title	description
Working with branches	Branching is one of Git's most powerful features, since it allows you to work on several things at once. This chapter shows you how to create and manage branches.

What is a branch?

type: PureMultipleChoiceExercise
key: 9db055a148
lang: bash
xp: 50

If you don't use version control, a common workflow is to create different subdirectories to hold different versions of your project in different states, for example development and final. Of course, then you always end up with final-updated and final-updated-revised as well. The problem with this is that it becomes difficult to work out if you have the right version of each file in the right subdirectory, and you risk losing work.

One of the reasons Git is popular is its support for creating branches, which allows you to have multiple versions of your work, and lets you track each version systematically.

Each branch is like a parallel universe: changes you make in one branch do not affect other branches (until you merge them back together).

Note: Chapter 2 described the three-part data structure Git uses to record a repository's history: blobs for files, trees for the saved states of the repositories, and commits to record the changes. Branches are the reason Git needs both trees and commits: a commit will have two parents when branches are being merged.

In the diagram below, each box is a commit and the arrows point to the next ("child") commit. How many merges have taken place?

@hint Look for boxes with two arrows coming into them.

@possible_answers

None
1
[2]
3

@feedback

No: some commits have more than one parent.
No: some commits have more than one parent.
Correct: two commits have more than one parent.
No: only two commits have more than one parent.

How can I see what branches my repository has?

type: MultipleChoiceExercise
key: f48d0ea4cf
lang: shell
xp: 50
skills:
  - 1

By default, every Git repository has a branch called master (which is why you have been seeing that word in Git's output in previous lessons). To list all of the branches in a repository, you can run the command git branch. The branch you are currently in will be shown with a * beside its name.

You are in the dental repository. How many branches are in this repository (including master)?

@possible_answers

None.

@hint Use git branch to list branches.

@pre_exercise_code

repl = connect('bash')
repl.run_command('cd dental')

@sct

Ex().has_chosen(4, ['No: every repository has at least one branch.',
                    'No: there are more branches than that.',
                    'No: there are more branches than that.',
                    'Correct!'])

How can I view the differences between branches?

type: MultipleChoiceExercise
key: 7333c66616
lang: shell
xp: 50
skills:
  - 1

Branches and revisions are closely connected, and commands that work on the latter usually work on the former. For example, just as git diff revision-1..revision-2 shows the difference between two versions of a repository, git diff branch-1..branch-2 shows the difference between two branches.

You are in the dental repository. How many files in the summary-statistics branch are different from their equivalents in the master branch?

@possible_answers

None.

@hint Use git diff branch..branch to find the differences.

@pre_exercise_code

repl = connect('bash')
repl.run_command('cd dental')

@sct

Ex().has_chosen(3, ['No: some files differ.',
                    'No: please count both files that have changed and files that are being added.',
                    'Correct!',
                    'No: please count the number of files that differ, not the number of lines that are different.'])

How can I switch from one branch to another?

type: BulletConsoleExercise
key: c418145d13
xp: 100

You previously used git checkout with a commit hash to switch the repository state to that hash. You can also use git checkout with the name of a branch to switch to that branch.

Two notes:

When you run git branch, it puts a * beside the name of the branch you are currently in.
Git will only let you do this if all of your changes have been committed. You can get around this, but it is outside the scope of this course.

In this exercise, you'll also make use of git rm. This removes the file (just like the shell command rm) then stages the removal of that file with git add, all in one step.

@pre_exercise_code

repl = connect('bash')
repl.run_command('cd dental')
repl.run_command('git branch')

type: ConsoleExercise
key: eee4722074
xp: 16

@instructions You are in the master branch of the dental repository. Switch to the summary-statistics branch.

@hint Use git checkout with the name of the branch as an argument.

