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### Version Control with Git

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# 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 Git commit workflow.

> - 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 `planets` directory.

    $ pwd

If you are still in `moons` navigate back up to `planets`

    $ cd ..

Let’s create a file called `mars.txt` that contains some notes about the Red Planet’s suitability as a base. 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 mars.txt

Type the text below into the `mars.txt` file:

    Cold and dry, but everything is my favorite color

`mars.txt` now contains a single line, which we can see by running:

    $ ls

    mars.txt

    $ cat mars.txt

    Cold and dry, but everything is my favorite color

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

    $ git status

    On branch master

    Initial commit

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

    	mars.txt
    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 mars.txt

and then check that the right thing happened:

    $ git status

    On branch master

    Initial commit

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

    	new file:   mars.txt

Git now knows that it’s supposed to keep track of `mars.txt`, 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 "Start notes on Mars as a base"

    [master (root-commit) f22b25e] Start notes on Mars as a base
     1 file changed, 1 insertion(+)
     create mode 100644 mars.txt

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) summary of changes made in the commit. If you want to go into more detail, add a blank line between the summary line and your additional notes.

If we run `git status` now:

    $ git status

    On branch master
    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 f22b25e3233b4645dabd0d81e651fe074bd8e73b
    Author: Vlad Dracula <vlad@tran.sylvan.ia>
    Date:   Thu Aug 22 09:51:46 2013 -0400

        Start notes on Mars as a base

`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 `mars.txt`. 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 suppose Dracula 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`.)

    $ nano mars.txt
    $ cat mars.txt

    Cold and dry, but everything is my favorite color
    The two moons may be a problem for Wolfman

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

    $ git status

    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:   mars.txt

    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/mars.txt b/mars.txt
    index df0654a..315bf3a 100644
    --- a/mars.txt
    +++ b/mars.txt
    @@ -1 +1,2 @@
     Cold and dry, but everything is my favorite color
    +The two moons may be a problem for Wolfman

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 concerns about effects of Mars' moons on Wolfman"
    $ git status

    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:   mars.txt

    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 mars.txt
    $ git commit -m "Add concerns about effects of Mars' moons on Wolfman"

    [master 34961b1] Add concerns about effects of Mars' moons on Wolfman
     1 file changed, 1 insertion(+)

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 our supervisor’s work to our thesis. We might want to commit those additions, and the corresponding addition to the bibliography, but not commit the work we’re doing on 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 for the picture! 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 snapshots, you might get the extra with incomplete makeup walking on the stage for the snapshot 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, we’ll add another line to the file:

    $ nano mars.txt
    $ cat mars.txt

    Cold and dry, but everything is my favorite color
    The two moons may be a problem for Wolfman
    But the Mummy will appreciate the lack of humidity

    $ git diff

    diff --git a/mars.txt b/mars.txt
    index 315bf3a..b36abfd 100644
    --- a/mars.txt
    +++ b/mars.txt
    @@ -1,2 +1,3 @@
     Cold and dry, but everything is my favorite color
     The two moons may be a problem for Wolfman
    +But the Mummy will appreciate the lack of humidity

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:

    $ git add mars.txt
    $ 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/mars.txt b/mars.txt
    index 315bf3a..b36abfd 100644
    --- a/mars.txt
    +++ b/mars.txt
    @@ -1,2 +1,3 @@
     Cold and dry, but everything is my favorite color
     The two moons may be a problem for Wolfman
    +But the Mummy will appreciate the lack of humidity

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 "Discuss concerns about Mars' climate for Mummy"

    [master 005937f] Discuss concerns about Mars' climate for Mummy
     1 file changed, 1 insertion(+)

check our status:

    $ git status

    On branch master
    nothing to commit, working directory clean

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

    $ git log

    commit 005937fbe2a98fb83f0ade869025dc2636b4dad5
    Author: Vlad Dracula <vlad@tran.sylvan.ia>
    Date:   Thu Aug 22 10:14:07 2013 -0400

