This post goes over how to inspect changes with git diff.

Prerequisites

Given the following:

echo 'Hello, world' > file.txt && git add . && git commit -m 'add file'

The text world is replaced with Mark:

sed -i.bak 's/world/Mark/' file.txt

git diff

Inspect the changes with git diff:

git diff
-Hello, world
+Hello, Mark

Additionally, --word-diff is useful for side-by-side comparisons per line:

git diff --word-diff
Hello, [-world-]{+Mark+}

If you stage the changes for commit:

git add file.txt

You can inspect the diff again with --cached or --staged:

git diff --cached

You can even compare branches:

git diff master..dev

As well as revisions:

git diff af5626b..6fd855f