UNB/ CS/ David Bremner/ teaching/ old/ cs4735/ milestones/ CS4735 Lab 1: Git Crash Course

Before you start.

This tutorial explains how to import a new project into Git, make changes to it, and share changes with other developers.

First, note that you can get documentation for a command such as git log --graph with:

$ man git-log

or:

$ git help log

With the latter, you can use the manual viewer of your choice; see git-help(1) for more information.

It is a good idea to introduce yourself to Git with your name and public email address before doing any operation. The easiest way to do so is:

$ git config --global user.name "Your Name Comes Here"
$ git config --global user.email you@yourdomain.example.com

You may also want to configure an editor to use with git. The default is probably vim. If that doesn't suit you, one way to configure it is

$ git config --global core.editor  <something>

Note that whatever editor you use should not put itself into the background right away as some gui editors do. A reasonable choice is gedit.

See also the hints at coursegit

Lab/Tutorial

Importing a new project

EXERCISE Create a git repository that will hold all of the labs for this term and place the examples under Git revision control as follows.

  1. Create a directory labs. This will be turned into a git repository below.

     $ mkdir labs
 $ cd labs

  2. Create the git repository

     $ git init

Git will reply

Initialized empty Git repository in /blah/blah/blah/.git/

You’ve now initialized the working directory—you may notice a new directory created, named ".git".

Download the textbook examples and unpack the zip archive into a new directory called lab1. Remember that linux (and git) are case sensitive.

Next, tell Git to take a snapshot of the contents of all files under the lab1, with git add:

$ git add lab1

This snapshot is now stored in a temporary staging area which Git calls the "index". You can permanently store the contents of the index in the repository with git commit:

$ git commit

This will prompt open and editor and prompt you for a commit message. Enter one and exit the editor. You’ve now stored the first version of your project in Git.

Making changes

You are now ready to commit. You can see what is about to be committed using git diff with the --cached option:

$ git diff --cached

(Without --cached, git diff will show you any changes that you’ve made but not yet added to the index.) You can also get a brief summary of the situation with git status:

$ git status
# On branch master
# Changes to be committed:
#   (use "git reset HEAD <file>..." to unstage)
#
#       modified:   file1
#       modified:   file2
#       modified:   file3
#

If you need to make any further adjustments, do so now, and then add any newly modified content to the index. Finally, commit your changes with:

$ git commit

This will again prompt you for a message describing the change, and then record a new version of the project.

which will automatically notice any modified (but not new) files, add them to the index, and commit, all in one step.

A note on commit messages: Though not required, it’s a good idea to begin the commit message with a single short (less than 50 character) line summarizing the change, followed by a blank line and then a more thorough description. The text up to the first blank line in a commit message is treated as the commit title, and that title is used throughout Git. For example, git-format-patch(1) turns a commit into email, and it uses the title on the Subject line and the rest of the commit in the body.

Git tracks content not files

Many revision control systems provide an add command that tells the system to start tracking changes to a new file. Git’s add command does something simpler and more powerful: git add is used both for new and newly modified files, and in both cases it takes a snapshot of the given files and stages that content in the index, ready for inclusion in the next commit.

EXERCISE

Viewing project history

At any point you can view the history of your changes using

$ git log

If you also want to see complete diffs at each step, use

$ git log -p

Often the overview of the change is useful to get a feel of each step

$ git log --stat --summary

EXERCISE Consulting the git-log(1), figure out the names of each file changed in each commit of your history in the main lab1 repository. Write the answer in to a file lab1/Q2. Commit that file.

Managing branches

A single Git repository can maintain multiple branches of development. To create a new branch named "quad", use

$ git branch quad

If you now run

$ git branch

you’ll get a list of all existing branches:

  quad
* master

The "quad" branch is the one you just created, and the "master" branch is a default branch that was created for you automatically. The asterisk marks the branch you are currently on; type

$ git checkout quad

to switch to the quad branch.

