Introduction to Revision Control
Before learning about Git, let us first understand what revision control is.
Given below is a general introduction to revision control, adapted from bryan-mercurial-guide:
Revision control is the process of managing multiple versions of a piece of information. In its simplest form, this is something that many people do by hand: every time you modify a file, save it under a new name that contains a number, each one higher than the number of the preceding version.
Manually managing multiple versions of even a single file is an error-prone task, though, so software tools to help automate this process have long been available. The earliest automated revision control tools were intended to help a single user to manage revisions of a single file. Over the past few decades, the scope of revision control tools has expanded greatly; they now manage multiple files, and help multiple people to work together. The best modern revision control tools have no problem coping with thousands of people working together on projects that consist of hundreds of thousands of files.
There are a number of reasons why you or your team might want to use an automated revision control tool for a project.
- It will track the history and evolution of your project, so you don't have to. For every change, you'll have a log of who made it; why they made it; when they made it; and what the change was.
- It makes it easier for you to collaborate when you're working with other people. For example, when people more or less simultaneously make potentially incompatible changes, the software will help you to identify and resolve those conflicts.
- It can help you to recover from mistakes. If you make a change that later turns out to be an error, you can revert to an earlier version of one or more files. In fact, a good revision control tool will even help you to efficiently figure out exactly when a problem was introduced.
- It will help you to work simultaneously on, and manage the drift between, multiple versions of your project.
Most of these reasons are equally valid, at least in theory, whether you're working on a project by yourself, or with a hundred other people.
A revision is the state of a piece of information at a specific point in time, resulting from changes made to it e.g., if you modify the code and save the file, you have a new revision (or a new version) of that file. Some seem to use this term interchangeably with version while others seem to distinguish the two -- here, let us treat them as the same, for simplicity.
Revision Control Software (RCS) are the software tools that automate the process of Revision Control i.e., managing revisions of software . RCS are also known as Version Control Software (VCS), and by a few other names.
Git is the most widely used RCS today. Other RCS tools include Mercurial, Subversion (SVN), Perforce, CVS (Concurrent Versions System), Bazaar, TFS (Team Foundation Server), and Clearcase.
Github is a web-based project hosting platform for projects using Git for revision control. Other similar services include GitLab, BitBucket, and SourceForge.
Putting a Folder Under Git's Control
To be able to save snapshots of a folder using Git, you must first put the folder under Git's control by initialising a Git repository in that folder.
Normally, we use Git to manage a revision history of a specific folder, which gives us the ability to revision-control any file in that folder and its subfolders.
To put a folder under the control of Git, we initialise a repository (short name: repo) in that folder. This way, we can initialise repos in different folders, to revision-control different clusters of files independently of each other e.g., files belonging to different projects.
You can follow the hands-on practical below to learn how to initialise a repo in a folder.
What is this? HANDS-ON panels contain hands-on activities you can do as you learn Git. If you are new to Git, we strongly recommend that you do them yourself (even if they appear straightforward), as hands-on usage will help you internalise the concepts and operations better.
1 First, choose a folder. The folder may or may not have any files in it already. For this practical, let us create a folder named things for this purpose.
cd my-projects
mkdir things
2 Then cd into it.
cd things
3 Run the git status command to check the status of the folder.
git status
fatal: not a git repository (or any of the parent directories): .git
Don't panic. The error message is expected. It confirms that the folder currently does not have a Git repo.
4 Now, initialise a repository in that folder.
Use the command git init which should initialise the repo.
git init
Initialized empty Git repository in things/.git/
The output might also contain a hint about a name for an initial branch (e.g., hint: Using 'master' as the name for the initial branch ...). You can ignore that for now.
Note how the output mentions the repo being created in things/.git/ (not things/). More on that later.
Windows: Click
File→Clone/New…→ Click on+ Createbutton on the top menu bar.
Enter the location of the directory and clickCreate.Mac:
New...→Create Local Repository(orCreate New Repository) → Click...button to select the folder location for the repository → click theCreatebutton.
done!
Initialising a repo results in two things:
- First, Git now recognises this folder as a Git repository, which means it can now help you track the version history of files inside this folder.
To confirm, you can run the git status command. It should respond with something like the following:
git status
On branch master
No commits yet
nothing to commit (create/copy files and use "git add" to track)
Don't worry if you don't understand the output (we will learn about them later); what matters is that it no longer gives an error message as it did before.
done!
- Second, Git created a hidden subfolder named
.gitinside thethingsfolder. This folder will be used by Git to store metadata about this repository.
A Git-controlled folder is divided into two main parts:
- The repository – stored in the hidden
.gitsubfolder, which contains all the metadata and history. - The working directory – everything else in that folder, where you create and edit files.
Specifying What to include in a Snapshot
To save a snapshot, you start by specifying what to include in it, also called staging.
Git considers new files that you add to the working directory as 'untracked' i.e., Git is aware of them, but they are not yet under Git's control. The same applies to files that existed in the working folder at the time you initialised the repo.
A Git repo has an internal space called the staging area which it uses to build the next snapshot. Another name for the staging area is the index.
We can stage an untracked file to tell Git that we want its current version to be included in the next snapshot. Once you stage an untracked file, it becomes 'tracked' (i.e., under Git's control). A staged file can be unstaged to indicate that we no longer want it to be included in the next snapshot.
In the example below, you can see how staging files change the status of the repo as you go from (a) to (c).
staging area
[empty]
other metadata ...
├─ fruits.txt (untracked!)
└─ colours.txt (untracked!)
(a) State of the repo, just after initialisation, and creating two files. Both are untracked.
staging area
└─ fruits.txt
other metadata ...
├─ fruits.txt (tracked)
└─ colours.txt (untracked!)
(b) State after staging
fruits.txt.staging area
├─ fruits.txt
└─ colours.txt
other metadata ...
├─ fruits.txt (tracked)
└─ colours.txt (tracked)
(c) State after staging
colours.txt.1 First, add a file (e.g., fruits.txt) to the things folder.
Here is an easy way to do that with a single terminal command.
echo -e "apples\nbananas\ncherries" > fruits.txt
apples
bananas
cherries
Windows users: Use the git-bash terminal to run the above command (and all commands given in these lessons). Some of them might not work in other terminals such as the PowerShell.
To see the content of the file, you can use the cat command:
cat fruits.txt
2 Stage the new file.
2.1 Check the status of the folder using the git status command.
git status
On branch master
No commits yet
Untracked files:
(use "git add <file>..." to include in what will be committed)
fruits.txt
nothing added to commit but untracked files present (use "git add" to track)
2.2 Use the git add <file> command to stage the file.
git add fruits.txt
You can replace the add with stage (e.g., git stage fruits.txt) and the result is the same (they are synonyms).
Windows users: When using the echo command to write to text files from Git Bash, you might see a warning LF will be replaced by CRLF the next time Git touches it when Git interacts with such a file. This warning is caused by the way line endings are handled differently by Git and Windows. You can simply ignore it, or suppress it in future by running the following command:
git config --global core.safecrlf false
2.3 Check the status again. You can see the file is no longer 'untracked'.
git status
On branch master
No commits yet
Changes to be committed:
(use "git rm --cached <file>..." to unstage)
new file: fruits.txt
As before, don't worry if you don't understand the content of the output (we'll unpack it in a later lesson). The point to note is that the file is no longer listed as 'untracked'.
2.1 Note how the file is shown as ‘unstaged’. The question mark icon indicates the file is untracked.
If the newly-added file does not show up in Sourcetree UI, refresh the UI (: F5
| ⌥+R)
Sourcetree screenshots/instructions: vs
Note that Sourcetree UI can vary slightly between Windows and Mac versions. Some of the screenshots given in our lessons are from the Windows version while some are from the Mac version.
In som cases, we have specified how they differ.
In other cases, you may need to adapt if the given screenshots/instructions are slightly different from what you are seeing in your Sourcetree.
2.2 Stage the file:
Select the fruits.txt and click on the Stage Selected button.
Staging can be done using tick boxes or the ... menu in front of the file.
2.3 Note how the file is staged now i.e., fruits.txt appears in the Staged files panel now.
If Sourcetree shows a \ No newline at the end of the file message below the staged lines (i.e., below the cherries line in the above screenshot), that is because you did not hit enter after entering the last line of the file (hence, Git is not sure if that line is complete). To rectify, move the cursor to the end of the last line in that file and hit enter (like you are adding a blank line below it). This new change will now appear as an 'unstaged' change. Stage it as well.
done!
If you modify a staged file, it goes into the 'modified' state i.e., the file contains modifications that are not present in the copy that is waiting (in the staging area) to be included in the next snapshot. If you wish to include these new changes in the next snapshot, you need to stage the file again, which will overwrite the copy of the file that was previously in the staging area.
The example below shows how the status of a file changes when it is modified after it was staged.
staging area
Alice
other metadata ...
Alice
(a) The file names.txt is staged. The copy in the staging area is an exact match to the one in the working directory.
staging area
Alice
other metadata ...
