Diffing (seeing what has changed)

Git is always looking to see what has changed in your working directory. Git can tell you what has changed by using the git diff command, e.g. type

git diff

You should see that nothing is printed, because, at the moment, nothing has changed since the last commit.

So, let us now make a change. Open up the file README.md and fix the error that we made in the text. Change the line that reads

will say that the cat goes woof.

to read

will say that the cat goes mieow.

Save and exit from the text editor, and then use the git status command to see if Git knows about this change. You should see output similar to

# 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.md
#
no changes added to commit (use "git add" and/or "git commit -a")

The important line here is modified: README.md. This shows that Git knows that the README.md file has been changed. To see what the change is, type the command

git diff

You should see output similar to this

diff --git a/README.md b/README.md
index 6c72b9d..9bf6053 100644
--- a/README.md
+++ b/README.md
@@ -6,5 +6,5 @@ We will use Git to record all of the versions of this file,
 letting us move back and forth through time.
 
 For example, in this first version of the file we
-will say that the cat goes woof.
+will say that the cat goes mieow.

(note that, if you are lucky, you should see all of the above in different colours. If you don’t see different colours, then type git config --global color.diff auto and then run git diff again).

What this (again overcomplicated) output shows, is that Git knows that the file README.md has changed, with the line will say that the cat goes woof. being removed (indicated by the - sign), and the line will say that the cat goes mieow. has been added (indicated by the + sign).

By default, git diff will show you all of the changes that have occurred since the last commit in all of the files in the working directory. You can limit the output to only a specific file by using git diff FILENAME, e.g. type

git diff README.md

which should show you the changes in README.md. Now type

git diff something.md

You should get no output, because something.md has not changed since the last commit.

Now that you have changed files in the working directory, you need to commit those changes. Do this by typing

git commit -a

and then adding in a suitable commit message, e.g.

Fixing a typo in README.md. Cats do not go woof.

# Please enter the commit message for your changes. Lines starting
# with '#' will be ignored, and an empty message aborts the commit.
# On branch master
# Changes to be committed:
#   (use "git reset HEAD <file>..." to unstage)
#
#   modified:   README.md
#

Save and exit the text editor, and you should see something like this output from Git

master 9253499] Fixing a typo in README.md. Cats do not go woof.
 1 file changed, 1 insertion(+), 1 deletion(-)

This shows that Git has recorded that one file has changed, which involved adding (inserting) one new line of text, and deleting one old line of text.

If we now run git diff we should see that nothing is printed. A quick check of git status should show us that the working directory is clean.


Exercise

Edit your file called something.md. Make some changes to the text (e.g. adding some new lines, or changing some words).

Use git status. Does Git know that you have changed something.md?

Use git diff. Does Git correctly find all of your changes?

Use git diff README.md. Does Git know that nothing has changed with README.md?

Use git commit -a to commit your changes, ensuring that you write a good commit message. Does git diff now give you no changes? Does git status now show that your working directory is clean?


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