@solution

git checkout summary-statistics

@sct

msg="You don't appear to be on the right branch. Have you used `git checkout` to go to the `summary-statistics` branch?"
Ex().multi(
    has_cwd('/home/repl/dental'),
    # Check we're on the right branch -- https://stackoverflow.com/a/12142066/1144523
    has_expr_output(expr='git rev-parse --abbrev-ref HEAD | grep summary-statistics',
                    output='summary-statistics', strict=True, incorrect_msg=msg)
)

type: ConsoleExercise
key: '6193872406'
xp: 16

@instructions Use git rm to delete report.txt.

@hint Use git rm with the name of the file as an argument.

@solution

git rm report.txt

@sct

msg1="Make sure you stay on the `summary-statistics` branch. Use `git checkout summary-statistics` to navigate back."
msg2="It seems that the deletion of `report.txt` was not staged. Have you used `git rm report.txt`?"
Ex().multi(
    has_cwd('/home/repl/dental'),
    has_expr_output(expr='git rev-parse --abbrev-ref HEAD | grep summary-statistics',
                    output='summary-statistics', strict=True, incorrect_msg=msg1),
    has_expr_output('git diff --name-only --staged | grep report.txt',
                    output='report.txt', strict=True, incorrect_msg=msg2)
)

type: ConsoleExercise
key: dcfdc86805
xp: 16

@instructions Commit your change with `-m "Removing report" as a message.

@hint Use git commit with -m and the message (in quotes to preserve the spaces).

@solution

git commit -m "Removing report"

@sct

branch_msg =" Make sure you stay on the `summary-statistics` branch. Use `git checkout summary-statistics` to navigate back."
not_committed_msg = "It seems that the staged changes to `report.txt` weren't committed. Have you used `git commit` with `-m \"Removing report\"`?"
bad_message_msg = 'It seems the commit message was incorrect. You can amend it with `git commit --amend -m "new message"`.'
Ex().multi(
    has_cwd('/home/repl/dental'),
    has_expr_output(
        expr='git rev-parse --abbrev-ref HEAD | grep summary-statistics',
        output='summary-statistics', strict=True, incorrect_msg=branch_msg
    ),
    check_or(
        has_expr_output(
            expr='git diff HEAD~ --name-only | grep report.txt',
            output = 'report.txt',
            strict=True,
            incorrect_msg=not_committed_msg
        ),
        has_code(r'\s*git\s+commit', incorrect_msg = not_committed_msg)            
    ),
    has_expr_output(
        expr='git log -1 | grep --only-matching "Removing report"',
        output = 'Removing report',
        strict=True,
        incorrect_msg=bad_message_msg
    )
)

type: ConsoleExercise
key: 99b72ed9cb
xp: 16

@instructions Use ls to check that it's gone.

@hint Run the command as shown.

@solution

ls

@sct

msg="Make sure you stay on the `summary-statistics` branch. Use `git checkout summary-statistics` to navigate back."
Ex().multi(
    has_cwd('/home/repl/dental'),
    has_expr_output(expr='git rev-parse --abbrev-ref HEAD | grep summary-statistics',
                    output='summary-statistics', strict=True, incorrect_msg=msg),
    has_expr_output(incorrect_msg="Have you used `ls` correctly? `report.txt` should have disappeared.")
)

type: ConsoleExercise
key: 4afc727945
xp: 16

@instructions Switch back to the master branch.

@hint Use git checkout and the name of the branch you want to switch to.

@solution

git checkout master

@sct

msg="You don't appear to be on the right branch. Have you used `git checkout master`?"
Ex().multi(
    has_cwd('/home/repl/dental'),
    has_expr_output(expr='git rev-parse --abbrev-ref HEAD | grep master',
                    output='master', strict=True, incorrect_msg=msg)
)

type: ConsoleExercise
key: b0b5946436
xp: 20

@instructions Use ls to ensure that report.txt is still there.

@hint Run the command as shown.

@solution

ls

@sct

Ex().has_expr_output(incorrect_msg="Have you used `ls` correctly? `report.txt` should have disappeared.")
Ex().success_msg("Quite a journey! In the next few exercises, you'll practice some more with navigating branches and staging and committing changes.")