Alice
Bob
(b) State after adding a line to the file. Git indicates it as 'modified' because it now differs from the version in the staged area.
staging area
Alice
Bob
other metadata ...
Alice
Bob
(c) After staging the file again, the staging area is updated with the latest copy of the file, and it is no longer marked as 'modified'.
1 First, add another line to fruits.txt, to make it 'modified'.
Here is a way to do that with a single terminal command.
echo "dragon fruits" >> fruits.txt
apples
bananas
cherries
dragon fruits
2 Now, verify that Git sees that file as 'modified'.
Use the git status command to check the status of the working directory.
$ git status
On branch master
No commits yet
Changes to be committed:
(use "git rm --cached <file>..." to unstage)
new file: fruits.txt
Changes not staged for commit:
(use "git add <file>..." to update what will be committed)
(use "git restore <file>..." to discard changes in working directory)
modified: fruits.txt
Note how fruits.txt now appears twice, once as new file: ... (representing the version of the file we staged earlier, which had only three lines) and once as modified: ... (representing the latest version of the file which now has a fourth line).
Note how fruits.txt appears in the Staged files panel as well as 'Unstaged files'.
3 Stage the file again, the same way you added/staged it earlier.
4 Verify that Git no longer sees it as 'modified', similar to step 2.
done!
Staging applies regardless of whether a file is currently tracked.
- Staging a tracked file will both begin tracking the file and include it in the next snapshot.
- Staging an already tracked file will simply mark its current changes for inclusion in the next commit.
Git also supports fine-grained selective staging i.e., staging only specific changes within a file while leaving other changes to the same file unstaged. This will be covered in a later lesson.
Git does not track empty folders. It tracks only folders that contain tracked files.
You can test this by adding an empty subfolder inside the things folder (e.g., things/more-things) and checking if it shows up as 'untracked' (it will not). If you add a file to that folder (e.g., things/more-things/food.txt) and then staged that file (e.g., git add more-things/food.txt), the folder will now be included in the next snapshot.
PRO-TIP: Applying a Git command to multiple files in one go
When a Git command expects a list of files or paths as a parameter (as the git add command does), these parameters are known as pathspecs — patterns that tell Git which files or directories to operate on. Pathspecs can be simple file names, directory names, or more complex patterns.
Here are some common ways to write them, shown with examples using the git add <pathspec> command:
Specify multiple files, separated by spaces:
git add f1.txt f2.txt data/lists/f3.txt # stages the specified three filesUse a glob pattern:
git add '*.txt' # stages all .txt files in the current directoryQuoting the glob pattern is recommended so your shell doesn’t expand it before Git sees it.
Use
.to indicate 'all in the current directory and subdirectories':git add . # stages all files in current directory and its subdirectoriesSpecific directory, to indicate 'this directory and its subdirectories':
git add path/to/dir # stages all files in path/to/dir and its subdirectoriesNegated pathspecs, to indicate 'except these':
git add . ':!*.log' # stage everything except .log files
Git supports combining these features — for example, you could add all .txt files except those in a certain folder using:
git add '*.txt' ':!docs/*.txt'
Saving a Snapshot
After staging, you can now proceed to save the snapshot, aka creating a commit.
Saving a snapshot is called committing and a saved snapshot is called a commit.
A Git commit is a full snapshot of your working directory based on the files you have staged, more precisely, a record of the exact state of all files in the staging area (index) at that moment -- even the files that have not changed since the last commit. This is in contrast to other revision control software that only store the in a commit. Consequently, a Git commit has all the information it needs to recreate the snapshot of the working directory at the time the commit was created.
A commit also includes metadata such as the author, date, and an optional commit message describing the change.
A Git commit is a snapshot of all tracked files, not simply a delta of what changed since the last commit.
Assuming you have previously staged changes to the fruits.txt, go ahead and create a commit.
1 First, let us do a sanity check using the git status command.
git status
On branch master
No commits yet
Changes to be committed:
(use "git rm --cached <file>..." to unstage)
new file: fruits.txt
2 Now, create a commit using the commit command. The -m switch is used to specify the commit message.
git commit -m "Add fruits.txt"
[master (root-commit) d5f91de] Add fruits.txt
1 file changed, 5 insertions(+)
create mode 100644 fruits.txt
3 Verify the staging area is empty using the git status command again.
git status
On branch master
nothing to commit, working tree clean
Note how the output says nothing to commit which means the staging area is now empty.
Click the Commit button, enter a commit message (e.g. add fruits.txt) into the text box, and click Commit.
done!
Examining the Revision History
It is useful to be able to visualise the commits timeline, aka the revision graph.
Git commits form a timeline, as each corresponds to a point in time when you asked Git to take a snapshot of your working directory. Each commit links to at least one previous commit, forming a structure that we can traverse.
A timeline of commits is called a branch. By default, Git names the initial branch master -- though many now use main instead. You'll learn more about branches in future lessons. For now, just be aware that the commits you create in a new repo will be on a branch called master (or main) by default.
gitGraph
%%{init: { 'theme': 'default', 'gitGraph': {'mainBranchName': 'master (or main)'}} }%%
commit id: "Add fruits.txt"
commit id: "Update fruits.txt"
commit id: "Add colours.txt"
commit id: "..."
Git can show you the list of commits in the Git history.
Preparation Use the things repo you created in a previous lesson. Alternatively, you can use the commands given below to create such a repo from scratch.
mkdir things # create a folder for the repo
cd things
git init
echo -e "apples\nbananas\ncherries" > fruits.txt
git add fruits.txt
git commit -m "Add fruits.txt"
You can copy-paste a list of commands (such as commands given above), including any comments, to the terminal. After that, hit enter to run them in sequence.
1 View the list of commits, which should show just the one commit you created just now.
You can use the git log command to see the commit history.
git log
commit ... (HEAD -> master)
Author: ... <...@...>
Date: ...
Add fruits.txt
Use the Q key to exit the output screen of the git log command.
Note how the output has some details about the commit you just created. You can ignore most of it for now, but notice it also shows the commit message you provided.
Expand the BRANCHES menu and click on the master to view the history graph, which contains only one node at the moment, representing the commit you just added. For now, ignore the label master attached to the commit.
2 Create a few more commits (i.e., a few rounds of add/edit files → stage → commit), and observe how the list of commits grows.
Here is an example list of bash commands to add two commits while observing the list of commits
echo "figs" >> fruits.txt # add another line to fruits.txt
git add fruits.txt # stage the updated file
git commit -m "Insert figs into fruits.txt" # commit the changes
git log # check commits list
echo "a file for colours" >> colours.txt # add a colours.txt file
echo "a file for shapes" >> shapes.txt # add a shapes.txt file
git add colours.txt shapes.txt # stage both files in one go
git commit -m "Add colours.txt, shapes.txt" # commit the changes
git log # check commits list
The output of the final git log should be something like this:
commit ... (HEAD -> master)
Author: ... <...@...>
Date: ...
Add colours.txt, shapes.txt
commit ...
Author: ... <...@...>
Date: ...
Insert figs into fruits.txt
commit ...
Author: ... <...@...>
Date: ...
Add fruits.txt
SIDEBAR: Working with the 'less' pager
Some Git commands — such as git log— may show their output through a pager. A pager is a program that lets you view long text one screen at a time, so you don’t miss anything that scrolls off the top. For example, the output of git log command will temporarily hide the current content of the terminal, and enter the pager view that shows output one screen at a time. When you exit the pager, the git log output will disappear from view, and the previous content of the terminal will reappear.
command 1
output 1
git log
→
commit f761ea63738a...
Author: ... <...@...>
Date: Sat ...
Add colours.txt
By default, Git uses a pager called less. Given below are some useful commands you can use inside the less pager.
| Command | Description |
|---|---|
q | Quit less and return to the terminal |
↓ or j | Move down one line |
↑ or k | Move up one line |
Space | Move down one screen |
b | Move up one screen |
G | Go to the end of the content |
g | Go to the beginning of the content |
/pattern | Search forward for pattern (e.g., /fix) |
n | Repeat the last search (forward) |
N | Repeat the last search (backward) |
h | Show help screen with all less commands |
If you’d rather see the output directly, without using a pager, you can add the --no-pager flag to the command e.g.,
git --no-pager log
It is possible to ask Git to not use less at all, use a different pager, or fine-tune how less is used. For example, you can reduce Git's use of the pager (recommended), using the following command:
git config --global core.pager "less -FRX"
Explanation:
-F: Quit if the output fits on one screen (don’t show pager unnecessarily)-R: Show raw control characters (for coloured Git output)-X: Keep content visible after quitting the pager (so output stays on the terminal)
To see the list of commits, click on the History item (listed under the WORKSPACE section) on the menu on the right edge of Sourcetree.
After adding two more commits, the list of commits should look something like this:
done!
The Git data model consists of two types of entities: objects and refs (short for references). In this lesson, you will encounter examples of both.
