Version Control with Git

As soon as people can work in parallel, they’ll likely step on each other’s toes. This will even happen with a single person: if we are working on a piece of software on both our laptop and a server in the lab, we could make different changes to each copy. Version control helps us manage these conflicts by giving us tools to resolve overlapping changes.

To see how we can resolve conflicts, we must first create one. The file forecast.md currently looks like this in both partners’ copies of our weather repository:

$ cat forecast.md
# Forecast
## Today
Cloudy with a chance of sun.
Mild temperatures around 16 °C.
## Tomorrow
Morning rainbows followed by light showers.

Let’s add a line to the collaborator’s copy only:

$ nano forecast.md
$ cat forecast.md
# Forecast
## Today
Cloudy with a chance of sun.
Mild temperatures around 16 °C.
## Tomorrow
Morning rainbows followed by light showers.
Ice forming overnight.

and then push the change to GitHub:

$ git add forecast.md
$ git commit -m "Add ice forecast for tomorrow evening"
[main 5ae9631] Add ice forecast for tomorrow evening
 1 file changed, 1 insertion(+)
$ git push origin main
Enumerating objects: 5, done.
Counting objects: 100% (5/5), done.
Delta compression using up to 8 threads
Compressing objects: 100% (3/3), done.
Writing objects: 100% (3/3), 331 bytes | 331.00 KiB/s, done.
Total 3 (delta 2), reused 0 (delta 0)
remote: Resolving deltas: 100% (2/2), completed with 2 local objects.
To https://github.com/mo-eormerod/weather.git
   29aba7c..dabb4c8  main -> main

Now let’s have the owner make a different change to their copy without updating from GitHub:

$ nano forecast.md
$ cat forecast.md
# Forecast
## Today
Cloudy with a chance of sun.
Mild temperatures around 16 °C.
## Tomorrow
Morning rainbows followed by light showers.
Snow forming overnight.

We can commit the change locally:

$ git add forecast.md
$ git commit -m "Add snow forecast for tomorrow evening"
[main 07ebc69] Add snow forecast for tomorrow evening
 1 file changed, 1 insertion(+)

but Git won’t let us push it to GitHub:

$ git push origin main
To https://github.com/mo-eormerod/weather.git
 ! [rejected]        main -> main (fetch first)
error: failed to push some refs to 'https://github.com/mo-eormerod/weather.git'
hint: Updates were rejected because the remote contains work that you do
hint: not have locally. This is usually caused by another repository pushing
hint: to the same ref. You may want to first integrate the remote changes
hint: (e.g., 'git pull ...') before pushing again.
hint: See the 'Note about fast-forwards' in 'git push --help' for details.

A diagram showing a conflict that might occur when two sets of independent changes are merged

Git rejects the push because it detects that the remote repository has new updates that have not been incorporated into the local branch. What we have to do is pull the changes from GitHub, merge them into the copy we’re currently working in, and then push that. Let’s start by pulling:

$ git pull origin main
remote: Enumerating objects: 5, done.
remote: Counting objects: 100% (5/5), done.
remote: Compressing objects: 100% (1/1), done.
remote: Total 3 (delta 2), reused 3 (delta 2), pack-reused 0
Unpacking objects: 100% (3/3), done.
From https://github.com/mo-eormerod/weather
 * branch            main     -> FETCH_HEAD
    29aba7c..dabb4c8  main     -> origin/main
Auto-merging forecast.md
CONFLICT (content): Merge conflict in forecast.md
Automatic merge failed; fix conflicts and then commit the result.
You may need to tell Git what to do

If you see the below in your output, Git is asking what it should do.

hint: You have divergent branches and need to specify how to reconcile them.
hint: You can do so by running one of the following commands sometime before
hint: your next pull:
hint:
hint:   git config pull.rebase false  # merge (the default strategy)
hint:   git config pull.rebase true   # rebase
hint:   git config pull.ff only       # fast-forward only
hint:
hint: You can replace "git config" with "git config --global" to set a default
hint: preference for all repositories. You can also pass --rebase, --no-rebase,
hint: or --ff-only on the command line to override the configured default per
hint: invocation.

In newer versions of Git it gives you the option of specifying different behaviours when a pull would merge divergent branches. In our case we want ‘the default strategy’. To use this strategy run the following command to select it as the default thing git should do.

$ git config pull.rebase false

Then attempt the pull again.

$ git pull origin main

The git pull command updates the local repository to include those changes already included in the remote repository. After the changes from remote branch have been fetched, Git detects that changes made to the local copy overlap with those made to the remote repository, and therefore refuses to merge the two versions to stop us from trampling on our previous work. The conflict is marked in in the affected file:

$ cat forecast.md
# Forecast
## Today
Cloudy with a chance of sun.
Mild temperatures around 16 °C.
## Tomorrow
Morning rainbows followed by light showers.
<<<<<<< HEAD
Snow forming overnight.
=======
Ice forming overnight.
>>>>>>> dabb4c8c450e8475aee9b14b4383acc99f42af1d

Our change is preceded by <<<<<<< HEAD. Git has then inserted ======= as a separator between the conflicting changes and marked the end of the content downloaded from GitHub with >>>>>>>. (The string of letters and digits after that marker identifies the commit we’ve just downloaded.)