        Discuss concerns about Mars' climate for Mummy

    commit 34961b159c27df3b475cfe4415d94a6d1fcd064d
    Author: Vlad Dracula <vlad@tran.sylvan.ia>
    Date:   Thu Aug 22 10:07:21 2013 -0400

        Add concerns about effects of Mars' moons on Wolfman

    commit f22b25e3233b4645dabd0d81e651fe074bd8e73b
    Author: Vlad Dracula <vlad@tran.sylvan.ia>
    Date:   Thu Aug 22 09:51:46 2013 -0400

        Start notes on Mars as a base

> ## Word-based diffing

> Sometimes, e.g. in the case of the text documments 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 the space bar.
> - To search for `some_word` in all pages, type `/some_word` and 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 005937fbe2a98fb83f0ade869025dc2636b4dad5
>     Author: Vlad Dracula <vlad@tran.sylvan.ia>
>     Date:   Thu Aug 22 10:14:07 2013 -0400

>        Discuss concerns about Mars' climate for Mummy

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

>     $ git log --oneline

>     * 005937f Discuss concerns about Mars' climate for Mummy
>     * 34961b1 Add concerns about effects of Mars' moons on Wolfman
>     * f22b25e Start notes on Mars as a base

> You can also combine the `--oneline` options with others. One useful combination is:

>     $ git log --oneline --graph --all --decorate

>     * 005937f Discuss concerns about Mars' climate for Mummy (HEAD, master)
>     * 34961b1 Add concerns about effects of Mars' moons on Wolfman
>     * f22b25e Start notes on Mars as a base

> ## 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:

>     $ mkdir directory
>     $ git status
>     $ git add directory
>     $ git status

> Note, our newly created empty directory `directory` 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.

> 2. 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:

>     git add <directory-with-files>

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`):

> ## Choosing a Commit Message

> Which of the following commit messages would be most appropriate for the last commit made to `mars.txt`?

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

> > ## Solution

> > Answer 1 is not descriptive enough, and answer 2 is too descriptive and redundant, but answer 3 is good: short but descriptive.

> ## 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.

> 1. Add some text to `mars.txt` 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.

> > ## Solution

> > First we make our changes to the `mars.txt` and `venus.txt` files:

> >     $ nano mars.txt
> >     $ cat mars.txt

> >     Maybe I should start with a base on Venus.

> >     $ nano venus.txt
> >     $ cat venus.txt

> >     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:

> >     $ git add mars.txt venus.txt

> > Or with multiple commands:

> >     $ git add mars.txt
> >     $ 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:

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

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

> ## Author and Committer

> For each of the commits you have done, Git stored your name twice. You are named as the author and as the committer. You can observe that by telling Git to show you more information about your last commits:

>     $ git log --format=full

> When commiting you can name someone else as the author:

>     $ git commit --author="Vlad Dracula <vlad@tran.sylvan.ia>"

> Create a new repository and create two commits: one without the `--author` option and one by naming a colleague of yours as the author. Run `git log` and `git log --format=full`. Think about ways how that can allow you to collaborate with your colleagues.

> > ## Solution

> >     $ git add me.txt
> >     $ git commit -m "Update Vlad's bio." --author="Frank N. Stein <franky@monster.com>"

> >     [master 4162a51] Update Vlad's bio.
> >     Author: Frank N. Stein <franky@monster.com>
> >     1 file changed, 2 insertions(+), 2 deletions(-)

> >     $ git log --format=full
> >     commit 4162a51b273ba799a9d395dd70c45d96dba4e2ff
> >     Author: Frank N. Stein <franky@monster.com>
> >     Commit: Vlad Dracula <vlad@tran.sylvan.ia>

> >     Update Vlad's bio.

> >     commit aaa3271e5e26f75f11892718e83a3e2743fab8ea
> >     Author: Vlad Dracula <vlad@tran.sylvan.ia>
> >     Commit: Vlad Dracula <vlad@tran.sylvan.ia>

> >     Vlad's initial bio.

> ## 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.

> - Always write a log message when committing changes.

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