A Git revision graph is a visualisation of a repo's revision history, consisting of one or more branches. First, let us learn to work with simpler revision graphs consisting of one branch, such as the one given below.
- Nodes in the revision graph represent commits. A commit is one of four main types of Git objects. For completeness, the other three are:
- blob (short for binary large object): stores the contents of a file
- tree: represents a directory and records the hierarchy of its contents by referencing blobs and other trees
- tag (specifically, annotated tag): a label-like object that can store additional metadata and point to a specific commit
- A commit is identified by its SHA value. A SHA (Secure Hash Algorithm) value is a unique identifier generated by Git to represent each commit. It is produced by using SHA-1 (i.e., one of the algorithms in the SHA family of cryptographic hash functions) on the entire content of the commit. It's a 40-character hexadecimal string (e.g.,
f761ea63738a67258628e9e54095b88ea67d95e2) that acts like a fingerprint, ensuring that every commit can be referenced unambiguously. That is, every commit has a unique SHA-1 hash value. - A commit is a full snapshot of the working directory, constructed based on the previous commit, and the changes staged. That means each commit (except the initial commit) is based on a another 'parent' commit. Some commits can have multiple parent commits -- we’ll cover that later.
Given every commit has a unique hash, the commit hash values you see in our examples will be different from the hash values of your own commits, for example, when following our hands-on practicals.
Edges in the revision graph represent links between a commit and its parent commit(s). In some revision graph visualisations, you might see arrows (instead of lines) showing how each commit points to its parent commit.
Git uses refs to name and keep track of various points in a repository’s history. These refs are essentially 'named-pointers' that can serve as bookmarks to reach a certain point in the revision graph using the ref name.
In the revision graph above, there are two refs master and ←HEAD.
- master is a branch ref. A branch ref points to the latest commit on a branch. In this visualisation, the commit shown alongside the ref is the one it points to i.e.,
C3.
When you create a new commit, the branch ref of the branch moves to the new commit. - ←HEAD is a special ref that typically points to the current branch and moves along with that branch ref. In this example, it is pointing to the
masterbranch.
In certain cases, theHEADmay point directly to a specific commit instead of a branch. This situation is called a "detachedHEAD", which will be covered in a later lesson.
Target Use Git features to examine the revision graph of a simple repo.
Preparation Use a repo with just a few commits and only one branch.
1 First, use a simple git log to view the list of commits.
git log
commit f761ea63738a... (HEAD -> master)
Author: ... <...@...>
Date: Sat ...
Add colours.txt, shapes.txt
commit 2bedace69990...
Author: ... <...@...>
Date: Sat ...
Add figs to fruits.txt
commit d5f91de5f0b5...
Author: ... <...@...>
Date: Fri ...
Add fruits.txt
Given below the visual representation of the same revision graph. As you can see, the log output shows the refs slightly differently, but it is not hard to see what they mean.
2 Use the --oneline flag to get a more concise view. Note how the commit SHA has been truncated to first seven characters (first seven characters of a commit SHA is enough for Git to identify a commit).
git log --oneline
f761ea6 (HEAD -> master, origin/master) Add colours.txt, shapes.txt
2bedace Add figs to fruits.txt
d5f91de Add fruits.txt
3 The --graph flag makes the result closer to a graphical revision graph. Note the * that indicates a node in a revision graph.
git log --oneline --graph
* f761ea6 (HEAD -> master, origin/master) Add colours.txt, shapes.txt
* 2bedace Add figs to fruits.txt
* d5f91de Add fruits.txt
The --graph option is more useful when examining a more complicated revision graph consisting of multiple parallel branches.
Click the History to see the revision graph.
- In some versions of Sourcetree, the
HEADref may not be shown -- it is implied that theHEADref is pointing to the same commit the currently active branch ref is pointing.
done!
Remote Repositories
To back up your Git repo on the cloud, you’ll need to use a remote repository service, such as GitHub.
A repo you have on your computer is called a local repo. A remote repo is a repo hosted on a remote computer and allows remote access. Some use cases for remote repositories:
- as a backup of your local repo
- as an intermediary repo to work on the same files from multiple computers
- for sharing the revision history of a codebase among team members of a multi-person project
It is possible to set up a Git remote repo on your own server, but an easier option is to use a remote repo hosting service such as GitHub.
Creating a Repo on GitHub
The first step of backing up a local repo on GitHub: create an empty repository on GitHub.
You can create a remote repository based on an existing local repository, to serve as a remote copy of your local repo. For example, suppose you created a local repo and worked with it for a while, but now you want to upload it onto GitHub. The first step is to create an empty repository on GitHub.
1 Login to your GitHub account and choose to create a new repo.
2 In the next screen, provide a name for your repo. Refer the screenshot below on some guidance on how to provide the required information.
Click Create repository button to create the new repository.
If you enable any of the three Add _____ options shown above, GitHub will not only create a repo, but will also initialise it with some initial content. That is not what we want here. To create an empty remote repo, keep those options disabled.
3 Note the URL of the repo. It will be of the form
https://github.com/{your_user_name}/{repo_name}.git.
e.g., https://github.com/johndoe/foobar.git (note the .git at the end)
done!
Linking a Local Repo With a Remote Repo
The second step of backing up a local repo on GitHub: link the local repo with the remote repo on GitHub.
A Git remote is a reference to a repository hosted elsewhere, usually on a server like GitHub, GitLab, or Bitbucket. It allows your local Git repo to communicate with another remote copy — for example, to upload locally-created commits that are missing in the remote copy.
By adding a remote, you are informing the local repo details of a remote repo it can communicate with, for example, where the repo exists and what name to use to refer to the remote.
The URL you use to connect to a remote repo depends on the protocol — HTTPS or SSH:
- HTTPS URLs use the standard web protocol and start with
https://github.com/(for GitHub users). e.g.,https://github.com/username/repo-name.git - SSH URLs use the secure shell protocol and start with
git@github.com:. e.g.,git@github.com:username/repo-name.git
A Git repo can have multiple remotes. You simply need to specify different names for each remote (e.g., upstream, central, production, other-backup ...).
Add the empty remote repo you created on GitHub as a remote of a local repo you have.
1 In a terminal, navigate to the folder containing the local repo things you created earlier.
2 List the current list of remotes using the git remote -v command, for a sanity check. No output is expected if there are no remotes yet.
3 Add a new remote repo using the git remote add <remote-name> <remote-url> command.
i.e., if using HTTPS, git remote add origin https://github.com/{YOUR-GITHUB-USERNAME}/things.git
git remote add origin https://github.com/JohnDoe/things.git # using HTTPS
git remote add origin git@github.com:JohnDoe/things.git # using SSH
4 List the remotes again to verify the new remote was added.
git remote -v
origin https://github.com/johndoe/things.git (fetch)
origin https://github.com/johndoe/things.git (push)
The same remote will be listed twice, to show that you can do two operations (fetch and push) using this remote. You can ignore that for now. The important thing is the remote you added is being listed.
1 Open the local repo in Sourcetree.
2 Open the dialog for adding a remote, as follows:
Choose Repository → Repository Settings menu option.
Choose Repository → Repository Settings... → Choose Remotes tab.
3 Add a new remote to the repo with the following values.
Remote name: the name you want to assign to the remote repo i.e.,originURL/path: the URL of your remote repo
i.e.,https://github.com/{YOUR-GITHUB-USERNAME}/things.gitUsername: your GitHub username
4 Verify the remote was added by going to Repository → Repository Settings again.
5 Add another remote, to verify that a repo can have multiple remotes. You can use any name (e.g., backup and any URL for this).
done!
Updating the Remote Repo
The third step of backing up a local repo on GitHub: push a copy of the local repo to the remote repo.
You can push content of one repository to another, usually from your local repo to a remote repo. Pushing transfers recorded Git history (such as past commits), but it does not transfer unstaged changes or untracked files.
- To push, you need to have to the remote repo.
- Pushing is performed one branch at a time; you must specify which branch you want to push.
You can configure Git to track a pairing between a local branch and a remote branch, so in future you can push from the same local branch to the corresponding remote branch without needing to specify them again. For example, you can set your local master branch to track the master branch on the remote repo origin i.e., local master branch will track the branch origin/master.
In the revision graph above, you see a new type of ref ( origin/master). This is a remote-tracking branch ref that represents the state of a corresponding branch in a remote repository (if you previously set up the branch to 'track' a remote branch). In this example, the master branch in the remote origin is also at the commit C3 (which means you have not created new commits after you pushed to the remote).
If you now create a new commit C4, the state of the revision graph will be as follows:
Explanation: When you create C4, the current branch master moves to C4, and HEAD moves along with it. However, the master branch in the remote origin remains at C3 (because you have not pushed C4 yet). That is, the remote-tracking branch origin/master is one commit behind the local branch master (or, the local branch is one commit ahead). The origin/master ref will move to C4 only after you push your local branch to the remote again.
