git-index.sh

$ git init .
Initialized empty Git repository in /Users/swittnekamp/tmp/git_index/.git/

$ cat <<EOF > foo.txt
Some example content
EOF

$ git add foo.txt

$ git status
On branch master

Initial commit

Changes to be committed:
  (use "git rm --cached <file>..." to unstage)

	new file:   foo.txt

$ git commit -m "Add foo"
[master (root-commit) 0cf172d] Add foo
 1 file changed, 1 insertion(+)
 create mode 100644 foo.txt

$ sed -i -e 's/example/banana/' foo.txt

$ git diff
diff --git a/foo.txt b/foo.txt
index e8ef7a2..162472c 100644
--- a/foo.txt
+++ b/foo.txt
@@ -1 +1 @@
-Some example content
+Some banana content

$ git add foo.txt

$ git status
On branch master
Changes to be committed:
  (use "git reset HEAD <file>..." to unstage)

	modified:   foo.txt

$ sed -i -e 's/Some/Yellow/' foo.txt

$ git diff
diff --git a/foo.txt b/foo.txt
index 162472c..366cb25 100644
--- a/foo.txt
+++ b/foo.txt
@@ -1 +1 @@
-Some banana content
+Yellow banana content

$ git status
On branch master
Changes to be committed:
  (use "git reset HEAD <file>..." to unstage)

	modified:   foo.txt

Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git checkout -- <file>..." to discard changes in working directory)

	modified:   foo.txt