Git diff

Diffing is a function that takes two input data sets and outputs the changes between them. git diff is a multi-use Git command that when executed runs a diff function on Git data sources. These data sources can be commits, branches, files and more. The git diff command is often used along with git status and git log to analyze the current state of a Git repo.

Example

Create and go to a directory

mkdir git_diff_example
cd  git_diff_example

Create a file
```
echo "This is a test" > a.txt
```

Initialise a git repo and commit the file

git init .
git add a.txt
git commit -m 'add(random file)'

If we execute git diff, there will be no output.
Let’s change the content of the file
```
echo "Another line" > a.txt
```

We can view the git diff output :

diff --git a/a.txt b/a.txt
index 90bfcb5..364133d 100644
--- a/a.txt
+++ b/a.txt
@@ -1 +1 @@
-this is a test
+Another line

The diff --git a/a.txt b/a.txt shows the sources of the diff.

The next line index 0527e6b..364133d 100644 represents git metadata.

The line --- a/a.txt and +++ b/a.txt displays the annotation of the input files.

The remaining content is a list of chunks. It only displays the section of the file that has changed.

The @@ -1 +1 @@ summarises the changes. It means that one line has been deleted and one has been added.

Finally, the -This is a test shows what is on the first file, and +Another line is what is on the second file.

Specify a file: The git diff command can have an explicit file path option.
```
git diff HEAD a.txt  
```
It will show the changes that are not in the stage index, which is the difference between the working directory and the local repository.
We can also see the diff between a file in the staging index and the local repository
```
echo "Original content" > b.txt
git add b.txt
git commit -m 'add(b file)'
echo "New line in stage index" > b.txt
git add b.txt
```
Here, we create a file b.txt and commit it in the local repository. Then, we modify the content.

If we do git diff b.txt, we will see nothing. This is because the change is in the staging index.

git diff --cached b.txt

It will output:

diff --git a/b.txt b/b.txt
index 9415d49..198feee 100644
--- a/b.txt
+++ b/b.txt
@@ -1 +1 @@
-Original content
+New line in stage index

It will output the differences between the file in the staging index and the local repository.

Diff between commits

We can see the differences between different branches, commit, etc.

For instance, if we have the following commits (obtained with git log --pretty=oneline)

e67ff745d10289508859f2b4780df1f69cc5e38b (HEAD -> main) update(b)
e0ecf97ef4fa81fdfbe195801b86fb00008c632f add(b file)

Then, to compare the differences between the two commits, we can just do:

git diff e67ff e0ecf

Diff between branches

Let’s create a branch and commit a file:

echo "Content from main branch" > a.txt
git add a.txt
git commit -m 'add(original file)'
git checkout -b dev
echo "Content from dev" > a.txt
git add a.txt
git commit -m 'update(a)'

We can then compare a specific file between the branch main and dev :

git diff main dev a.txt

We see the differences of the file between the two branches.

diff --git a/a.txt b/a.txt
index 55f8e97..103c3c3 100644
--- a/a.txt
+++ b/a.txt
@@ -1 +1 @@
-Content from main branch
+Content from dev