Tracking Changes

Overview

Teaching: 20 min
Exercises: 0 min

Questions

• How do I record changes in Git?

• How do I check the status of my version control repository?

• How do I record notes about what changes I made and why?

Objectives

• Go through the modify-add-commit cycle for one or more files.

• Explain where information is stored at each stage of that cycle.

• Distinguish between descriptive and non-descriptive commit messages.

Adding files

Let’s create a text file called index.md and add it to our repository. The file will later be converted into a webpage by GitHub pages. We’ll write the file using a syntax called Markdown, which is why we use the .md extensions.

Markdown

Markdown is a language used to simplify writing HTML. Plain text characters like # and * are used in place of HTML tags. These characters are then processed (by GitHub pages) and transformed into HTML tags. As the name Markdown suggests, the language has been trimmed down to a minimum. The most frequently used elements, like headings, paragraphs, lists, tables and basic text formatting (i.e. bold, italic) are part of Markdown. Markdown’s simplified syntax keeps content human-readable.

We’ll use nano to edit the file; you can use whatever editor you like. In particular, this does not have to be the core.editor you set globally earlier. But remember, the bash command to create or edit a new file will depend on the editor you choose (it might not be nano). For a refresher on text editors, check out “Which Editor?” in The Unix Shell lesson.

$ nano index.md

Type a few lines about yourself. (Use the # to create a section header).

# Seth Erickson

I am a data services librarian at UCSB

Let’s first verify that the file was properly created by running the list command (ls):

$ ls

index.md

index.md is a text file, which we can see by running:

$ cat index.md

# Seth Erickson

I am a data services librarian at UCSB

If we check the status of our project again, Git tells us that it’s noticed the new file:

$ git status

On branch main

No commits yet

Untracked files:
  (use "git add <file>..." to include in what will be committed)
	index.md

nothing added to commit but untracked files present (use "git add" to track)

The “untracked files” message means that there’s a file in the directory that Git isn’t keeping track of. We can tell Git to track a file using git add:

$ git add index.md

and then check that the right thing happened:

$ git status

On branch main

No commits yet

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

Git now knows that it’s supposed to keep track of index.md, but it hasn’t recorded these changes as a commit yet. To get it to do that, we need to run one more command:

$ git commit -m "adding initial page content (commit #1)"

[main (root-commit) f14106c] adding initial page content (commit #1)
 1 file changed, 3 insertions(+)
 create mode 100644 index.md

When we run git commit, Git takes everything we have told it to save by using git add and stores a copy permanently inside the special .git directory. This permanent copy is called a commit (or revision) and its short identifier is f14106c. Your commit may have another identifier.

We use the -m flag (for “message”) to record a short, descriptive, and specific comment that will help us remember later on what we did and why. If we just run git commit without the -m option, Git will launch nano (or whatever other editor we configured as core.editor) so that we can write a longer message.

Good commit messages start with a brief (<50 characters) statement about the changes made in the commit. Generally, the message should complete the sentence “If applied, this commit will” . If you want to go into more detail, add a blank line between the summary line and your additional notes. Use this additional space to explain why you made changes and/or what their impact will be.

If we run git status now:

$ git status

On branch main
nothing to commit, working directory clean

it tells us everything is up to date. If we want to know what we’ve done recently, we can ask Git to show us the project’s history using git log:

$ git log

commit f14106cb34f6983fa329168c97042739e4deec77 (HEAD -> main)
Author: Seth Erickson <xxx@yyy.com>
Date:   Mon Apr 24 11:01:07 2023 -0700

    adding initial page content (commit #1)

git log lists all commits made to a repository in reverse chronological order. The listing for each commit includes the commit’s full identifier (which starts with the same characters as the short identifier printed by the git commit command earlier), the commit’s author, when it was created, and the log message Git was given when the commit was created.

Where Are My Changes?

If we run ls at this point, we will still see just one file called index.md. That’s because Git saves information about files’ history in the special .git directory mentioned earlier so that our filesystem doesn’t become cluttered (and so that we can’t accidentally edit or delete an old version).

Now let’s add more to index.md. (Again, we’ll edit with nano and then cat the file to show its contents; you may use a different editor, and don’t need to cat.)

$ nano index.md

Let’s add a list of job responsibilities. (In Markdown, a line beginning with a - and space is converted into an HTML list with bullet points.)