Check that the change you made is no longer visible, since it was made on the quad branch and you’re back on the master branch.

at this point the two branches have diverged, with different changes made in each. To merge the changes made in quad into master, run

$ git merge quad

If the changes don’t conflict, you’re done. If there are conflicts, markers will be left in the problematic files showing the conflict;

$ git diff

will show this. Once you’ve edited the files to resolve the conflicts, use git add and git commit to commit the changes.

$ gitk

will show a nice graphical representation of the resulting history. If gitk is missing you can use

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

At this point you could delete the quad branch with

$ git branch -d quad

This command ensures that the changes in the quad branch are already in the current branch.

If you develop on a branch crazy-idea, then regret it, you can always delete the branch with

$ git branch -D crazy-idea

Branches are cheap and easy, so this is a good way to try something out.

Sharing changes with a central repo

  1. Create a central copy of your repository. This step depends a bit on your hosting service. Since we are using the FCS git repositories, there is an existing repository for all students who registered early enough. If it turns out there is no repository for you, you may have to talk to Sean Seeley.

    • First add the remote. This something like a nickname for the URL where the repo will be stored.

        $ git remote add origin https://<username>@vcs.cs.unb.ca/git/cs4735-labs-<username>

      origin is the default name for a remote, but we could have used a different name here. ⟨username;⟩ is your FCS Linux account name.

    • Now upload your local copy of the repo.

         $ git push --all origin

      you should see something like

             Counting objects: 388, done.
       Delta compression using up to 8 threads.
       Compressing objects: 100% (350/350), done.
       Writing objects: 100% (388/388), 71.63 KiB | 0 bytes/s, done.
       Total 388 (delta 223), reused 59 (delta 33)
       To https://username@vcs.cs.unb.ca/git/cs4735-labs-username
          e7e5311..f1ae959  master -> master

  2. Now move to the parent directory, and make a clone of the central repo (Imagine you are now on a different machine, for example).

      $ git clone https://<username>@vcs.cs.unb.ca/git/cs4735-labs-<username> labs-clone

    This creates a new directory "labs-clone" containing a clone of your repository. The clone is on an equal footing with the original project, possessing its own copy of the original project’s history.

  3. enter the clone, change the canvas colour you previously changed to white to cyan and commit your changes:

     (edit files)
 [labs-clone] $ git commit -a
 (repeat as necessary)

  4. Push your changes back to the central repository:

     [labs-clone] $ git push origin master

  5. Now pull your changes back to the original repo

     [labs-clone] $ cd ../labs
 [labs] $ git pull origin master

This merges the changes from the central copy of "master" branch into the current branch. branch. If you made other changes in the meantime, then you may need to manually fix any conflicts.

The "pull" command thus performs two operations: it fetches changes from a remote branch, then merges them into the current branch.

Pull is fetch+merge

Suppose we have two users Alice and Bob collaborating via git. We will simulate this by assuming that labs is the repository of Alice and labs-clone belongs to Bob.

EXERCISE Before starting this section, go to the labs-clone repository and make some trivial change (e.g. add comment to one of the javascript files). Commit your change, but don't push it anywhere.

Note that in general, Alice would want her local changes committed before initiating a "pull". If Bob’s work conflicts with what Alice did since their histories forked, Alice will use her working tree and the index to resolve conflicts, and existing local changes will interfere with the conflict resolution process (Git will still perform the fetch but will refuse to merge --- Alice will have to get rid of her local changes in some way and pull again when this happens).

Alice can peek at what Bob did without merging first, using the "fetch" command; this allows Alice to inspect what Bob did, using a special symbol "FETCH_HEAD", in order to determine if he has anything worth pulling, like this:

alice$ git fetch /blah/blah/blah/labs-clone master
alice$ git log -p HEAD..FETCH_HEAD

This operation is safe even if Alice has uncommitted local changes. The range notation "HEAD..FETCH_HEAD" means "show everything that is reachable from the FETCH_HEAD but exclude anything that is reachable from HEAD". Alice already knows everything that leads to her current state (HEAD), and reviews what Bob has in his state (FETCH_HEAD) that she has not seen with this command.