How can I create a branch?

type: BulletConsoleExercise
key: 51c4cb1dc0
xp: 100

You might expect that you would use git branch to create a branch, and indeed this is possible. However, the most common thing you want to do is to create a branch then switch to that branch.

In the previous exercise, you used git checkout branch-name to switch to a branch. To create a branch then switch to it in one step, you add a -b flag, calling git checkout -b branch-name,

The contents of the new branch are initially identical to the contents of the original. Once you start making changes, they only affect the new branch.

@pre_exercise_code

repl = connect('bash')
repl.run_command('cd dental')
repl.run_command('git branch')

type: ConsoleExercise
key: 71253d1c95
xp: 25

@instructions You are in the master branch of the dental repository. Create a new branch called deleting-report.

@hint Use git checkout with -b and the name of the branch you want to create.

@solution

git checkout -b deleting-report

@sct

msg="Have you used `git checkout -b deleting-report?` to create a new branch and switch to it?"
Ex().multi(
    has_cwd('/home/repl/dental'),
    check_correct(
        has_expr_output(expr='git rev-parse --abbrev-ref HEAD | grep deleting-report',
                        output='deleting-report', strict=True, incorrect_msg=msg),
        multi(
            has_code(r'git\s+checkout', incorrect_msg="Have you used `git checkout`?"),
            has_code(r'-b', incorrect_msg="Make sure to use the flag `-b`."),
            has_code(r'deleting-report', incorrect_msg="Your new branch should be called `deleting-report`. Beware of typos!")
        )
    )
)

type: ConsoleExercise
key: 4dc64f3a09
xp: 25

@instructions Use git rm report.txt to delete the report.

@hint Run the command as shown.

@solution

git rm report.txt

@sct

msg1="Make sure you stay on the `deleting-report` branch. Use `git checkout deleting-report` to navigate back."
msg2="It seems that the deletion of `report.txt` was not staged. Have you used `git rm report.txt`?"
Ex().multi(
    has_cwd('/home/repl/dental'),
    has_expr_output(expr='git rev-parse --abbrev-ref HEAD | grep deleting-report',
                    output='deleting-report', strict=True, incorrect_msg=msg1),
    has_expr_output('git diff --name-only --staged | grep report.txt',
                    output='report.txt', strict=True, incorrect_msg=msg2)
)

type: ConsoleExercise
key: a7e82dfb0c
xp: 25

@instructions Commit your changes with a log message.

@hint Use git commit with -m and a message in quotes.

@solution

git commit -m "Deleting report"

@sct

msg1="Make sure you stay on the `deleting-report` branch. Use `git checkout deleting-report` to navigate back."
msg2="It seems that the staged deletion of `report.txt` wasn't committed. Use `git commit` with `-m \"any message\"`."
Ex().multi(
    has_cwd('/home/repl/dental'),
    has_expr_output(expr='git rev-parse --abbrev-ref HEAD | grep deleting-report',
                    output='deleting-report', strict=True, incorrect_msg=msg1),
    has_expr_output(expr='git diff HEAD~ --name-only | grep report.txt',
                    output = 'report.txt', incorrect_msg=msg2)
)

type: ConsoleExercise
key: ec7d242138
xp: 25

@instructions Use git diff with appropriate arguments to compare the master branch with the new state of the deleting-report branch.

@hint The two branches master and deleting-report should be separated by .. with no spaces.