It is now up to us to edit this file to remove these markers and reconcile the changes. We can do anything we want: keep the change made in the local repository, keep the change made in the remote repository, write something new to replace both, or get rid of the change entirely. Let’s replace both so that the file looks like this:

$ cat forecast.md
# Forecast
## Today
Cloudy with a chance of sun.
Mild temperatures around 16 °C.
## Tomorrow
Morning rainbows followed by light showers.
Snow forming overnight with a chance of icy patches.

To finish merging, we add forecast.md to the changes being made by the merge and then commit:

$ git add forecast.md
$ git status
On branch main
All conflicts fixed but you are still merging.
  (use "git commit" to conclude merge)

Changes to be committed:

    modified:   forecast.md

$ git commit -m "Merge changes from GitHub"
[main 2abf2b1] Merge changes from GitHub

Now we can push our changes to GitHub:

$ git push origin main
Enumerating objects: 10, done.
Counting objects: 100% (10/10), done.
Delta compression using up to 8 threads
Compressing objects: 100% (6/6), done.
Writing objects: 100% (6/6), 645 bytes | 645.00 KiB/s, done.
Total 6 (delta 4), reused 0 (delta 0)
remote: Resolving deltas: 100% (4/4), completed with 2 local objects.
To https://github.com/mo-eormerod/weather.git
   dabb4c8..2abf2b1  main -> main

Git keeps track of what we’ve merged with what, so we don’t have to fix things by hand again when the collaborator who made the first change pulls again:

$ git pull origin main
remote: Enumerating objects: 10, done.
remote: Counting objects: 100% (10/10), done.
remote: Compressing objects: 100% (2/2), done.
remote: Total 6 (delta 4), reused 6 (delta 4), pack-reused 0
Unpacking objects: 100% (6/6), done.
From https://github.com/mo-eormerod/weather
 * branch            main     -> FETCH_HEAD
    dabb4c8..2abf2b1  main     -> origin/main
Updating dabb4c8..2abf2b1
Fast-forward
 forecast.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

We get the merged file:

$ cat forecast.md
# Forecast
## Today
Cloudy with a chance of sun.
Mild temperatures around 16 °C.
## Tomorrow
Morning rainbows followed by light showers.
Snow forming overnight with a chance of icy patches.

We don’t need to merge again because Git knows someone has already done that.

Git’s ability to resolve conflicts is very useful, but conflict resolution costs time and effort, and can introduce errors if conflicts are not resolved correctly. If you find yourself resolving a lot of conflicts in a project, consider these technical approaches to reducing them:

  • Pull from upstream more frequently, especially before starting new work
  • Use topic branches to segregate work, merging to main when complete
  • Make smaller more atomic commits
  • Push your work when it is done and encourage your team to do the same to reduce work in progress and, by extension, the chance of having conflicts
  • Where logically appropriate, break large files into smaller ones so that it is less likely that two authors will alter the same file simultaneously

Conflicts can also be minimized with project management strategies:

  • Clarify who is responsible for what areas with your collaborators
  • Discuss what order tasks should be carried out in with your collaborators so that tasks expected to change the same lines won’t be worked on simultaneously
  • If the conflicts are stylistic churn (e.g. tabs vs. spaces), establish a project convention that is governing and use code style tools (e.g. htmltidy, perltidy, rubocop, etc.) to enforce, if necessary

Solving Conflicts that You Create

Clone the repository created by your instructor. Add a new file to it, and modify an existing file (your instructor will tell you which one). When asked by your instructor, pull her changes from the repository to create a conflict, then resolve it.

Conflicts on Non-textual files

What does Git do when there is a conflict in an image or some other non-textual file that is stored in version control?

Solution (Solution). Let’s try it. Suppose you take a picture of the television forecast and call it forecast.jpg.

If you do not have an image file of forecast available, you can create a dummy binary file like this:

$ head --bytes 1024 /dev/urandom > forecast.jpg
$ ls -lh forecast.jpg
-rw-r--r-- 1 mo-eormerod 57095 1.0K Mar  8 20:24 forecast.jpg

ls shows us that this created a 1-kilobyte file. It is full of random bytes read from the special file, /dev/urandom.

