$ git status
examples/out/status_03.txt
# On branch master
# 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:   README.txt
#
no changes added to commit (use "git add" and/or "git commit -a")

What has changed?

$ git diff
WARNING: terminal is not fully functional
diff --git a/README.txt b/README.txt
index e51ca0d..a697828 100644
--- a/README.txt
+++ b/README.txt
@@ -1 +1,2 @@
-Hello Git
+Hello Git
+Second line
$ git add README.txt
$ git status
examples/out/status_05.txt
# On branch master
# Changes to be committed:
#   (use "git reset HEAD <file>..." to unstage)
#
#       modified:   README.txt
#

What did we change?

$ git diff
$ git diff --cached    (or --staged)
WARNING: terminal is not fully functional
diff --git a/README.txt b/README.txt
index e51ca0d..62567d0 100644
--- a/README.txt
+++ b/README.txt
@@ -1 +1,2 @@
-Hello Git
\ No newline at end of file
+Hello Git
+Second line
$ git commit -m "update README"
[master 1251a45] update README
 1 file changed, 2 insertions(+), 1 deletion(-)