@solution

git diff master..deleting-report

@sct

msg = 'Use `git diff branch-1..branch-2` to compare branches. Replace `branch-1` and `branch-2` with the appropriate names for this context.'
# The sensible solution below causes the exercise to hang if the student types git diff with no arguments.
# So just do regex matching instead.
# Ex().check_or(
#     has_expr_output(strict = True, incorrect_msg = msg),
#     has_expr_output(code = 'git diff deleting-report..master', incorrect_msg = msg, strict = True),
#     has_code(r'\s*git\s+diff\s*(deleting-report\.\.master|master\.\.deleting-report)', incorrect_msg = msg)
# )
Ex().check_or(
has_code(r'\s*git\s+diff\s*(deleting-report\.\.master|master\.\.deleting-report)', incorrect_msg = msg),
has_code(r'\s*git\s+diff\s*(deleting-report master|master deleting-report)', incorrect_msg = msg)
)

Ex().success_msg("Well done. Let's have a look at merging branches now.")

How can I merge two branches?

type: BulletConsoleExercise
key: 3812382b52
xp: 100

Branching lets you create parallel universes; merging is how you bring them back together. When you merge one branch (call it the source) into another (call it the destination), Git incorporates the changes made to the source branch into the destination branch. If those changes don't overlap, the result is a new commit in the destination branch that includes everything from the source branch (the next exercises describe what happens if there are conflicts).

To merge two branches, you run git merge source destination (without .. between the two branch names). Git automatically opens an editor so that you can write a log message for the merge; you can either keep its default message or fill in something more informative.

@pre_exercise_code

repl = connect('bash')
repl.run_command('cd dental')

type: ConsoleExercise
key: 7a4bb39d31
xp: 100

@instructions You are in the master branch of the dental repository. Merge the changes from the summary-statistics branch (the source) into the master branch (the destination) with the message "Merging summary statistics."

@hint Use git merge with summary-statistics as the first argument (the "source" branch) and master as the second (the "destination" branch).

@solution

# Run this command *without* the '-no-edit' flag:
git merge --no-edit summary-statistics master

@sct

msg1 = "Make sure you are on the `master` branch when you do the `git merge` command. Use `git checkout master` first."
msg2 = "Have you used `git merge summary-statistics master` to merge `summary-statistics` into `master`? An editor will show up. You can use Ctrl+O and then Ctrl+X to exit this. If you want to avoid this, use the `--no-edit` flag in the `git merge` command."
Ex().multi(
    has_cwd('/home/repl/dental'),
    has_expr_output( expr='git rev-parse --abbrev-ref HEAD | grep master',
                    output='master', strict=True, incorrect_msg=msg1),
    has_expr_output( expr='git diff HEAD~ --name-only | grep bin/summary',
                    output = 'bin/summary', strict=True, incorrect_msg=msg2),
    has_expr_output( expr='git diff HEAD~ --name-only | grep results/summary.txt',
                    output = 'results/summary.txt', strict=True, incorrect_msg=msg2)
)

Ex().success_msg("Merged it! Well done!")

What are conflicts?

type: MultipleChoiceExercise
key: 354f733f81
xp: 50

Sometimes the changes in two branches will conflict with each other: for example, bug fixes might touch the same lines of code, or analyses in two different branches may both append new (and different) records to a summary data file. In this case, Git relies on you to reconcile the conflicting changes.

The file todo.txt initially contains these two lines:

A) Write report.
B) Submit report.

You create a branch called update and modify the file to be:

A) Write report.
B) Submit final version.
C) Submit expenses.

You then switch back to the master branch and delete the first line, so that the file contains:

B) Submit report.

When you try to merge update and master, what conflicts does Git report? You can use git diff master..update to view the difference between the two branches.

@possible_answers

[Just line B, since it is the only one to change in both branches.]
Lines A and B, since one was deleted and the other changed.
Lines B and C, since one was changed and the other deleted.
All three lines, since all were either added, deleted, or changed.

@hint What line(s) changed in both branches?