Now, suppose you add forecast.jpg to your repository:

$ git add forecast.jpg
$ git commit -m "Add picture of forecast"
[main 8e4115c] Add picture of forecast
 1 file changed, 0 insertions(+), 0 deletions(-)
 create mode 100644 forecast.jpg

Suppose that Jimmy has added a similar picture in the meantime. His is a picture of a forecast from Chile, but it is also called forecast.jpg. When you try to push, you get a familiar message:

$ git push origin main
To https://github.com/mo-eormerod/weather.git
 ! [rejected]        main -> main (fetch first)
error: failed to push some refs to 'https://github.com/mo-eormerod/weather.git'
hint: Updates were rejected because the remote contains work that you do
hint: not have locally. This is usually caused by another repository pushing
hint: to the same ref. You may want to first integrate the remote changes
hint: (e.g., 'git pull ...') before pushing again.
hint: See the 'Note about fast-forwards' in 'git push --help' for details.

We’ve learned that we must pull first and resolve any conflicts:

$ git pull origin main

When there is a conflict on an image or other binary file, git prints a message like this:

$ git pull origin main
remote: Counting objects: 3, done.
remote: Compressing objects: 100% (3/3), done.
remote: Total 3 (delta 0), reused 0 (delta 0)
Unpacking objects: 100% (3/3), done.
From https://github.com/mo-eormerod/weather.git
 * branch            main     -> FETCH_HEAD
   6a67967..439dc8c  main     -> origin/main
warning: Cannot merge binary files: forecast.jpg (HEAD vs. 439dc8c08869c342438f6dc4a2b615b05b93c76e)
Auto-merging forecast.jpg
CONFLICT (add/add): Merge conflict in forecast.jpg
Automatic merge failed; fix conflicts and then commit the result.

The conflict message here is mostly the same as it was for forecast.md, but there is one key additional line:

warning: Cannot merge binary files: forecast.jpg (HEAD vs. 439dc8c08869c342438f6dc4a2b615b05b93c76e)

Git cannot automatically insert conflict markers into an image as it does for text files. So, instead of editing the image file, we must check out the version we want to keep. Then we can add and commit this version.

On the key line above, Git has conveniently given us commit identifiers for the two versions of forecast.jpg. Our version is HEAD, and Jimmy’s version is 439dc8c0.... If we want to use our version, we can use git checkout:

$ git checkout HEAD forecast.jpg
$ git add forecast.jpg
$ git commit -m "Use image of just the local forecast"
[main 21032c3] Use image of just the local forecast

If instead we want to use Jimmy’s version, we can use git checkout with Jimmy’s commit identifier, 439dc8c0:

$ git checkout 439dc8c0 forecast.jpg
$ git add forecast.jpg
$ git commit -m "Use image of just the local forecast"
[main da21b34] Use image of forecast with nachos instead of just forecast

We can also keep both images. The catch is that we cannot keep them under the same name. But, we can check out each version in succession and rename it, then add the renamed versions. First, check out each image and rename it:

$ git checkout HEAD forecast.jpg
$ git mv forecast.jpg forecast-only.jpg
$ git checkout 439dc8c0 forecast.jpg
$ mv forecast.jpg forecast-chile.jpg

Then, remove the old forecast.jpg and add the two new files:

$ git rm forecast.jpg
$ git add forecast-only.jpg
$ git add forecast-chile.jpg
$ git commit -m "Use two images: local forecast and Chile forecast"
[main 94ae08c] Use two images: local forecast and Chile forecast
 2 files changed, 0 insertions(+), 0 deletions(-)
 create mode 100644 forecast-chile.jpg
 rename forecast.jpg => forecast-only.jpg (100%)

Now both images of forecast are checked into the repository, and forecast.jpg no longer exists.

A Typical Work Session

You sit down at your computer to work on a shared project that is tracked in a remote Git repository. During your work session, you take the following actions, but not in this order:

  • Make changes by appending the number 100 to a text file numbers.txt
  • Update remote repository to match the local repository
  • Celebrate your success with some fancy beverage(s)
  • Update local repository to match the remote repository
  • Stage changes to be committed
  • Commit changes to the local repository

In what order should you perform these actions to minimize the chances of conflicts? Put the commands above in order in the action column of the table below. When you have the order right, see if you can write the corresponding commands in the command column. A few steps are populated to get you started.

order action . . . . . . . . . . command . . . . . . . . . .
1
2 echo 100 >> numbers.txt
3
4
5
6 Celebrate!

Solution (Solution).

order action . . . . . . command . . . . . . . . . . . . . . . . . . .
1 Update local git pull origin main
2 Make changes echo 100 >> numbers.txt
3 Stage changes git add numbers.txt
4 Commit changes git commit -m "Add 100 to numbers.txt"
5 Update remote git push origin main
6 Celebrate!

Keypoints

  • Conflicts occur when two or more people change the same lines of the same file.
  • The version control system does not allow people to overwrite each other’s changes blindly, but highlights conflicts so that they can be resolved.

Back To Top