If Alice wants to visualize what Bob did since their histories forked she can issue the following command:

$ gitk HEAD..FETCH_HEAD

This uses the same two-dot range notation we saw earlier with git log.

Alice may want to view what both of them did since they forked. She can use three-dot form instead of the two-dot form:

$ gitk HEAD...FETCH_HEAD

This means "show everything that is reachable from either one, but exclude anything that is reachable from both of them".

Please note that these range notation can be used with both gitk and "git log".

After inspecting what Bob did, if there is nothing urgent, Alice may decide to continue working without pulling from Bob. If Bob’s history does have something Alice would immediately need, Alice may choose to stash her work-in-progress first, do a "pull", and then finally unstash her work-in-progress on top of the resulting history.

When you are working in a small closely knit group, it is not unusual to interact with the same repository over and over again. By defining remote repository shorthand, you can make it easier:

alice$ git remote add bob /blah/blah/blah/labs-clone

With this, Alice can perform the first part of the "pull" operation alone using the git fetch command without merging them with her own branch, using:

alice$ git fetch bob

Unlike the longhand form, when Alice fetches from Bob using a remote repository shorthand set up with git remote, what was fetched is stored in a remote-tracking branch, in this case bob/master. So after this:

alice$ git log -p master..bob/master

shows a list of all the changes that Bob made since he branched from Alice’s master branch.

After examining those changes, Alice could merge the changes into her master branch:

alice$ git merge bob/master

This merge can also be done by pulling from her own remote-tracking branch, like this:

alice$ git pull . remotes/bob/master

Note that git pull always merges into the current branch, regardless of what else is given on the command line.

Later, Bob can update his repo with Alice’s latest changes using

bob$ git pull

Note that he doesn’t need to give the path to Alice’s repository; when Bob cloned Alice’s repository, Git stored the location of her repository in the repository configuration, and that location is used for pulls:

bob$ git config --get remote.origin.url  /blah/blah/blah/labs

(The complete configuration created by git clone is visible using git config -l, and the git-config(1) man page explains the meaning of each option.)

Git also keeps a pristine copy of Alice’s master branch under the name "origin/master":

bob$ git branch -r
    origin/master

If Bob later decides to work from a different host, he can still perform clones and pulls using the ssh protocol:

bob$ git clone alice.org:/home/alice/project myrepo

Alternatively, Git has a native protocol, or can use rsync or http; see git-pull(1) for details.

EXERCISE Merge Bob's changes into Alice's master branch. Use the git diff command to save the difference between Alice's branch before and after this merge into a file lab1/Q3. Commit that file.

Meet the parents

Git history is represented as a series of interrelated commits. We have already seen that the git log command can list those commits. Note that first line of each git log entry also gives a name for the commit:

$ git log
commit c82a22c39cbc32576f64f5c6b3f24b99ea8149c7
Author: Junio C Hamano <junkio@cox.net>
Date:   Tue May 16 17:18:22 2006 -0700

    merge-base: Clarify the comments on post processing.

We can give this name to git show to see the details about this commit.

$ git show c82a22c39cbc32576f64f5c6b3f24b99ea8149c7

But there are other ways to refer to commits. You can use any initial part of the name that is long enough to uniquely identify the commit:

$ git show c82a22c39c   # the first few characters of the name are
                        # usually enough
$ git show HEAD         # the tip of the current branch
$ git show experimental # the tip of the "experimental" branch

Every commit usually has one "parent" commit which points to the previous state of the project:

$ git show HEAD^  # to see the parent of HEAD
$ git show HEAD^^ # to see the grandparent of HEAD
$ git show HEAD~4 # to see the great-great grandparent of HEAD

Note that merge commits may have more than one parent:

$ git show HEAD^1 # show the first parent of HEAD (same as HEAD^)
$ git show HEAD^2 # show the second parent of HEAD

EXERCISE

In a file lab1/Q4, create a text file containing list of the hash of each commit, and if applicable, the parent commits of that commit. The line corresponding to a merge commit might look like:

abc123 deadb33f ffff0000

The name(s) of the commit

You can also give commits names of your own; after running

$ git tag v2.5 1b2e1d63ff

you can refer to 1b2e1d63ff by the name "v2.5". If you intend to share this name with other people (for example, to identify a release version), you should create a "tag" object, and perhaps sign it; see git-tag(1) for details.

