git diff Explained: How to See What Changed

git status tells you which files changed. git diff tells you what changed inside them. Reading a diff before every commit is one of the best habits you can build.

See unstaged changes#

Here notes.txt has three lines. Change the second and add a fourth:

cat notes.txt

Buy milk
Call the bank
Pay rent

sed -i 's/Call the bank/Call the bank before 5 pm/' notes.txt
echo "Water the plants" >> notes.txt
git diff

diff --git a/notes.txt b/notes.txt
index 62e4899..a7233bc 100644
--- a/notes.txt
+++ b/notes.txt
@@ -1,3 +1,4 @@
 Buy milk
-Call the bank
+Call the bank before 5 pm
 Pay rent
+Water the plants

Reading the output#

Git has no idea of an edited line: a changed line appears as one removed line and one added line.

Staged changes#

After git add, plain git diff shows nothing, because it compares your files with the staging area:

git add notes.txt
git diff
git diff --staged

diff --git a/notes.txt b/notes.txt
index 62e4899..a7233bc 100644
--- a/notes.txt
+++ b/notes.txt
@@ -1,3 +1,4 @@
 Buy milk
-Call the bank
+Call the bank before 5 pm
 Pay rent
+Water the plants

git diff --staged compares the staging area with the last commit, so it shows exactly what the next commit will contain. Run it before committing.

Compare commits#

Give two commits, and Git shows the changes from the first to the second:

git diff HEAD~1 HEAD --stat

 notes.txt | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--stat shows a summary instead of every line. --name-only lists only the file names.

Compare branches#

The same works with branch names, which is how you review a feature before merging:

git diff main feature/login
git diff main...feature/login

With three dots, Git shows only what feature/login changed since it split off from main, which is usually what you want. This is also what a pull request shows.

Compare one word at a time#

For long lines of text, this highlights just the words that changed:

sed -i 's/before 5 pm/before 4 pm/' notes.txt
git diff --word-diff

diff --git a/notes.txt b/notes.txt
index a7233bc..d205443 100644
--- a/notes.txt
+++ b/notes.txt
@@ -1,4 +1,4 @@
Buy milk
Call the bank before [-5-]{+4+} pm
Pay rent
Water the plants

Common mistakes#

Practice#

echo "Book tickets" >> notes.txt
git add notes.txt
git diff --staged --stat

 notes.txt | 1 +
 1 file changed, 1 insertion(+)

Key takeaway#

Next: Ignoring Files with .gitignore.