Preparation Use a local repo that is connected to an empty remote repo e.g., the things repo from previous hands-on practicals:
1 Push the master branch to the remote. Also instruct Git to track this branch pair.
Use the git push -u <remote-repo-name> <local-branch-name> to push the commits to a remote repository.
git push -u origin master
Explanation:
push: the Git sub-command that pushes the current local repo content to a remote repoorigin: name of the remotemaster: branch to push-u(or--set-upstream): the flag that tells Git to track that this localmasteris trackingorigin/masterbranch
Click the Push button on the buttons ribbon at the top.
In the next dialog, ensure the settings are as follows, ensure the Track option is selected, and click the Push button on the dialog.
2 Observe the remote-tracking branch origin/master is now pointing at the same commit as the master branch.
Use the git log --oneline --graph to see the revision graph.
* f761ea6 (HEAD -> master, origin/master) Add colours.txt, shapes.txt
* 2bedace Add figs to fruits.txt
* d5f91de Add fruits.txt
Click the History to see the revision graph.
- In some versions of Sourcetree, the
HEADref may not be shown -- it is implied that theHEADref is pointing to the same commit the currently active branch ref is pointing. - If the remote-tracking branch ref (e.g.,
origin/master) is not showing up, you may need to enable theShow Remote Branchesoption.
done!
The push command can be used repeatedly to send further updates to another repo e.g., to update the remote with commits you created since you pushed the first time.
Target Add a few more commits to the same local repo, and push those commits to the remote repo.
1 Commit some changes in your local repo.
Use the git commit command to create commits, as you did before.
Optionally, you can run the git status command, which should confirm that your local branch is 'ahead' by one commit (i.e., the local branch has commits that are not present in the corresponding branch in the remote repo).
git status
On branch master
Your branch is ahead of 'origin/master' by 1 commit.
(use "git push" to publish your local commits)
nothing to commit, working tree clean
You can also use the git log --oneline --graph command to see where the branch refs are. Note how the remote-tracking branch origin/master is one commit behind the local master.
e60deae (HEAD -> master) Update fruits list
f761ea6 (origin/master) Add colours.txt, shapes.txt
2bedace Add figs to fruits.txt
d5f91de Add fruits.txt
Create commits as you did before.
Before pushing the new commit, Sourcetree will indicate that your local branch is 'ahead' by one commit (i.e., the local branch has one new commit that is not in the corresponding branch in the remote repo).
2 Push the new commits to your fork on GitHub.
To push the newer commit(s) to the remote, any of the following commands should work:
git push origin mastergit push origin
(due to tracking you set up earlier, Git will assume you are pushing themasterbranch)git push
(due to tracking, Git will assume you are pushing to the remoteoriginand to the branchmasteri.e.,origin/master)
After pushing, the revision graph should look something like the following (note how both local and remote-tracking branch refs are pointing to the same commit again).
e60deae (HEAD -> master, origin/master) Update fruits list
f761ea6 Add colours.txt, shapes.txt
2bedace Add figs to fruits.txt
d5f91de Add fruits.txt
To push, click the Push button on the top buttons ribbon, ensure the settings are as follows in the next dialog, and click the Push button on the dialog.
After pushing the new commit to the remote, the remote-tracking branch ref should move to the new commit:
done!
Note that you can push between two repos only if those repos have a shared history among them (i.e., one should have been created by copying the other).
Omitting Files from Revision Control
Git allows you to specify which files should be omitted from revision control.
You can specify which files Git should ignore from revision control. While you can always omit files from revision control simply by not staging them, having an 'ignore-list' is more convenient, especially if there are files inside the working folder that are not suitable for revision control (e.g., temporary log files) or files you want to prevent from accidentally including in a commit (files containing confidential information).
A repo-specific ignore-list of files can be specified in a .gitignore file, stored in the root of the repo folder.
The .gitignore file itself can be either revision controlled or ignored.
- To version control it (the more common choice – which allows you to track how the
.gitignorefile changes over time), simply commit it as you would commit any other file. - To ignore it, simply add its name to the
.gitignorefile itself.
The .gitignore file supports file patterns e.g., adding temp/*.tmp to the .gitignore file prevents Git from tracking any .tmp files in the temp directory.
SIDEBAR: .gitignore File Syntax
Blank lines: Ignored and can be used for spacing.
Comments: Begin with
#(lines starting with # are ignored).# This is a commentWrite the name or pattern of files/directories to ignore.
log.txt # Ignores a file named log.txtWildcards:
*matches any number of characters, except/(i.e., for matching a string within a single directory level):abc/*.tmp # Ignores all .tmp files in abc directory**matches any number of characters (including/)**/foo.tmp # Ignores all foo.tmp files in any directory?matches a single characterconfig?.yml # Ignores config1.yml, configA.yml, etc.[abc]matches a single character (a, b, or c)file[123].txt # Ignores file1.txt, file2.txt, file3.txt
Directories:
- Add a trailing
/to match directories.logs/ # Ignores the logs directory - Patterns without
/match files/folders recursively.*.bak # Ignores all .bak files anywhere - Patterns with
/are relative to the.gitignorelocation./secret.txt # Only ignores secret.txt in the root directory
- Add a trailing
Negation: Use
!at the start of a line to not ignore something.*.log # Ignores all .log files !important.log # Except important.log
Example:
# Ignore all log files
*.log
# Ignore node_modules folder
node_modules/
# Don’t ignore main.log
!main.log
1 Add a file into your repo's working folder that you presumably do not want to revision-control e.g., a file named temp.txt. Observe how Git has detected the new file.
Add a few other files with .tmp extension.
2 Configure Git to ignore those files:
Create a file named .gitignore in the working directory root and add the text temp.txt into it.
echo "temp.txt" >> .gitignore
temp.txt
Observe how temp.txt is no longer detected as 'untracked' by running the git status command (but now it will detect the .gitignore file as 'untracked'.
Update the .gitignore file as follows:
temp.txt
*.tmp
Observe how .tmp files are no longer detected as 'untracked' by running the git status command.
The file should be currently listed under Unstaged files. Right-click it and choose Ignore.... Choose Ignore exact filename(s) and click OK.
Also take note of other options available e.g., Ignore all files with this extension etc. They may be useful in future.
Note how the temp.text is no longer listed under Unstaged files. Observe that a file named .gitignore has been created in the working directory root and has the following line in it. This new file is now listed under Unstaged files.
temp.txt
Right-click on any of the .tmp files you added, and choose Ignore... as you did previously. This time, choose the option Ignore files with this extension.
Note how .temp files are no longer shown as unstaged files, and the .gitignore file has been updated as given below:
temp.txt
*.tmp
3 Optionally, stage and commit the .gitignore file.
done!
Files recommended to be omitted from version control
- Binary files generated when building your project e.g.,
*.class,*.jar,*.exe
Reasons:- no need to version control these files as they can be generated again from the source code
- Revision control systems are optimized for tracking text-based files, not binary files.
- Temporary files e.g., log files generated while testing the product
- Local files i.e., files specific to your own computer e.g., local settings of your IDE (
.idea/) - Sensitive content i.e., files containing sensitive/personal information e.g., credential files, personal identification data (especially if there is a possibility of those files getting leaked via the revision control system).
Duplicating a Remote Repo on the Cloud
GitHub allows you to create a remote copy of another remote repo, called forking.
A fork is a copy of a remote repository created on the same hosting service such as GitHub, GitLab, or Bitbucket. On GitHub, you can fork a repository from another user or organisation into your own space (i.e., your user account or an organisation you have sufficient access to). Forking is particularly useful if you want to experiment with a repo but don’t have write permissions to the original -- you can fork it and work on your own remote copy without affecting the original repository.
Preparation Create a GitHub account if you don't have one yet.
1 Go to the GitHub repo you want to fork e.g., samplerepo-things
2 Click on the 
button in the top-right corner. In the next step,
- choose to fork to your own account or to another GitHub organization that you are an admin of.
- un-tick the
[ ] Copy the master branch onlyoption, so that you get copies of other branches (if any) in the repo.
done!
Forking is not a Git feature, but a feature provided by hosted Git services like GitHub, GitLab, or Bitbucket.
GitHub does not allow you to fork the same repo more than once to the same destination. If you want to re-fork, you need to delete the previous fork.
Creating a Local Copy of a Repo
The next step is to create a local copy of the remote repo, by cloning the remote repo.
You can clone a repository to create a full copy of it on your computer. This copy includes the entire revision history, branches, and files of the original, so it behaves just like the original repository. For example, you can clone a repository from a hosting service like GitHub to your computer, giving you a complete local version to work with.
Cloning a repo automatically creates a remote named origin which points to the repo you cloned from.
The repo you cloned from is often referred to as the upstream repo.
1 Clone the remote repo to your computer. For example, you can clone the samplerepo-things repo, or the fork you created from it in a previous lesson.