# Seth Erickson

I am a data services librarian at UCSB

My responsibilities include:

- Teaching Carpentry Workshops
- Helping students learn Git

When we run git status now, it tells us that a file it already knows about has been modified:

$ git status

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:   index.md

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

The last line is the key phrase: “no changes added to commit”. We have changed this file, but we haven’t told Git we will want to save those changes (which we do with git add) nor have we saved them (which we do with git commit). So let’s do that now. It is good practice to always review our changes before saving them. We do this using git diff. This shows us the differences between the current state of the file and the most recently saved version:

$ git diff

diff --git a/index.md b/index.md
index 9b284ec..c826feb 100644
--- a/index.md
+++ b/index.md
@@ -1,3 +1,9 @@
 # Seth Erickson
 
 I am a data services librarian at UCSB
+
+My responsibilities include:
+
+- Teaching Carpentry Workshops
+- Helping students learn Git
+

The output is cryptic because it is actually a series of commands for tools like editors and patch telling them how to reconstruct one file given the other. If we break it down into pieces:

1. The first line tells us that Git is producing output similar to the Unix diff command comparing the old and new versions of the file.
2. The second line tells exactly which versions of the file Git is comparing; df0654a and 315bf3a are unique computer-generated labels for those versions.
3. The third and fourth lines once again show the name of the file being changed.
4. The remaining lines are the most interesting, they show us the actual differences and the lines on which they occur. In particular, the + marker in the first column shows where we added a line.

After reviewing our change, it’s time to commit it:

$ git commit -m "add list of responsibilities (commit #2)"

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:   index.md

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

Whoops: Git won’t commit because we didn’t use git add first. Let’s fix that:

$ git add index.md
$ git commit -m "add list of responsibilities (commit #2)"

[main bc5ac3e] add list of responsibilities (commit #2)
 1 file changed, 6 insertions(+)

Git insists that we add files to the set we want to commit before actually committing anything. This allows us to commit our changes in stages and capture changes in logical portions rather than only large batches. For example, suppose we’re adding a few citations to relevant research to our thesis. We might want to commit those additions, and the corresponding bibliography entries, but not commit some of our work drafting the conclusion (which we haven’t finished yet).

To allow for this, Git has a special staging area where it keeps track of things that have been added to the current changeset but not yet committed.

Staging Area

If you think of Git as taking snapshots of changes over the life of a project, git add specifies what will go in a snapshot (putting things in the staging area), and git commit then actually takes the snapshot, and makes a permanent record of it (as a commit). If you don’t have anything staged when you type git commit, Git will prompt you to use git commit -a or git commit --all, which is kind of like gathering everyone to take a group photo! However, it’s almost always better to explicitly add things to the staging area, because you might commit changes you forgot you made. (Going back to the group photo simile, you might get an extra with incomplete makeup walking on the stage for the picture because you used -a!) Try to stage things manually, or you might find yourself searching for “git undo commit” more than you would like!

Let’s watch as our changes to a file move from our editor to the staging area and into long-term storage. First, let’s make another change: I’ll fix a typo by adding a period to the bio line.

$ nano index.md

# Seth Erickson

I am a data services librarian at UCSB.

My responsibilities include:

- Teaching Carpentry Workshops
- Helping students learn Git

$ git diff

diff --git a/index.md b/index.md
index c826feb..93d5098 100644
--- a/index.md
+++ b/index.md
@@ -1,6 +1,6 @@
 # Seth Erickson
 
-I am a data services librarian at UCSB
+I am a data services librarian at UCSB.
 
 My responsibilities include:

So far, so good. Now let’s put that change in the staging area and see what git diff reports:

$ git add index.md
$ git diff

There is no output: as far as Git can tell, there’s no difference between what it’s been asked to save permanently and what’s currently in the directory. However, if we do this:

$ git diff --staged

diff --git a/index.md b/index.md
index c826feb..93d5098 100644
--- a/index.md
+++ b/index.md
@@ -1,6 +1,6 @@
 # Seth Erickson
 
-I am a data services librarian at UCSB
+I am a data services librarian at UCSB.
 
 My responsibilities include:

it shows us the difference between the last committed change and what’s in the staging area. Let’s save our changes:

$ git commit -m "fixing a typo (commit #3)"

[main d8dd09e] fixing a typo (commit #3)
 1 file changed, 1 insertion(+), 1 deletion(-)

check our status:

$ git status

On branch main
nothing to commit, working directory clean

and look at the history of what we’ve done so far:

$ git log

commit d8dd09e46f76a9e8560d57bd31ba99059eb27bad (HEAD -> main)
Author: Seth Erickson <...>
Date:   Mon Apr 24 11:19:41 2023 -0700

    fixing a typo (commit #3)

commit bc5ac3ec2412f7e4bfb4da2d7085d3df4cc514ea
Author: Seth Erickson <...>
Date:   Mon Apr 24 11:11:40 2023 -0700

    add list of responsibilities (commit #2)

commit f14106cb34f6983fa329168c97042739e4deec77
Author: Seth Erickson <...>
Date:   Mon Apr 24 11:01:07 2023 -0700

    adding initial page content (commit #1)

Word-based diffing

Sometimes, e.g. in the case of the text documents a line-wise diff is too coarse. That is where the --color-words option of git diff comes in very useful as it highlights the changed words using colors.

