useful reading: https://en.wikipedia.org/wiki/Diff

Hopefully this was of some help.