This page contains a number of frequently asked questions (at the moment only related to Git, but we may add more FAQs to this page)


I didn’t specify a commit message, and now I’m in a screen I don’t know how to get out of

When you forget to specify the -m option to the git commit command, Git will open a command-line text editor for you to specify a commit message. You will typically end up in either the nano text editor or the vim text editor. While you are welcome to learn how to use either editor (and use them to type in more elaborate commit messages), here we will simply explain how to exit the editor so you can specify the message using the -m option.

If you see something like this at the bottom of the screen:

^G Get Help      ^O Write Out     ^W Where Is      ^K Cut Text      ^J Justify       ^C Cur Pos       ^Y Prev Page     M-\ First Line   M-W WhereIs Next
^X Exit          ^R Read File     ^\ Replace       ^U Uncut Text    ^T To Spell      ^_ Go To Line    ^V Next Page     M-/ Last Line    M-] To Bracket

you’re in the nano editor. Just press Control+X to exit. You will return to the shell, and this message will appear:

Aborting commit due to empty commit message.

This is normal. Just re-run git with the -m option.

If you see something like this at the bottom of the screen:

"~/cmsc12100-aut-20-borja/.git/COMMIT_EDITMSG" 67L, 2572C

you’re in the vim editor. Press Escape, and then type in the following:


And press Enter. As with nano, you will exit into the shell and you will get a message telling you the commit was aborted.

Trying to do a git pull upstream master takes me to a screen saying “Merge branch…”

When you run git pull upstream master you may be taken to a screen that starts with something like this:

Merge branch 'master' of mit.cs.uchicago.edu:cmsc12100-aut-20/cmsc12100-aut-20

# Please enter a commit message to explain why this merge is necessary,
# especially if it merges an updated upstream into a topic branch.
# Lines starting with '#' will be ignored, and an empty message aborts
# the commit.

Doing a git pull upstream master fetches the latest changes from our upstream repository, and merges them into your repository. This merge is done by wrapping up all of our latest changes into a new commit, which must have some message. Because this commit was automatically created, Git hasn’t yet given you a chance to specify what the message should be, so it opens up a text editor for you to edit a message.

First, read our answer to the previous question to identify what editor you are using. Once you’ve done so, simply do the following:

  • If you are using nano: Press Control+O (the letter, not 0 the number), then Enter to save the message. Then, press Control+X to exit.

  • If you are using vim: Press Escape and then type :wq and press Enter.

Doing this will save the commit and will complete the pull operation.

How can I recover an accidentally-deleted file?

Let’s say you accidently deleted a file named something.py. If that file has been previously added and committed to your repository, you can recover it by running this command:

git checkout -- something.py

What does it mean when Git tells me my current branch is behind?

When trying to do a git push, you may get the following error message:

To git@mit.cs.uchicago.edu:cmsc12100-aut-20/username.git
! [rejected] master -> master (non-fast-forward)
error: failed to push some refs to 'git@mit.cs.uchicago.edu:cmsc12100-aut-20/username.git'
hint: Updates were rejected because the tip of your current branch is behind
hint: its remote counterpart. Integrate the remote changes (e.g.
hint: 'git pull ...') before pushing again.
hint: See the 'Note about fast-forwards' in 'git push --help' for details.

This error means that there are commits on the server that you pushed from somewhere else, and did not pull to your current location. Before you can push, you need to pull those commits from the server:

git pull

To avoid this problem from happening in the future, it is generally good practice to always run git pull before you start working on your code, to make sure you always have the latest version of the code from the Git server.

What are unmerged files?

When running Git, you may sometimes get an error message like this:

Pull is not possible because you have unmerged files.
Please, fix them up in the work tree, and then use
'git add/rm <file>' as appropriate to mark resolution,
or use 'git commit -a'

This error message means that, at some point in the past, you did a git pull that included code that conflicted with the code on your machine. This situation is called a merge conflict, and Git needs you to resolve it manually, because it cannot tell whether you want the code on the server or the code on your machine to take precedence.

You can run git status to see what files need to be updated manually (under “Unmerged paths”). If you open these file(s) with an editor, you should see some parts of the file that look like this:


What Git is basically saying is “In one version, this line contained ‘foobar’, but in another version it contained ‘foobaz’, and I don’t know how to reconcile that difference”. You need to choose one of the two and determine which one you want to keep. If you wanted to keep foobar in the example above, you would replace everything above with foobar (i.e., remove the “<<<”, “===”, “>>>” lines too).

Once you’ve resolved these conflicts, just git add the files and commit as usual.

You may also want to check out this handy guide from GitHub. Please note that it explains merge conflicts in terms of Git “branches”, which you do not need to use in this course. For the purposes of reading this guide, you can think of your local repository as one branch and the copy on the Git server as the other branch.

How do I look at a previous version of a file I pushed to the Git server?

Go to your project on https://mit.cs.uchicago.edu, and then click on “Commits” on the left bar. This will show you a list of all the commits you’ve pushed to the server. In each of them, there is a “Browse code” link that will allow you to see the state of the code at that precise commit.