Note that the URL of the GitHub project is different from the URL you need to clone a repo in that GitHub project. e.g.
https://github.com/se-edu/samplerepo-things # GitHub project URL
https://github.com/se-edu/samplerepo-things.git # the repo URL
You can use the git clone <repository-url> [directory-name] command to clone a repo.
<repository-url>: The URL of the remote repository you want to copy.[directory-name](optional): The name of the folder where you want the repository to be cloned. If you omit this, Git will create a folder with the same name as the repository.
git clone https://github.com/se-edu/samplerepo-things.git # if using HTTPS
git clone git@github.com:se-edu/samplerepo-things.git # if using SSH
git clone https://github.com/foo/bar.git my-bar-copy # also specifies a dir to use
For exact steps for cloning a repo from GitHub, refer to this GitHub document.
File → Clone / New ... and provide the URL of the repo and the destination directory.
File → New ... → Choose as shown below → Provide the URL of the repo and the destination directory in the next dialog.
2 Verify the clone has a remote named origin pointing to the upstream repo.
Use the git remote -v command that you learned earlier.
Choose Repository → Repository Settings menu option.
done!
Downloading Data Into a Local Repo
When there are new changes in the remote, you need to pull those changes down to your local repo.
There are two steps to bringing over changes from a remote repository into a local repository: fetch and merge.
- Fetch is the act of downloading the latest changes from the remote repository, but without applying them to your current branch yet. It updates metadata in your repo so that it knows what has changed in the remote repo, but your own local branch remains untouched.
- Merge is what you do after fetching, to actually incorporate the fetched changes into your local branch. It combines your local branch with the changes from the corresponding branch from the remote repo.
1 Clone the repo se-edu/samplerepo-finances. It has 3 commits. Your clone now has a remote origin pointing to the remote repo you cloned from.
2 Change the remote origin to point to samplerepo-finances-2. This remote repo is a copy of the one you cloned, but it has two extra commits.
git remote set-url origin https://github.com/se-edu/samplerepo-finances-2.git
Go to Repository → Repository settings ... to update remotes.
3 Verify the local repo is unaware of the extra commits in the remote.
git status
On branch master
Your branch is up to date with 'origin/master'.
nothing to commit, working tree clean
The revision graph should look like the below:
If it looks like the below, it is possible that Sourcetree is auto-fetching data from the repo periodically.
4 Fetch from the new remote.
Use the git fetch <remote> command to fetch changes from a remote. If the <remote> is not specified, the default remote origin will be used.
git fetch origin
remote: Enumerating objects: 8, done.
... # more output ...
afbe966..cc6a151 master -> origin/master
* [new tag] beta -> beta
Click on the Fetch button on the top menu:
5 Verify the fetch worked i.e., the local repo is now aware of the two missing commits. Also observe how the local branch ref of the master branch, the staging area, and the working directory remain unchanged after the fetch.
Use the git status command to confirm the repo now knows that it is behind the remote repo.
git status
On branch master
Your branch is behind 'origin/master' by 2 commits, and can be fast-forwarded.
(use "git pull" to update your local branch)
nothing to commit, working tree clean
Now, the revision graph should look something like the below. Note how the origin/master ref is now two commits ahead of the master ref.
6 Merge the fetched changes.
Use the git merge <remote-tracking-branch> command to merge the fetched changes. Check the status and the revision graph to verify that the branch tip has now moved by two more commits.
git merge origin/master
git status
git log --oneline --decorate
To merge the fetched changes, right-click on the latest commit on origin/remote branch and choose Merge.
In the next dialog, choose as follows:
The final result should be something like the below (same as the repo state before we started this hands-on practical):
Note that merging the fetched changes can get complicated if there are multiple branches or the commits in the local repo conflict with commits in the remote repo. We will address them when we learn more about Git branches, in a later lesson.
done!
Pull is a shortcut that combines fetch and merge — it fetches the latest changes from the remote and immediately merges them into your current branch. In practice, Git users typically use the pull instead of the fetch-then-merge.
pull = fetch + merge
1 Similar to the previous hands-on practical, clone the repo se-edu/samplerepo-finances (to a new location).
Change the remote origin to point to samplerepo-finances-2.
2 Pull the newer commits from the remote, instead of a fetch-then-merge.
Use the git pull <remote> <branch> command to pull changes.
git pull origin master
The following works too. If the <remote> and <branch> are not specified, Git will pull to the current branch from the remote branch it is tracking.
git pull
Click on the Pull button on the top menu:
3 Verify the outcome is same as the fetch + merge steps you did in the previous hands-on practical.
done!
You can pull from any number of remote repos, provided the repos involved have a shared history. This can be useful when the upstream repo you forked from has some new commits that you wish to bring over to your copies of the repo (i.e., your fork and your local repo).
Preparation Fork se-edu/samplerepo-finances to your GitHub account.
Clone your fork to your computer.
Now, let's pretend that there are some new commits in upstream repo that you would like to bring over to your fork, and your local repo. Here are the steps:
1 Add the upstream repo se-edu/samplerepo-finances as remote named upstream in your local repo.
Adding remotes was covered in Lesson T2L4. Linking a Local Repo With a Remote Repo
2 Pull from the upstream repo. If there are new commits (in this case, there will be none), those will come over to your local repo. For example:
git pull upstream master
3 Push to your fork. Any new commits you pulled from the upstream repo will now appear in your fork as well. For example:
git push origin master
The method given above is the more 'standard' method of synchronising a fork with the upstream repo. In addition, platforms such as GitHub can provide other ways (example: GitHub's Sync fork feature).
4 For good measure, let's pull from another repo.
- Add the upstream repo se-edu/samplerepo-finances-2 as remote named
other-upstreamin your local repo. - Pull from it to your local repo; this will bring some new commits.
- Now, you can push those new commits to your fork.
git remote add other-upstream https://github.com/se-edu/samplerepo-finances-2.git
git pull other-upstream master
git push origin master
done!
Examining a Commit
It is useful to be able to see what changes were included in a specific commit.
When you examine a commit, normally what you see is the 'changes made since the previous commit'. This does not mean that a Git commit contains only the changes made since the previous commit. As you recall, a Git commit contains a full snapshot of the working directory. However, tools used to examine commits typically show only the changes, as that is the more informative part.
Git shows changes included in a commit by dynamically calculating the difference between the snapshots stored in the target commit and the parent commit. This is because Git commits store snapshots of the working directory, not changes themselves.
Although each commit represents a copy of the entire working directory, Git uses space efficiently in two main ways:
- Reuse of unchanged data: If a file hasn’t changed since a previous commit, the commit simply points to the already stored version of that file instead of making another copy. This means only new or changed files take up extra space, while unchanged files are reused.
- Compression: Git also compresses all the files and data it stores using an algorithm (zlib). So, even the objects that are stored (whether reused or new) take up less disk space because they are saved in a compressed format.
To address a specific commit, you can use its SHA (e.g., e60deaeb2964bf2ebc907b7416efc890c9d4914b). In fact, just the first few characters of the SHA is enough to uniquely address a commit (e.g., e60deae), provided the partial SHA is long enough to uniquely identify the commit (i.e., only one commit has that partial SHA).
Naturally, a commit can be addressed using any ref pointing to it too (e.g., HEAD, master).
Another related technique is to use the <ref>~<n> notation (e.g., HEAD~1) to address the commit that is n commits prior to the commit pointed by <ref> i.e., "start with the commit pointed by <ref> and go back n commits".
A related alternative notation is HEAD~, HEAD~~, HEAD~~~, ... to mean HEAD~1, HEAD~2, HEAD~3 etc.
HEAD or masterHEAD~1 or master~1 or HEAD~ or master~HEAD~2 or master~2Git uses the diff format to show file changes in a commit. The diff format was originally developed for Unix. It was later extended with headers and metadata to show changes between file versions and commits. Here is an example diff showing the changes to a file.
diff --git a/fruits.txt b/fruits.txt
index 7d0a594..f84d1c9 100644
--- a/fruits.txt
+++ b/fruits.txt
@@ -1,6 +1,6 @@
-apples
+apples, apricots
bananas
cherries
dragon fruits
-elderberries
figs
@@ -20,2 +20,3 @@
oranges
+pears
raisins
diff --git a/colours.txt b/colours.txt
new file mode 100644
index 0000000..55c8449
--- /dev/null
+++ b/colours.txt
@@ -0,0 +1 @@
+a file for colours
A Git diff can consist of multiple file diffs, one for each changed file. Each file diff can contain one or more hunk i.e., a localised group of changes within the file — including lines added, removed, or left unchanged (included for context).
