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.

Let’s start off by learning some simple Python. Python is a scripting language, which has become very popular. We’ll start off by writing a very basic program.

It’s probably worth pointing out here, that git (and revision control systems in general) are really only designed for text files, since they “track changes” line by line. If you add a binary file (e.g. a JPEG photo, or a Word document), it will still record them, but not very efficiently, and some features will be unavailable. As a general rule, don’t add binary files to a git repository!

Now, let’s create a file called loop.py that contains a simple loop. (Python files usually have the .py suffix). (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.)

$ gedit loop.py

Type the following into the loop.py file:

for i in range(1, 100):
    print(i)

Note that the second line begins with 4 spaces (not a tab).

loop.py now contains two lines, which we can see by running:

$ ls

loop.py

$ cat loop.py

for i in range(1, 100):
    print(i)

You can also run the program by typing

$ python loop.py

which should print out the numbers from 1 to 99.

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)

	loop.py
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 loop.py

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:   loop.py

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

$ git commit -m "Start python program"

[master (root-commit) 373fb47] Start python program
 1 file changed, 5 insertions(+)
 create mode 100644 loop.py

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 373fb47 (Your commit will have a different 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 gedit (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 373fb4773ff2ab36416485bb5e86270d46ad415b
Author: Chris Richardson <chris@bpi.cam.ac.uk>
Date:   Tue Dec 5 09:53:58 2017 +0000

    Start python program

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 loop.py. 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 I add more information to the file. (Again, we’ll edit with gedit and then cat the file to show its contents; you may use a different editor, and don’t need to cat.)

$ gedit loop.py
$ cat loop.py

# Written by Chris
for i in range(1, 100):
    print(i)

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:   loop.py

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/loop.py b/loop.py
index eafc011..77e5f25 100644
--- a/loop.py
+++ b/loop.py
@@ -1,2 +1,3 @@
+# Written by Chris
 for i in range(1, 100):
      print(i)

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; eafc011 and 77e5f25 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 a comment"

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:   loop.py

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 loop.py
$ git commit -m "Add a comment"

[master 34961b1] Add a comment
 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 change set 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 a few more lines to the file:

$ gedit loop.py
$ cat loop.py

# Written by Chris
a = 0.0
for i in range(1, 100):
    a = a + i
    print(i, a)

$ git diff

diff --git a/loop.py b/loop.py
index 77e5f25..573eea0 100644
--- a/loop.py
+++ b/loop.py
@@ -1,3 +1,5 @@
 # Written by Chris
+a = 0.0
 for i in range(1, 100):
-    print(i)
+    a = a + i
+    print(i, a)

So far, so good: we’ve added some lines to 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 loop.py
$ 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/loop.py b/loop.py
index 77e5f25..573eea0 100644
--- a/loop.py
+++ b/loop.py
@@ -1,3 +1,5 @@
 # Written by Chris
+a = 0.0
 for i in range(1, 100):
-    print(i)
+    a = a + i
+    print(i, a)

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 "Add up numbers in 'a'"

[master 941ecf6] Add up numbers in 'a'
 1 file changed, 3 insertions(+), 1 deletion(-)

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 941ecf6d4a945bf88d86e4e3791dbd43b2558b6e
Author: Chris Richardson <chris@bpi.cam.ac.uk>
Date:   Tue Dec 5 10:03:18 2017 +0000

    Add up numbers in 'a'

commit 171e6b2d2f52fe45f8a353536d22c189125229dc
Author: Chris Richardson <chris@bpi.cam.ac.uk>
Date:   Tue Dec 5 10:00:09 2017 +0000

    Add a comment

commit 373fb4773ff2ab36416485bb5e86270d46ad415b
Author: Chris Richardson <chris@bpi.cam.ac.uk>
Date:   Tue Dec 5 09:53:58 2017 +0000

    Start python program

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 throught matches pressing n.

Limit Log Size

To avoid that git log cover all your terminal screen you can limit the numbers of commit that Git will list by using -N where N is the number of commits that you want to receive the information. For example, if you only want the information from the last commit you can use

$ git log -1

commit 941ecf6d4a945bf88d86e4e3791dbd43b2558b6e
Author: Chris Richardson <chris@bpi.cam.ac.uk>
Date:   Tue Dec 5 10:03:18 2017 +0000

   Add up numbers in 'a'

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

$ git log --oneline

* 941ecf6 Add up numbers in 'a'
* 171e6b2 Add a comment
* 373fb47 Start python program

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

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

* 941ecf6 (HEAD -> master) Add up numbers in 'a'
* 171e6b2 Add a comment
* 373fb47 Start python program

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 loop.py?

1. “Changes”
2. “Added line ‘a = a + i’ to loop.py”
3. “Add up numbers in ‘a’”

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 loop.py (e.g. another comment line).
2. Create a new file example.py with some different content of your choice.
3. Add changes from both files to the staging area, and commit those changes.

Solution

First we make our changes to the loop.py and example.py files:

$ gedit loop.py
$ cat loop.py

# Written by Chris
# December 2017
a = 0.0
for i in range(1, 100):
    a = a + i
    print(i, a)

$ gedit example.py
$ cat example.py

# Written by Chris
a = 2.0
if (a > 1.0):
    print ("a is greater than 1")

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

$ git add loop.py example.py

Or with multiple commands:

$ git add loop.py
$ git add example.py

Now the files are ready to commit. You can check that using git status. If you are ready to commit use: ~~~ $ git commit -m “Updated loop.py and started new example code” ~~~

[master cc127c2]
Updated loop.py and started new example code
2 files changed, 5 insertions(+)

~~~

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 "Updated Vlad's bio." --author="Frank N. Stein <franky@monster.com>"

[master 4162a51] Updated 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>

Updated 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.