Diffs

What is a diff?

A diff is the output of using the git diff command. Is a text that allows to view differences or changes between commits, branches, files, working directory and many other things.

Example:

diff --git a/rainbow.txt b/rainbow.txt
index 0b75516..26ec8e7 100644
--- a/rainbow.txt
+++ b/rainbow.txt
@@ -3,4 +3,5 @@ orange
 yellow
 green
 blue
-purple
+indigo
+violet

How to read a diff

Here are the more meaningful lines in the example below:

The first line shows which two files are being compared. Is normally the same file but can be set up to be different. Original is going to be A and new is B

diff --git a/rainbow.txt b/rainbow.txt

- will be used for representing changes in file a, and + for changes in file b

--- a/rainbow.txt
+++ b/rainbow.txt

Next the chunks will start. A diff will only show portions or chunks of files that were modified together with some context (lines before and after). The first thing is the header. In this case it means: From file a, 4 lines are extracted starting from line 3 (in the chunk) and from file b, 5 lines are extracted starting from line 3 (in the chunk)

@@ -3,4 +3,5 @@ orange

Finally, the changes are shown with the symbols relating to the files as explained above

 yellow
 green
 blue
-purple
+indigo
+violet

In this case, in file A, the last line was purple. Meanwhile in file B, purple was removed and indigo and violet were added

How to create a diff

There a different ways to run the git diff command according to what you want to compare:

git diff will show changes in the working directory that are not in staging yet. In other words, compares the working directory with the staging area
git diff HEAD will compare HEAD and the working directory. In other words, it shows changes in the working directory since the last commit. Different to above it will show staged and unstaged changes.
git diff --staged or git diff --cached will compare HEAD and the staging area. It will show the changes that will be committed.

It is also possible to combine this options with different targets:

git diff <OPS> file shows changes for a specific file
git diff <OPS> branch1..branch2 or git diff <OPS> branch1 branch2 will compare two branches. The order in which they are passed determines which is considered A and B (First is always the “base”). In addition, adding a file name at the end, will limit the comparison to a specific file
git diff commit1..commit2 will compare changes between two commits, passing the hash of the commits. Same as above, the order influences which is A and B.