Given below is how the above diff is divided into its components:
File diff for fruits.txt:
diff --git a/fruits.txt b/fruits.txt
index 7d0a594..f84d1c9 100644
--- a/fruits.txt
+++ b/fruits.txt
Hunk 1:
@@ -1,6 +1,6 @@
-apples
+apples, apricots
bananas
cherries
dragon fruits
-elderberries
figs
Hunk 2:
@@ -20,2 +20,3 @@
oranges
+pears
raisins
File diff for colours.txt:
diff --git a/colours.txt b/colours.txt
new file mode 100644
index 0000000..55c8449
--- /dev/null
+++ b/colours.txt
Hunk 1:
@@ -0,0 +1 @@
+a file for colours
Here is an explanation of the diff:
| Part of Diff | Explanation |
|---|---|
diff --git a/fruits.txt b/fruits.txt | The diff header, indicating that it is comparing the file fruits.txt between two versions: the old (a/) and new (b/). |
index 7d0a594..f84d1c9 100644 | Shows the before and after the change, and the file mode (100 means a regular file, 644 are file permission indicators). |
--- a/fruits.txt+++ b/fruits.txt | Marks the old version of the file (a/fruits.txt) and the new version of the file (b/fruits.txt). |
| This hunk header shows that lines 1-6 (i.e., starting at line 1, showing 6 lines) in the old file were compared with lines 1–6 in the new file. |
-apples+apples, apricots | Removed line apples and added line apples, apricots. |
bananascherriesdragon fruits | Unchanged lines, shown for context. |
-elderberries | Removed line: elderberries. |
figs | Unchanged line, shown for context. |
| Hunk header showing that lines 20-21 in the old file were compared with lines 20–22 in the new file. |
oranges+pearsraisins | Unchanged line. Added line: pears.Unchanged line. |
diff --git a/colours.txt b/colours.txt | The usual diff header, indicates that Git is comparing two versions of the file colours.txt: one before and one after the change. |
new file mode 100644 | This is a new file being added. 100644 means it’s a normal, non-executable file with standard read/write permissions. |
index 0000000..55c8449 | The usual SHA hashes for the two versions of the file. 0000000 indicates the file did not exist before. |
--- /dev/null+++ b/colours.txt | Refers to the "old" version of the file (/dev/null means it didn’t exist before), and the new version. |
@@ -0,0 +1 @@ | Hunk header, saying: “0 lines in the old file were replaced with 1 line in the new file, starting at line 1.” |
+a file for colours | Added line |
Points to note:
+indicates a line being added.
-indicates a line being deleted.- Editing a line is seen as deleting the original line and adding the new line.
TargetView contents of specific commits in a repo.
Preparation You can use any repo that has commits e.g., the things repo.
1 Locate the commits to view, using the revision graph.
git log --oneline --decorate
e60deae (HEAD -> master, origin/master) Update fruits list
f761ea6 Add colours.txt, shapes.txt
2bedace Add figs to fruits.txt
d5f91de Add fruits.txt
2 Use the git show command to view specific commits.
git show # shows the latest commit
commit e60deaeb2964bf2ebc907b7416efc890c9d4914b (HEAD -> master, origin/master)
Author: damithc <...@...>
Date: Sat Jun ...
Update fruits list
diff --git a/fruits.txt b/fruits.txt
index 7d0a594..6d502c3 100644
--- a/fruits.txt
+++ b/fruits.txt
@@ -1,6 +1,6 @@
-apples
+apples, apricots
bananas
+blueberries
cherries
dragon fruits
-elderberries
figs
To view the parent commit of the latest commit, you can use any of these commands:
git show HEAD~1
git show master~1
git show e60deae # first few characters of the SHA
git show e60deae..... # run git log to find the full SHA and specify the full SHA
To view the commit that is two commits before the latest commit, you can use git show HEAD~2 etc.
Click on the commit. The remaining panels (indicated in the image below) will be populated with the details of the commit.
done!
PRO-TIP: Use Git Aliases to Work Faster
The Git alias feature allows you to create custom shortcuts for frequently used Git commands. This saves time and reduces typing, especially for long or complex commands. Once an alias is defined, you can use the alias just like any other Git command e.g., use git lodg as an alias for git log --oneline --decorate --graph.
To define a global git alias, you can use the git config --global alias.<alias> "command" command. e.g.,
git config --global alias.lodg "log --oneline --graph --decorate"
You can also create shell-level aliases using your shell configuration (e.g., .bashrc, .zshrc) to make even shorter aliases. This lets you create shortcuts for any command, including Git commands, and even combine them with other tools. e.g., instead of the Git alias git lodg, you can define a shorter shell-level alias glodg.
1. Locate your .bash_profile file (likely to be in : C:\Users\<YourName>\.bash_profile -- if it doesn’t exist, create it.)
1. Locate your shell's config file e.g., .bashrc or .zshrc (likely to be in your ~ folder)
1. Locate your shell's config file e.g., .bashrc or .zshrc (likely to be in your ~ folder)
Oh-My-Zsh for Zsh terminal supports a Git plugin that adds a wide array of Git command aliases to your terminal.
2. Add aliases to that file:
alias gs='git status'
alias glod='git log --oneline --graph --decorate'
3. Apply changes by running the command source ~/.zshrc or source ~/.bash_profile or source ~/.bashrc, depending on which file you put the aliases in.
Tagging Commits
When working with many commits, it helps to tag specific commits with custom names so they’re easier to refer to later.
Git lets you tag commits with names, making them easy to reference later. This is useful when you want to mark specific commits -- such as releases or key milestones (e.g., v1.0 or v2.1). Using tags to refer to commits is much more convenient than using SHA hashes. In the diagram below, v1.0 and interim are tags.
A tag stays fixed to the commit. Unlike branch refs or HEAD, tags do not move automatically as new commits are made. As you see below, after adding a new commit, tags stay in the previous commits while master←HEAD has moved to the new commit.
Git supports two kinds of tags:
- A lightweight tag is just a ref that points directly to a commit, like a branch that doesn’t move.
- An annotated tag is a full Git object that stores a reference to a commit along with metadata such as the tagger’s name, date, and a message.
Annotated tags are generally preferred for versioning and public releases, while lightweight tags are often used for less formal purposes, such as marking a commit for your own reference.
Target Add a few tags to a repository.
Preparation Fork and clone the samplerepo-preferences. Use the cloned repo on your computer for the following steps.
1 Add a lightweight tag to the current commit as v1.0:
git tag v1.0
2 Verify the tag was added. To view tags:
git tag
v1.0
To view tags in the context of the revision graph:
git log --oneline --decorate
507bb74 (HEAD -> master, tag: v1.0, origin/master, origin/HEAD) Add donuts
de97f08 Add cake
5e6733a Add bananas
3398df7 Add food.txt
3 Use the tag to refer to the commit e.g., git show v1.0 should show the changes in the tagged commit.
4 Add an annotated tag to an earlier commit. The example below adds a tag v0.9 to the commit HEAD~2 with the message First beta release. The -a switch tells Git this is an annotated tag.
git tag -a v0.9 HEAD~2 -m "First beta release"
5 Check the new annotated tag. While both types of tags appear similarly in the revision graph, the show command on an annotated tag will show the details of the tag and the details of the commit it points to.
git show v0.9
tag v0.9
Tagger: ... <...@...>
Date: Sun Jun ...
First beta release
commit ....999087124af... (tag: v0.9)
Author: ... <...@...>
Date: Sat Jun ...
Add figs to fruits.txt
diff --git a/fruits.txt b/fruits.txt
index a8a0a01..7d0a594 100644
# rest of the diff goes here
Right-click on the commit (in the graphical revision graph) you want to tag and choose Tag….
Specify the tag name e.g., v1.0 and click Add Tag.
Configure tag properties in the next dialog and press Add. For example, you can choose whether to make it a lightweight tag or an annotated tag (default).
Tags will appear as labels in the revision graph, as seen below. To see the details of an annotated tag, you need to use the menu indicated in the screenshot.
done!
If you need to change what a tag points to, you must delete the old one and create a new tag with the same name. This is because tags are designed to be fixed references to a specific commit, and there is no built-in mechanism to 'move' a tag.
Preparation Continue with the same repo you used for the previous hands-on practical.
Move the v1.0 tag to the commit HEAD~1, by deleting it first and creating it again at the destination commit.
Delete the previous v1.0 tag by using the -d . Add it again to the other commit, as before.
git tag -d v1.0
git tag v1.0 HEAD~1
The same dialog used to add a tag can be used to delete and even move a tag. Note that the 'moving' here translates to deleting and re-adding behind the scene.
done!
Tags are different from commit messages, in purpose and in form. A commit message is a description of the commit that is part of the commit itself. A tag is a short name for a commit, which you can use to address a commit.
Pushing commits to a remote does not push tags automatically. You need to push tags specifically.
Target Push tags you created earlier to the remote.
Preparation Continue with the same repo you used for the previous hands-on practical.
You can go to your remote on GitHub link https://github.com/{USER}/{REPO}/tags (e.g., https://github.com/johndoe/samplerepo-preferences/tags) to verify the tag is present there.
Note how GitHub assumes these tags are meant as releases, and automatically provides zip and tar.gz archives of the repo (as at that tag).
