Tracking Changes

Last updated on 2023-10-19 | Edit this page

Overview

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.

First let’s make sure we’re still in the right directory. You should be in the simple-site directory.

BASH 

$ cd ~/Desktop/simple-site

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.

BASH 

$ nano index.md

Type the text below into the index.md file:

OUTPUT 

I am a ___ at UCSB

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

BASH 

$ ls

OUTPUT 

index.md

index.md contains a single line, which we can see by running:

BASH 

$ cat index.md

OUTPUT 

I am a ___ at UCSB

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

BASH 

$ git status

OUTPUT 

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:

BASH 

$ git add index.md

and then check that the right thing happened:

BASH 

$ git status

OUTPUT 

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:

BASH 

$ git commit -m "Start new webpage"

OUTPUT 

[main (root-commit) f22b25e] Start new webpage
 1 file changed, 1 insertion(+)
 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 f22b25e. 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:

BASH 

$ git status

OUTPUT 

On branch main
nothing to commit, working tree 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:

BASH 

$ git log

OUTPUT 

commit f22b25e3233b4645dabd0d81e651fe074bd8e73b
Author: ...
Date:   Thu Aug 22 09:51:46 2013 -0400

    Start new webpage

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).

Let’s adds more information to the file. (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.)

BASH 

$ nano index.md
$ cat index.md

OUTPUT 

# Your Name

I am a ___ at UCSB

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

BASH 

$ git status

OUTPUT 

On branch main
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:   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:

BASH 

$ git diff

OUTPUT 

diff --git a/index.md b/index.md
index 7d781a7..bbb33fe 100644
--- a/index.md
+++ b/index.md
@@ -1 +1,3 @@
+# Your Name
+
 I am a ___ at UCSB

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; 7d781a7 and bbb33fe 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:

BASH 

$ git commit -m "Add header"

OUTPUT 

On branch main
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:   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:

BASH 

$ git add index.md
$ git commit -m "Add header"

OUTPUT 

[main 019f377] add header
 1 file changed, 2 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!

The Git Staging Area

Let’s watch as our changes to a file move from our editor to the staging area and into long-term storage. First, we’ll add another line to the file:

BASH 

$ nano index.md
$ cat index.md

OUTPUT 

# Your Name

I am a ___ at UCSB. My responsibilities include:

- Carpentry Workshops

BASH 

$ git diff

OUTPUT 

diff --git a/index.md b/index.md
index bbb33fe..22a33eb 100644
--- a/index.md
+++ b/index.md
@@ -1,3 +1,5 @@
 # Your Name

-I am a ___ at UCSB
+I am a ___ at UCSB. My responsibilities include:
+
+- Carpentry Workshops

So far, so good: we’ve added one line to the end of the file (shown with a + in the first column). Now let’s put that change in the staging area and see what git diff reports:

BASH 

$ 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:

BASH 

$ git diff --staged

OUTPUT 

diff --git a/index.md b/index.md
index bbb33fe..22a33eb 100644
--- a/index.md
+++ b/index.md
@@ -1,3 +1,5 @@
 # Your Name

-I am a ___ at UCSB
+I am a ___ at UCSB. My responsibilities include:
+
+- Carpentry Workshops

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

BASH 

$ git commit -m "add responsibilities"

OUTPUT 

[main d11d7e5] add responsibilities
 1 file changed, 3 insertions(+), 1 deletion(-)

check our status:

BASH 

$ git status

OUTPUT 

On branch main
nothing to commit, working tree clean

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

BASH 

$ git log

OUTPUT 

commit d11d7e52ab98d3d4c18cde4c4a0bbeea3fe40983 (HEAD -> main)
Author: ...
Date:   Thu Oct 19 12:07:51 2023 -0400

    add responsibilities

commit 019f37773f9f18b77f508990df65e56a34df45de
Author: ...
Date:   Thu Oct 19 12:03:04 2023 -0400

    add header

commit 8defaab26aa641a4233896ec68e603c541aa77b4
Author: ...
Date:   Thu Oct 19 12:01:17 2023 -0400

    add page

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:

BASH 

$ git log -1

OUTPUT 

commit d11d7e52ab98d3d4c18cde4c4a0bbeea3fe40983 (HEAD -> main)
Author: ...
Date:   Thu Oct 19 12:07:51 2023 -0400

    add responsibilities

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

BASH 

$ git log --oneline

OUTPUT 

d11d7e5 (HEAD -> main) add responsibilities
019f377 add header
8defaab add page

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:

BASH 

$ git log --oneline --graph

OUTPUT 

* d11d7e5 (HEAD -> main) add responsibilities
* 019f377 add header
* 8defaab add page

Directories

Two important facts you should know about directories in Git.

  1. Git does not track directories on their own, only files within them. Try it for yourself:

BASH 

$ mkdir spaceships
$ git status
$ git add spaceships
$ git status

Note, our newly created empty directory spaceships 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.

  1. If you create a directory in your Git repository and populate it with files, you can add all files in the directory at once by:

BASH 

git add <directory-with-files>

Try it for yourself:

BASH 

$ touch spaceships/apollo-11 spaceships/sputnik-1
$ git status
$ git add spaceships
$ git status

Before moving on, we will commit these changes.

BASH 

$ git commit -m "Add some initial thoughts on spaceships"

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):

The Git Commit Workflow

Choosing a Commit Message

Which of the following commit messages would be most appropriate for the last commit made to index.md?

  1. “Changes”
  2. “Added line ‘But the Mummy will appreciate the lack of humidity’ to index.md”
  3. “Discuss effects of Mars’ climate on the Mummy”

Answer 1 is not descriptive enough, and the purpose of the commit is unclear; and answer 2 is redundant to using “git diff” to see what changed in this commit; but answer 3 is good: short, descriptive, and imperative.

Committing Changes to Git

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

  1. BASH 

      $ git commit -m "my recent changes"

  2. BASH 

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

  3. BASH 

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

  4. BASH 

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

  1. Add some text to index.md noting your decision to consider Venus as a base
  2. Create a new file venus.txt with your initial thoughts about Venus as a base for you and your friends
  3. Add changes from both files to the staging area, and commit those changes.

The output below from cat index.md reflects only content added during this exercise. Your output may vary.

First we make our changes to the index.md and venus.txt files:

BASH 

$ nano index.md
$ cat index.md

OUTPUT 

Maybe I should start with a base on Venus.

BASH 

$ nano venus.txt
$ cat venus.txt

OUTPUT 

Venus is a nice planet and I definitely should consider it as a base.

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

BASH 

$ git add index.md venus.txt

Or with multiple commands:

BASH 

$ git add index.md
$ git add venus.txt

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

BASH 

$ git commit -m "Write plans to start a base on Venus"

OUTPUT 

[main cc127c2]
 Write plans to start a base on Venus
 2 files changed, 2 insertions(+)
 create mode 100644 venus.txt

bio Repository

  • Create a new Git repository on your computer called bio.
  • Write a three-line biography for yourself in a file called me.txt, commit your changes
  • Modify one line, add a fourth line
  • Display the differences between its updated state and its original state.

If needed, move out of the simple-site folder:

BASH 

$ cd ..

Create a new folder called bio and ‘move’ into it:

BASH 

$ mkdir bio
$ cd bio

Initialise git:

BASH 

$ git init

Create your biography file me.txt using nano or another text editor. Once in place, add and commit it to the repository:

BASH 

$ git add me.txt
$ git commit -m "Add biography file"

Modify the file as described (modify one line, add a fourth line). To display the differences between its updated state and its original state, use git diff:

BASH 

$ git diff me.txt

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.