@pre_exercise_code

repl = connect('bash')
repl.run_command('cd dental')
repl.run_command('cd init')
repl.run_command('git checkout -b master')
repl.run_command('touch todo.txt')
repl.run_command('echo "A) Write report." >> todo.txt')
repl.run_command('echo "B) Submit report." >> todo.txt')
repl.run_command('git add todo.txt')
repl.run_command('git commit -m "Master revisions 1."')
repl.run_command('git checkout -b update')
repl.run_command('cat /dev/null > todo.txt')
repl.run_command('echo "A) Write report." >> todo.txt')
repl.run_command('echo "B) Submit final version." >> todo.txt')
repl.run_command('echo "C) Submit expenses." >> todo.txt')
repl.run_command('git add todo.txt')
repl.run_command('git commit -m "Update revisions."')
repl.run_command('git checkout master')
repl.run_command('cat /dev/null > todo.txt')
repl.run_command('echo "B) Submit report." >> todo.txt')
repl.run_command('git add todo.txt')
repl.run_command('git commit -m "Master revisions 2."')
repl.run_command('clear')

@sct

Ex().has_chosen(1, ['Correct: Git can merge the deletion of line A and the addition of line C automatically.',
'No: Git can merge the deletion of line A automatically.',
'No: Git can merge the addition of line C automatically.',
'No: Git can merge the deletion of line A and the addition of line C automatically.'])

How can I merge two branches with conflicts?

type: BulletConsoleExercise
key: 3a4ba0eda1
xp: 100

When there is a conflict during a merge, Git tells you that there's a problem, and running git status after the merge reminds you which files have conflicts that you need to resolve by printing both modified: beside the files' names.

Inside the file, Git leaves markers that look like this to tell you where the conflicts occurred:

<<<<<<< destination-branch-name
...changes from the destination branch...
=======
...changes from the source branch...
>>>>>>> source-branch-name

In many cases, the destination branch name will be HEAD because you will be merging into the current branch. To resolve the conflict, edit the file to remove the markers and make whatever other changes are needed to reconcile the changes, then commit those changes.

@pre_exercise_code

repl = connect('bash')
repl.run_command('cd dental')
repl.run_command('git branch')

type: ConsoleExercise
key: 1e11825f95
xp: 20

@instructions You are in the master branch of the dental repository. Merge the changes from the alter-report-title branch (the source) into the master branch (the destination).

@hint Use git merge with the "from" and "to" branch names in that order.

@solution

# Run this command *without* the '-no-edit' flag:
git merge --no-edit -m "Merging altered report title" alter-report-title master

@sct

msg1 = "Make sure you are on the `master` branch when you do the `git merge` command. Use `git checkout master` first."
msg2 = "Have you used `git merge source destination` to merge `alter-report-title` into `master`?"
Ex().multi(
    has_cwd('/home/repl/dental'),
    has_expr_output(expr='git rev-parse --abbrev-ref HEAD | grep master',
                    output='master', strict=True, incorrect_msg=msg1),
    # git diff -name-only should return report.txt twice after unsuccessful merge
    has_expr_output(expr='git diff --name-only | grep report.txt | wc -w', output = '2', incorrect_msg=msg2)
)

type: ConsoleExercise
key: 9ecd08b08b
xp: 20

@instructions Use git status to see which file has conflicts.

@hint Run the command as shown.

@solution

git status

@sct

msg1 = "Make sure you are on the `master` branch when you do the `git merge` command. Use `git checkout master` first."
Ex().multi(
    has_cwd('/home/repl/dental'),
    has_expr_output(expr='git rev-parse --abbrev-ref HEAD | grep master',
                    output='master', strict=True, incorrect_msg=msg1),
    has_expr_output(incorrect_msg="Have you used `git status` to see the status of your repo?")
)

type: ConsoleExercise
key: 4acfdd12bd
xp: 20

@instructions It turns out that report.txt has some conflicts. Use nano report.txt to open it and remove some lines so that only the second title is kept. Save your work with Ctrl+O and Enter, and then leave the editor with Ctrl+X. You can easily remove entire lines with Ctrl+K.

@hint Use nano and the filename. Remove the following lines:

<<<<<<< HEAD
# Seasonal Dental Surgeries (2017) 2017-18
=======
>>>>>>> alter-report-title

@solution

# This solution uses another command
# because our automated tests can't edit files interactively.
echo -e '
# Dental Work by Season 2017-18

TODO: write executive summary.