Paging the Log

When the output of git log is too long to fit in your screen, git uses a program to split it into pages of the size of your screen. When this “pager” is called, you will notice that the last line in your screen is a :, instead of your usual prompt.

• To get out of the pager, press Q.
• To move to the next page, press Spacebar.
• To search for some_word in all pages, press / and type some_word. Navigate through matches pressing N.

Limit Log Size

To avoid having git log cover your entire terminal screen, you can limit the number of commits that Git lists by using -N, where N is the number of commits that you want to view. For example, if you only want information from the last commit you can use:

$ git log -1

commit d8dd09e46f76a9e8560d57bd31ba99059eb27bad (HEAD -> main)
Author: Seth Erickson <...>
Date:   Mon Apr 24 11:19:41 2023 -0700

    fixing a typo (commit #3)
(END)

You can also reduce the quantity of information using the --oneline option:

$ git log --oneline

d8dd09e (HEAD -> main) fixing a typo (commit #3)
bc5ac3e add list of responsibilities (commit #2)
f14106c adding initial page content (commit #1)

You can also combine the --oneline option with others. One useful combination adds --graph to display the commit history as a text-based graph and to indicate which commits are associated with the current HEAD, the current branch main, or other Git references:

$ git log --oneline --graph

* d8dd09e (HEAD -> main) fixing a typo (commit #3)
* bc5ac3e add list of responsibilities (commit #2)
* f14106c adding initial page content (commit #1)

Directories in Git

Two important facts you should know about directories in Git:

1. Git does not track directories on their own, only files within them.
2. If you can create a directory in your Git repository and populate it with files, you can add all the files in the directory at once.

Create a new directory named images We will be using this directory in the following lessons so please follow along.

$ mkdir images
$ git status
$ git add images
$ git status

Note, our newly created empty directory images does not appear in the list of untracked files even if we explicitly add it (via git add) to our repository. This is the reason why you will sometimes see .gitkeep files in otherwise empty directories. Unlike .gitignore, these files are not special and their sole purpose is to populate a directory so that Git adds it to the repository. In fact, you can name such files anything you like.

Try adding an empty file in our new directory:

$ touch images/.gitkeep
$ git status
$ git add images
$ git status

Before moving on, we will commit these changes.

$ git commit -m "Add images folder"

To recap, when we want to add changes to our repository, we first need to add the changed files to the staging area (git add) and then commit the staged changes to the repository (git commit):

Committing Changes to Git

Which command(s) below would save the changes of myfile.txt to my local Git repository?

1. $ git commit -m "my recent changes"
   

2. $ git init myfile.txt
   $ git commit -m "my recent changes"
   

3. $ git add myfile.txt
   $ git commit -m "my recent changes"
   

4. $ git commit -m myfile.txt "my recent changes"
   

Solution

1. Would only create a commit if files have already been staged.
2. Would try to create a new repository.
3. Is correct: first add the file to the staging area, then commit.
4. Would try to commit a file “my recent changes” with the message myfile.txt.

Committing Multiple Files

The staging area can hold changes from any number of files that you want to commit as a single snapshot. The goals for the rest of this lesson are:

1. Add some text to index.md
2. Create a new file reading-list.md with a list of books you want to read
3. Add changes from both files to the staging area, and commit those changes.

You can add both files to the staging area. We can do that in one line:

$ git add index.md reading-list.md

Or with multiple commands:

$ git add index.md
$ git add reading-list.md

Now the files are ready to commit. You can check that using git status. If you are ready to commit use:

$ git commit -m "update page and add reading list"

[main 86abfdb] add reading list
 2 files changed, 2 insertions(+)
 create mode 100644 reading-list.md

Key Points

• git status shows the status of a repository.

• Files can be stored in a project’s working directory (which users see), the staging area (where the next commit is being built up) and the local repository (where commits are permanently recorded).

• git add puts files in the staging area.

• git commit saves the staged content as a new commit in the local repository.

• Write a commit message that accurately describes your changes.