Getting Started

On UNIX-like operating systems (OS X or Linux) open the Terminal application to use these commands.

On Windows you should install Git for Windows from the Git site. Then run the program Git Bash to open a terminal where you can run all these commands, in addition to Git commands.

Here are some of the most common commands:

• cd (stands for “change directory”)

  Changes the current directory. In you’re in a directory that has a subdirectory called hello, then cd hello moves into that subdirectory.

  Use cd .. to move to the parent directory of your current directory.

• pwd (“print working directory”)

  Prints out the current directory, if you’re not sure where you are.

  On a well-configured system, your current directory is displayed as part of the prompt that the system shows when it’s ready to receive a command. If that’s not the case on your system, post on Piazza to get help configuring your prompt.

• ls (“list”)

  Lists the files in the current directory.

  Use ls -l for extra information (a “long” listing) about the files. Use ls -a (stands for “all”) to show hidden files, which are files and subdirectories whose names begin with a period.

• mkdir (“make directory”)

  Creates a new directory in the current directory. To create a directory called goodbye, use mkdir goodbye.

• up and down arrow

  Use up arrow to put the command you just ran back on the command line. You can now edit that command to fix a typo, or just press enter to run it again.

  Use the up and down arrow keys to navigate through your history of commands, so you never have to re-type a long command line.

Git is a Version Control System (VCS). If you have used other version control software before, like SVN or CVS, many of the concepts and procedures of git will be familiar to you. The Pro Git book (which you can read online) describes what Git is used for:

What is version control, and why should you care? Version control is a system that records changes to a file or set of files over time so that you can recall specific versions later. […] It allows you to revert files back to a previous state, revert the entire project back to a previous state, compare changes over time, see who last modified something that might be causing a problem, who introduced an issue and when, and more. Using a VCS also generally means that if you screw things up or lose files, you can easily recover.

Some of the most important Git concepts:

• repository: A folder containing all the files associated with a project (e.g. a 6.005 problem set or team project), as well as the entire history of commits to those files.

• commit (or “revision”): A snapshot of the files in a repository at a given point in time.

• add (or “stage”): Before changes to a file can be committed to a repository, the files in question must be added or staged (before each commit). This lets you commit changes to only certain files of your choosing at a time, but can also be a bit of a pain if you accidentally forget to add all the files you wanted to commit before committing.

• clone: Since git is a “distributed” version control system, there is no concept of a centralized git “server” that holds the latest official version of your code. Instead, developers “clone” remote repositories that contain files they want access to, and then commit to their local clones. Only when they push their local commits to the original remote repository are other developers able to see their changes.

• push: The act of sending your local commits to a remote repository. Again, until you add, commit, and push your changes, no one else can see them.

• pull: The act of retrieving commits made to a remote repository and writing them into your local repository. This is how you are able to see commits made by others after the time at which you made an initial clone.

You start working with Git repos in 6.005 by cloning a remote repository into a local repository on your computer.

To do this, open the terminal (or the Git Bash for Windows) and use the cd command to change to the directory where you would like to store your code. Then run:

git clone URI-of-remote-repo

or

git clone URI-of-remote-repo project-name

Replace URI-of-remote-repo with the location of the remote repository, and replace project-name with the appropriate project name, like ps0. The result will be a new directory project-name with the contents of the repository. This is your local repository.

Cloning problem sets: for each problem set in 6.005, you will have a Git repository on Athena.

Initially this remote repository only contains some template code.

To start working on the problem set, you will clone that repository onto your machine.

As you complete each part of the problem set, you will commit your changes to the local repository and then push them to the remote repository.

When the time comes for grading your assignment, we will clone the remote repository and look at the last commit you made and pushed there before the deadline.

After you have cloned the repository, you should navigate into the repository on your command prompt using cd. This lets you run git commands on the repository.

For example, you can see the last commit on the repository using git show. This will show you the commit message as well as all the modifications.

You can see the list of all the commits you made (along with their commit messages) with git log. If you do git log -p, it will show you the full commit history, including the changes each commit made.

Long output: if git show or git log generate more output than fits on one page, you will see a colon (:) symbol at the bottom of the screen. You will not be able to type another command! Use the arrow keys to scroll up and down, and quit the output viewer by pressing q.

Commit IDs: every Git commit has a unique ID, the long hexadecimal numbers you see in git log or git show. These numbers are in fact a cryptographic hash of the contents of your commit. One neat feature is that this ID is unique not just within your repository, but actually within the universe of Git commits. In other words, if your commit ID is something like ab1312313febc241..., that commit is (extremely likely) to be the only commit in the world with that name.

You can reference a commit by its ID (or frequently just by the first several characters). This is most useful with something like git show, where you can look at a particular commit, rather than just the most recent one.

You will also see commits identified by ID in tools like gitweb and Didit.

The basic building block of data in Git is called a “commit”. A commit represents some change to one or more files (or the creation of one or more files).

When you first create a file or change a file, that data is unknown. To add it, run:

git add file.txt (where file.txt is the file you want to add)

You’ll either need to run that command from the same directory as the file, or include directory names in the file path.

This “stages” the file. Once you’ve staged all your changes, run:

git commit

This will pop up an editor that will give you a chance to write a commit message. When you save and close the editor, the commit will be created.

Git has some nice commands for seeing the status of your repository.

The most basic of these is git status. You can run this at any point to see which files Git sees have been modified and are still unstaged and which files have been modified and staged (so that if you git commit those changes will be included in the commit). Note that the same file might have both staged and unstaged changes, if you changed the file more after running git add.