1 Push a specific tag in the local repo to the remote (e.g., v1.0) using the git push <origin> <tag-name> command.
git push origin v1.0
In addition to verifying the tag's presence via GitHub, you can also use the following command to list the tags presently in the remote.
git ls-remote --tags origin
2 Delete a tag in the remote, using the git push --delete <remote> <tag-name> command.
git push --delete origin v1.0
3 Push all tags to the remote repo, using the git push <remote> --tags command.
git push origin --tags
To push a specific tag, use the following menu:
To push all tags, you can tick the Push all tags option when pushing commits:
done!
Comparing Points of History
Git can tell you the net effect of changes between two points of history.
Git's diff feature can show you what changed between two points in the revision history. Given below are some use cases.
Usage 1: Examining changes in the working directory
Example use case: To verify the next commit will include exactly what you intend it to include.
Preparation For this, you can use the things repo you created earlier. If you don't have it, you can clone a copy of a similar repo given here.
1 Do some changes to the working directory. Stage some (but not all) changes. For example, you can run the following commands.
echo -e "blue\nred\ngreen" >> colours.txt
git add . # a shortcut to stage all changes
echo "no shapes added yet" >> shapes.txt
2 Examine the staged and unstaged changes.
The git diff command shows unstaged changes in the working directory (tracked files only). The output of the diff command, is a diff view (introduced in this lesson).
git diff
diff --git a/shapes.txt b/shapes.txt
index 5c2644b..949c676 100644
--- a/shapes.txt
+++ b/shapes.txt
@@ -1 +1,2 @@
a file for shapes
+no shapes added yet!
The git diff --staged command shows the staged changes (same as git diff --cached).
git diff --staged
Select the two commits: Click on one commit, and Ctrl-Click (or Cmd-Click) on the second commit. The changes between the two selected commits will appear in the other panels, as shown below:
done!
Usage 2: Comparing two commits at different points of the revision graph
Example use case: Suppose you’re trying to improve the performance of a piece of software by experimenting with different code tweaks. You commit after each change (as you should). After several commits, you now want to review the overall effect of all those changes on the code.
Target Compare two commits in a repo.
Preparation You can use any repo with multiple commits e.g., the things repo.
You can use the git diff <commit1> <commit2> command for this.
- You may use any valid way to refer to commits (e.g., SHA, tag, HEAD~n etc.).
- You may also use the
..notation to specify the commit range too e.g.,0023cdd..fcd6199,HEAD~2..HEAD
git diff v0.9 HEAD
diff --git a/colours.txt b/colours.txt
new file mode 100644
index 0000000..55c8449
--- /dev/null
+++ b/colours.txt
@@ -0,0 +1 @@
+a file for colours
# rest of the diff ...
Swap the commit order in the command and see what happens.
git diff HEAD v0.9
diff --git a/colours.txt b/colours.txt
deleted file mode 100644
index 55c8449..0000000
--- a/colours.txt
+++ /dev/null
@@ -1 +0,0 @@
-a file for colours
# rest of the diff ...
As you can see, the diff is directional i.e., diff <commit1> <commit2> shows what changes you need to do to go from the <commit1> to <commit2>. If you swap <commit1> and <commit2>, the output will change accordingly e.g., lines previously shown as 'added' will now be shown as 'deleted'.
Select the two commits: Click on one commit, and Ctrl-Click (or Cmd-Click) on the second commit. The changes between the two selected commits will appear in the other panels, as shown below:
The same method can be used to compare the current state of the working directory (which might have uncommitted changes) to a point in the history.
done!
Usage 3: Examining changes to a specific file
Example use case: Similar to other use cases but when you are interested in a specific file only.
Target Examine the changes done to a file between two different points in the version history (including the working directory).
Preparation Use any repo with multiple commits e.g. the things repo.
Add the -- path/to/file to a previous diff command to narrow the output to a specific file. Some examples:
git diff -- fruits.txt # unstaged changes to fruits.txt
git diff --staged -- src/main.java # staged changes to src/main.java
git diff HEAD~2..HEAD -- fruits.txt # changes to fruits.txt between commits
Sourcetree UI shows changes to one file at a time by default; just click on the file to view changes to that file. To view changes to multiple files, Ctrl-Click (or Cmd-Click) on multiple files to select them.
done!
Traversing to a Specific Commit
Another useful feature of revision control is to be able to view the working directory as it was at a specific point in history, by checking out a commit created at that point.
Suppose you added a new feature to a software product, and while testing it, you noticed that another feature added two commits ago doesn’t handle a certain edge case correctly. Now you’re wondering: did the new feature break the old one, or was it already broken? Can you go back to the moment you committed the old feature and test it in isolation, and come back to the present after you found the answer? With Git, you can.
To view the working directory at a specific point in history, you can check out a commit created at that point.
When you check out a commit, Git:
- Updates your working directory to match the snapshot in that commit, overwriting current files as needed.
- Moves the
HEADref to that commit, marking it as the current state you’re viewing.
→
[check out commit C2...]
Checking out a specific commit puts you in a "detached HEAD" state: i.e., the HEAD no longer points to a branch, but directly to a commit (see the above diagram for an example). This isn't a problem by itself, but any commits you make in this state can be lost, unless certain follow-up actions are taken. It is perfectly fine to be in a detached state if you are only examining the state of the working directory at that commit.
To get out of a "detached HEAD" state, you can simply check out a branch, which "re-attaches" HEAD to the branch you checked out.
→
[check out master...]
Target Checkout a few commits in a local repo, while examining the working directory to verify that it matches the state when you created the corresponding commit
Preparation Use any repo with commits e.g., the things repo
1 Examine the revision tree, to get your bearing first.
git log --oneline --decorate
Reminder: You can use aliases to reduce typing Git commands.
e60deae (HEAD -> master, origin/master) Update fruits list
f761ea6 (tag: v1.0) Add colours.txt, shapes.txt
2bedace (tag: v0.9) Add figs to fruits.txt
d5f91de Add fruits.txt
2 Use the checkout <commit-identifier> command to check out a commit other than the one currently pointed by HEAD. You can use any of the following methods:
git checkout v1.0: checks out the commit taggedv1.0git checkout 0023cdd: checks out the commit with the hash0023cddgit checkout HEAD~2: checks out the commit 2 commits behind the most recent commit.
git checkout HEAD~2
Note: switching to 'HEAD~2'.
You are in 'detached HEAD' state.
# rest of the warning about the detached head ...
HEAD is now at 2bedace Add figs to fruits.txt
3 Verify HEAD and the working directory have updated as expected.
HEADshould now be pointing at the target commit- The working directory should match the state it was in at that commit (e.g., files added after that commit -- such as
shapes.txtshould not be in the folder).
git log --oneline --decorate
2bedace (HEAD, tag: v0.9) Add figs to fruits.txt
d5f91de Add fruits.txt
HEAD is indeed pointing at the target commit.
But note how the output does not show commits you added after the checked-out commit.
The --all switch tells git log to show commits from all refs, not just those reachable from the current HEAD. This includes commits from other branches, tags, and remotes.
git log --oneline --decorate --all
e60deae (origin/master, master) Update fruits list
f761ea6 (tag: v1.0) Add colours.txt, shapes.txt
2bedace (HEAD, tag: v0.9) Add figs to fruits.txt
d5f91de Add fruits.txt
4 Go back to the latest commit by checking out the master branch again.
git checkout master
In the revision graph, double-click the commit you want to check out, or right-click on that commit and choose Checkout....
Click OK to the warning about ‘detached HEAD’ (similar to below).
The specified commit is now loaded onto the working folder, as indicated by the HEAD label.
To go back to the latest commit on the master branch, double-click the master branch.
If you check out a commit that comes before the commit in which you added a certain file (e.g., temp.txt) to the .gitignore file, and if the .gitignore file is version controlled as well, Git will now show it under ‘unstaged modifications’ because at Git hasn’t been told to ignore that file yet.
done!
If there are uncommitted changes in the working directory, Git proceeds with a checkout only if it can preserve those changes.
- Example 1: There is a new file in the working directory that is not committed yet.
→ Git will proceed with the checkout and will keep the uncommitted file as well. - Example 2: There is an uncommitted change to a file that conflicts with the version of that file in the commit you wish to check out.
→ Git will abort the checkout, and the repo will remain in the current commit.
The Git stash feature temporarily sets aside uncommitted changes you’ve made (in your working directory and staging area), without committing them. This is useful when you’re in the middle of some work, but need to switch to another state (e.g., checkout a previous commit), and your current changes are not yet ready to be committed or discarded. You can later reapply the stashed changes when you’re ready to resume that work.
Rewriting History to Start Over
Git can also reset the revision history to a specific point so that you can start over from that point.
Suppose you realise your last few commits have gone in the wrong direction, and you want to go back to an earlier commit and continue from there — as if the “bad” commits never happened. Git’s reset feature can help you do that.
Git reset moves the tip of the current branch to a specific commit, optionally adjusting your staged and unstaged changes to match. This effectively rewrites the branch's history by discarding any commits that came after that point.
