Overview
Teaching: 20 min
Exercises: 0 minQuestions
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 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.)
$ 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 calledmars.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:
diff
command
comparing the old and new versions of the file.df0654a
and 315bf3a
are unique computer-generated labels for those versions.+
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 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), andgit 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 typegit commit
, Git will prompt you to usegit commit -a
orgit 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
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 pressingn
.
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
?
- “Changes”
- “Added line ‘But the Mummy will appreciate the lack of humidity’ to mars.txt”
- “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?
$ git commit -m "my recent changes"
$ git init myfile.txt
$ git commit -m "my recent changes"
$ git add myfile.txt
$ git commit -m "my recent changes"
$ git commit -m myfile.txt "my recent changes"
Solution
- Would only create a commit if files have already been staged.
- Would try to create a new repository.
- Is correct: first add the file to the staging area, then commit.
- 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.
- Add some text to
mars.txt
noting your decision to consider Venus as a base- Create a new file
venus.txt
with your initial thoughts about Venus as a base for you and your friends- Add changes from both files to the staging area, and commit those changes.
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. Rungit log
andgit 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.