Any Git command that needs to know a commit can take any of these names. For example:

$ git diff v2.5 HEAD     # compare the current HEAD to v2.5
$ git branch stable v2.5 # start a new branch named "stable" based
                         # at v2.5
$ git reset --hard HEAD^ # reset your current branch and working
                         # directory to its state at HEAD^

Be careful with that last command: in addition to losing any changes in the working directory, it will also remove all later commits from this branch. If this branch is the only branch containing those commits, they will be lost. Also, don’t use git reset on a publicly-visible branch that other developers pull from, as it will force needless merges on other developers to clean up the history. If you need to undo changes that you have pushed, use git revert instead.

EXERCISE Make another trivial change in Bob's repository. Commit that change, and make a tag. Use git-fetch(1) to fetch that tag (and the corresponding commit). Merge that tag into Alice's repository.

Get a grep

The git grep command can search for strings in any version of your project, so

$ git grep "hello" v2.5

searches for all occurrences of "hello" in v2.5.

If you leave out the commit name, git grep will search any of the files it manages in your current directory. So

$ git grep "hello"

is a quick way to search just the files that are tracked by Git.

EXERCISE

Use git grep to count the occurances of "clearColor" in your repository. Write that number to a file lab1/Q5, and commit that file.

Ranges of Commits

Many Git commands also take sets of commits, which can be specified in a number of ways. Here are some examples with git log:

$ git log v2.5..v2.6            # commits between v2.5 and v2.6
$ git log v2.5..                # commits since v2.5
$ git log --since="2 weeks ago" # commits from the last 2 weeks
$ git log v2.5.. Makefile       # commits since v2.5 which modify
                                # Makefile

You can also give git log a "range" of commits where the first is not necessarily an ancestor of the second; for example, if the tips of the branches "stable" and "master" diverged from a common commit some time ago, then

$ git log stable..master

will list commits made in the master branch but not in the stable branch, while

$ git log master..stable

will show the list of commits made on the stable branch but not the master branch.

The git log command has a weakness: it must present commits in a list. When the history has lines of development that diverged and then merged back together, the order in which git log presents those commits is meaningless.

Most projects with multiple contributors (such as the Linux kernel, or Git itself) have frequent merges, and gitk does a better job of visualizing their history. For example,

$ gitk --since="2 weeks ago" drivers/

allows you to browse any commits from the last 2 weeks of commits that modified files under the "drivers" directory. (Note: you can adjust gitk’s fonts by holding down the control key while pressing "-" or "+".)

EXERCISE

Make a file lab1/Q6 containing the git command(s) to list a one line summary for all commits in master not yet pushed to the remote origin

Space Time Coordinates

Finally, most commands that take filenames will optionally allow you to precede any filename by a commit, to specify a particular version of the file:

$ git diff v2.5:Makefile HEAD:Makefile.in

You can also use git show to see any such file:

$ git show v2.5:Makefile

Next Steps

This tutorial should be enough to perform basic distributed revision control for your projects. However, to fully understand the depth and power of Git you need to understand two simple ideas on which it is based:

Part two of this tutorial explains the object database, the index file, and a few other odds and ends that you’ll need to make the most of Git. You can find it at gittutorial-2(7).

If you don’t want to continue with that right away, a few other digressions that may be interesting at this point are:

SEE ALSO

gittutorial-2(7), gitcvs-migration(7), gitcore-tutorial(7), gitglossary(7), git-help(1), gitworkflows(7), Everyday Git, The Git User’s Manual

GIT

Part of the git(1) suite.