When you have unstaged changes, you can see what the changes were (relative to the last commit) by running git diff. Note that this will not include changes that were staged (but not committed). You can see those if you run git diff --staged.

After you’ve made some commits, you might want to push them to a remote repository. Again, in 6.005, you really only have one remote repository to push to, called origin. To push to it, you run the command:

git push origin master

The origin in the command specifies that you’re pushing to the origin remote. The master refers to the master branch. Branches are an advanced feature of Git that we’re not going to be using in 6.005, but since Git has them, you do have to specify a branch. For now, just include this part when you push.

Once you run this, you will be prompted for your password and hopefully everything will push. You’ll get a line like this:

a67cc45..b4db9b0  master -> master

Sometimes, when you try to push, things will go wrong. You might get an output like this:

! [rejected]      master -> master (non-fast-forward)

What’s going on here is that Git won’t let you push to a repository unless all your commits come after all the ones already in your remote repository. If you get an error message like that, it means that there is a commit in your remote repository that you don’t have in your local one (probably because a teammate pushed before you did). If you find yourself in this situation, you have to pull first and then push.

To perform a pull, you should run git pull. When you run this, Git actually does two things:

1. It downloads the changes and stores them in its internal state. At this point, your repository doesn’t appear any different—it just knows what the state of the remote repository is and what the state of your repository is.
2. It incorporates the changes from the remote repository into the new repository via a process called merging (see next section).

If you made some changes to your repository and you’re trying to incorporate the changes from another repository, you need to merge them together somehow. In terms of commits, what actually needs to happen is that you have to create a special merge commit which encompasses both changes. How this process actually happens depends on the changes.

If you’re lucky, then the changes you made and the changes that you downloaded from the remote repository don’t conflict. For example, maybe you changed one file and your partner changed another. In this case, it’s safe to just include both changes. Similarly, maybe you changed different functions of the same file. In these cases, Git can do the merge automatically. When you run git pull, it will pop up an editor as if you were making a commit—in fact, this is the commit message of the merge commit that Git automatically generated. Once you save and close this editor, the merge commit will be made and you will have incorporated the changes. At this point, you can try to git push again and hopefully it will work this time.

Sometimes, you’re not so lucky. If the changes you made and the changes you pulled edit the same part of the same file, Git won’t know how to resolve it. This is called a merge conflict. In this case, you will get an output that says CONFLICT in big letters. If you run git status, it will show the conflicting files with the label Both modified. You now have to edit these files and resolve them by hand.

First, open them up in your text editor (probably Eclipse for 6.005). The parts that are conflicted will be really obviously marked with obnoxious <<<<<<<<<<<<<<<<<<, ==================, and >>>>>>>>>>>>>>>>>> lines. Everything between the <<<< and the ==== lines are the changes you made. Everything between the ==== and the >>>> lines are the changes you pulled in. It’s your job to figure out how to combine these. The answer will of course depend on the situation. Maybe one change logically supercedes the other, or maybe they can be merged somehow. You should edit the file to your satisfaction and remove the <<<</====/>>>> markers when you’re done.

Once you have resolved all the conflicts (note that there can be several conflicting files, and also several conflicts per file), git add all the affected files and then git commit. You will have an opportunity to write the merge commit message (where you should describe how you did the merge). Now you should be able to push.

If you’d like to practice using the version history to undo a change:

Unlike other similar systems, Git doesn’t have built-in a notion of a “central repository.” Instead, any repository can push to any other repository by specifying it as a “remote.” A “remote” is just a pair of a name (which can be anything) and a URI, which is a string indicating how it can find the other repository. The URI might look something like this:

ssh://USERNAME@athena.dialup.mit.edu/mit/6.005/git/SEMESTER/psets/PSET/USERNAME.git

Breaking that down:

• ssh:// — this specifies the protocol git should use to transfer the data. SSH is a protocol that lets you send data securely, which is useful to us because we have to type in a password. But in principle this is totally analogous to, for example, the http:// which you see in web browsers (HTTP is a protocol commonly used for data on the Web).
• USERNAME@athena.dialup.mit.edu — this actually has two parts. The USERNAME is the username you use to log in to the server. The athena.dialup.mit.edu is the address of the server itself. athena.dialup.mit.edu is the name of an Athena server IS&T runs. It accepts Kerberos logins, so your username can just be your Kerberos name.
• /mit/6.005/git/SEMESTER/psets/PSET/USERNAME.git — this is the path on the server where the repository is stored. (Note that you should use the actual path specified in the pset that you’re about to work on, so that SEMESTER and PSET are replaced with the right things.) One noteworthy thing here is the USERNAME.git part at the end. In 6.005, this will always be your username, or the usernames of your group members in a group project. However, there’s nothing in Git to say that the username you log in with (the thing before the @ sign) and repository name at the end of the path have to match. In 6.005, we just set it up that way.

Now, even though Git doesn’t have the idea of a central repository, it’s very useful for 6.005. Thus, in 6.005, all of your repositories are actually created by cloning a remote repository which we create (and which acts as the “central” repository). You’ve done this with the git clone URI directory command. This actually does a couple of things:

1. Create an empty directory called directory (i.e. the last argument to git clone).
2. Initialize it as an empty Git repository.
3. Add a remote with the URI you specified and the name origin.
4. Download the data from the remote.

So for those of you who were wondering, that’s what the origin means. It’s just the default name of the remote repository that you cloned your repository from.