TODO: include link to raw data.

TODO: remember to cite funding sources!

' > report.txt

@sct

msg1 = "Make sure you are on the `master` branch when you do the `git merge` command. Use `git checkout master` first."
edit=" Edit the file again with `nano report.txt` and make some more changes."
msg2 = "The title `# Dental Work by Season 2017-18` disappeared from `report.txt`, while that was the title to keep!" + edit
msg3patt = "It seems that there is a still a conflict marker (`%s`) in the `report.txt` file. Make sure to remove this line entirely." + edit
Ex().multi(
    has_cwd('/home/repl/dental'),
    has_expr_output(expr='git rev-parse --abbrev-ref HEAD | grep master',
                    output='master', strict=True, incorrect_msg=msg1),
    check_file('/home/repl/dental/report.txt').multi(
        has_code('# Dental Work by Season 2017-18', fixed=True, incorrect_msg=msg2),
        check_not(has_code(r'<<<<<'), incorrect_msg=msg3patt%'<<<<<<<'),
        check_not(has_code(r'>>>>>'), incorrect_msg=msg3patt%'>>>>>>>'),
        check_not(has_code(r'====='), incorrect_msg=msg3patt%'=======')
    )
)

type: ConsoleExercise
key: 323f9a133c
xp: 20

@instructions Add the merged file to the staging area.

@hint Use git add with report.txt as an argument.

@solution

git add report.txt

@sct

msg1 = "Make sure you are on the `master` branch when you do the `git merge` command. Use `git checkout master` first."
# Ex().multi(
#     has_cwd('/home/repl/dental'),
#     has_expr_output(expr='git rev-parse --abbrev-ref HEAD | grep master',
#                 output='master', strict=True, incorrect_msg=msg1),
#     has_expr_output(expr='git diff --name-only --staged | grep report.txt',
#                     output = 'report.txt',
#                     incorrect_msg = "It seems that `report.txt` was not added to the staging area. Have you used `git add report.txt`?")
# )

Ex().multi(
has_cwd('/home/repl/dental'),
has_code(r'\s*git\s+add\s+report\.txt', incorrect_msg = "It seems that `report.txt` was not added to the staging area. Have you used `git add report.txt`?")
)

type: ConsoleExercise
key: d9b25e272f
xp: 20

@instructions Commit your changes with a log message.

@hint Use git commit with -m and a message in quotes.

@solution

git commit -m "Reconciling"

@sct

msg1 = "Make sure you are on the `master` branch when you do the `git merge` command. Use `git checkout master` first."
msg2 = "It seems that the changes to `report.txt` that were staged with the `git add` command weren't committed. Have you used `git commit` with `-m \"any message\"`?"
Ex().multi(
    has_cwd('/home/repl/dental'),
    has_expr_output(expr='git rev-parse --abbrev-ref HEAD | grep master',
            output='master', strict=True, incorrect_msg=msg1),
    has_expr_output(expr='git diff HEAD~ --name-only | grep report.txt', output='report.txt',     incorrect_msg=msg2),
    has_expr_output(expr='git diff --name-only --staged | wc -w', output='0', incorrect_msg=msg2)
)
Ex().success_msg("Well done! This was quite a challenge. A final yet extremely important aspect about Git is collaboration. Learn all about it in the fifth and final chapter of this course!")

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chapter4.md

chapter4.md

What is a branch?

How can I see what branches my repository has?

How can I view the differences between branches?

How can I switch from one branch to another?

How can I create a branch?

How can I merge two branches?

What are conflicts?

How can I merge two branches with conflicts?

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What is a branch?

How can I see what branches my repository has?

How can I view the differences between branches?

How can I switch from one branch to another?

How can I create a branch?

How can I merge two branches?

What are conflicts?

How can I merge two branches with conflicts?