Introducing Version Control with Git

Seeing the changes (diffing)

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 meow.

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 main
Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git restore <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..1abd0a1 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 meow.

(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 (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 meow. 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 version controlled directory (which we also call 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

Should print no output, because something.md has not changed since the last commit.

Reverting a change

Maybe you don't want to save this change? Remember, the old version of the file is safely saved in git. We can revert back to the old version of README.md by using the command git checkout. Typing git checkout main FILENAME will revert back to the last saved version of FILENAME. Let's revert back to our original README.md. Type;

git checkout main README.md

Run git status again. You should now see that the working tree is clean, and README.md has been reverted back to the old version, where the cat goes woof.

Let's now change README.md again. You are free to either fix the file to make the dog goes woof or the cat goes meow (it's your choice ;-)).

Saving the change

Once you've fixed the file, you will need to commit the change using git commit -a and add a suitable commit message. For example, I will make the cat goes meow and will use this commit message;

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 main
# Changes to be committed:
#       modified:   README.md
#

When I save and exit from the text editor I see this output from git. You should see something similar (but not identical)

[main 3bdce37] Fixing a typo in README.md. Cats do not go woof.
 1 file changed, 1 insertion(+), 1 deletion(-)

Git Cheat Sheet

We have two new commands, git diff for printing differences, and git checkout for reverting files.

Git Cheat Sheet

(1)  git init             : Tell git to start version controlling the files in a directory
                            (initialises git in a directory)
(2)  git status           : Tell git to print the status of the files in the version 
                            controlled directory.
(3)  git add              : Tell git to start monitoring (tracking) the versions of a new
                            file, e.g. `git add README.md` will tell git to track `README.md`   
(4)  git commit -a        : Tell git to save a new snapshot version of all of the tracked
                            files in the directory. The `-a` means "all files". You can
                            commit new versions of individual files if you want, but this
                            is not recommended.
(5)  git diff             : Tell git to show the differences between the files in the working
                            directory and the last saved version in the git repository. This will
                            show the differences for all tracked files. Use
                            `git diff FILENAME` to limit to only the file `FILENAME`
(6) git checkout main FILENAME : Tell git to revert a file back to the last version of `FILENAME` 
                                 saved in the repository

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 checkout something.md to revert the file back to the last saved version. Has this correctly restored the old version of the file?

Make some more changes to something.md.