Resetting is different from the checkout feature:
- Reset: Lets you start over from a past state. It rewrites history by moving the branch ref to a new location.
- Checkout: Lets you explore a past state without rewriting history. It just moves the
HEADref.
→
[reset to C2...]
master branch!There are three types of resets: soft, mixed, hard. All three move the branch pointer to a new commit, but they vary based on what happens to the staging area and the working directory.
- soft reset: Moves the cumulative changes from the discarded commits into the staging area, waiting to be committed again. Any staged and unstaged changes that existed before the reset will remain untouched.
- mixed reset: Cumulative changes from the discarded commits, and any existing staged changes, are moved into the working directory.
- hard reset: All staged and unstaged changes are discarded. Both the working directory and the staging area are aligned with the target commit (as if no changes were done after that commit).
Preparation First, set the stage as follows (e.g., in the things repo):
i) Add four commits that are supposedly 'bad' commits.
ii) Do a 'bad' change to one file and stage it.
iii) Do a 'bad' change to another file, but don't stage it.
The following commands can be used to add commits B1-B4:
echo "bad colour" >> colours.txt
git commit -am "Incorrectly update colours.txt"
echo "bad shape" >> shapes.txt
git commit -am "Incorrectly update shapes.txt"
echo "bad fruit" >> fruits.txt
git commit -am "Incorrectly update fruits.txt"
echo "bad line" >> incorrect.txt
git add incorrect.txt
git commit -m "Add incorrect.txt"
echo "another bad colour" >> colours.txt
git add colours.txt
echo "another bad shape" >> shapes.txt
Now we have some 'bad' commits and some 'bad' changes in both the staging area and the working directory. Let's use the reset feature to get rid of all of them, but do it in three steps so that you can learn all three types of resets.
1 Do a soft reset to B2 (i.e., discard last two commits). Verify,
- the
masterbranch is now pointing atB2, and, - the changes that were in the discarded commits (i.e.,
B3andB4) are now in the staging area.
Use the git reset --soft <commit> command to do a soft reset.
git reset --soft HEAD~2
You can run the following commands to verify the current status of the repo is as expected.
git status # check overall status
git log --oneline --decorate # check the branch tip
git diff # check unstaged changes
git diff --staged # check staged changes
Right-click on the commit that you want to reset to, and choose Reset <branch-name> to this commit option.
In the next dialog, choose Soft - keep all local changes.
2 Do a mixed reset to commit B1. Verify,
- the
masterbranch is now pointing atB1. - the staging area is empty.
- the accumulated changes from all three discarded commits (including those from the previous soft reset) are now appearing as unstaged changes in the working directory.
Note howincorrect.txtappears as an 'untracked' file -- this is because unstaging a change of type 'add file' results in an untracked file.
Use the git reset --mixed <commit> command to do a mixed reset. The --mixed flag is the default, and can be omitted.
git reset HEAD~1
Verify the repo status, as before.
Similar to the previous reset, but choose the Mixed - keep working copy but reset index option in the reset dialog.
3 Do a hard reset to commit C4. Verify,
- the
masterbranch is now pointing atC4i.e., all 'bad' commits are gone. - the staging area is empty.
- there are no unstaged changes (except for the untracked files
incorrect.txt-- Git leaves untracked files alone, as untracked files are not meant to be under Git's control).
Use the git reset --hard <commit> command.
git reset --hard HEAD~1
Verify the repo status, as before.
Similar to the previous reset, but choose the Hard - discard all working copy changes option.
done!
Rewriting history can cause your local repo to diverge from its remote counterpart. For example, if you discard earlier commits and create new ones in their place, and you’ve already pushed the original commits to a remote repository, your local branch history will no longer match the corresponding remote branch. Git refers to this as a diverged history.
To protect the integrity of the remote, Git will reject attempts to push a diverged branch using a normal push. If you want to overwrite the remote history with your local version, you must perform a force push.
Preparation Choose a local-remote repo pair under your control e.g., the things repo from Tour 2: Backing up a Repo on the Cloud.
1 Rewrite the last commit: Reset the current branch back by one commit, and add a new commit.
For example, you can use the following commands.
git reset --hard HEAD~1
echo "water" >> drinks.txt
git add .
git commit -m "Add drinks.txt"
2 Observe how the local branch is diverged.
git log --oneline --graph --all
* fc1d04e (HEAD -> master) Add drinks.txt
| * e60deae (upstream/master, origin/master) Update fruits list
|/
* f761ea6 (tag: v1.0) Add colours.txt, shapes.txt
* 2bedace (tag: v0.9) Add figs to fruits.txt
* d5f91de Add fruits.txt
3 Attempt to push to the remote. Observe Git rejects the push.
git push origin master
To https://github.com/.../things.git
! [rejected] master -> master (non-fast-forward)
error: failed to push some refs to 'https://github.com/.../things.git'
hint: Updates were rejected because the tip of your current branch is behind
hint: its remote counterpart. If you want to integrate the remote changes,
hint: ...
4 Do a force-push.
You can use the --force (or -f) flag to force push.
git push -f origin master
A safer alternative to --force is --force-with-lease which overwrites the remote branch only if it hasn’t changed since you last fetched it (i.e., only if remote doesn't have recent changes that you are unaware of):
git push --force-with-lease origin master
done!
Controlling What Goes Into a Commit
To create well-crafted commits, you need to know how to control which precise changes go into a commit.
Crafting a commit involves two aspects:
- What changes to include in it: deciding what changes belong together in a single commit — this is about commit granularity, ensuring each commit represents a meaningful, self-contained change.
- How to include those changes: carefully staging just those changes — this is about using Git’s tools to precisely control what ends up in the commit.
SIDEBAR: Guidelines on what to include in a commit
A good commit represents a single, logical unit of change — something that can be described clearly in one sentence. For example, fixing a specific bug, adding a specific feature, or refactoring a specific function. If each commit tells a clear story about why the change was made and what it achieves, your repository history becomes a valuable narrative of the project’s development. Here are some (non-exhaustive) guidelines:
- No more than one change per commit: Avoid lumping unrelated changes into one commit, as this makes the history harder to understand, review, or revert (if each commit contains one standalone change, to reverse that change can be done by deleting or reverting that specific commit entirely, without affecting any other changes).
- Make the commit standalone: Don’t split a single logical change across multiple commits unnecessarily, as this can clutter the history and make it harder to follow the evolution of an idea or feature.
- Small enough to review easily, but large enough to stand on its own: For example, fixing the same typo in five files can be one commit — splitting it into five separate commits is excessive. Conversely, implementing a big feature may be too much for one commit — instead, break it down into a series of commits, each containing a meaningful yet standalone step towards the final goal.
Git can let you choose not just which files, but which specific changes within those files, to include in a commit. Most Git tools — including the command line and many GUIs — let you interactively select which "hunks" or even individual lines of a file to stage. This allows you to separate unrelated changes and avoid committing unnecessary edits. If you make multiple changes in the same file, you can selectively stage only the parts that belong to the current logical change.
This level of control is particularly useful when:
- You noticed and fixed a small, unrelated issue while working on something else.
- You experimented with multiple approaches in the same file and now want to commit only the final, clean solution.
- You want your commit history to clearly separate concerns, even when the edits touch the same files.
Preparation You can use any repo for this.
1 Do several changes to some tracked files. Change multiple files. Also change multiple locations in the same file.
2 Stage some changes in some files while keeping other changes in the same files unstaged.
As you know, you can use git add <filename> to stage changes to an entire file.
To select which hunks to stage, you can use the git add -p command instead (-p stands for 'by patch'):
git add -p
This command will take you to an interactive mode in which you can go through each hunk and decide if you want to stage it. The video below contains a demonstration of how this feature works:
To stage a hunk, you can click the Stage button above the hunk in question:
Unstaging can be done similarly:Most git operations can be done faster through the CLI than equivalent Git GUI clients, once you are familiar enough with the CLI commands.
However, selective staging is one exception where a good GUI can do better than the CLI, if you need to do many fine-grained staging operations (e.g., frequently staging only parts of hunks).
done!
Writing Good Commit Messages
Detailed and well-written commit messages can increase the value of Git revision history.
Every commit you make in Git also includes a commit message that explains the change. While one-line messages are fine for small or obvious changes, as your revision history grows, good commit messages become an important source of information — for example, to understand the rationale behind a specific change made in the past.
A commit message is meant to explain the intent behind the changes, not just what was changed. The code (or diff) already shows what changed. Well-written commit messages make collaboration, code reviews, debugging, and future maintenance easier by helping you and others quickly understand the project’s history without digging into the code of every commit.
A complete commit message can include a short summary line (the subject) followed by a more detailed body if needed. The subject line should be a concise description of the change, while the body can elaborate on the context, rationale, side effects, or other details if the change is more complex.
A commit message has the following structure (note how the subject and the body are separated by a blank line):
Subject line
<blank line>
Body
# lines starting with '#' are ignored (they will not be included in the commit message)
Here is an example commit message:
Find command: make matching case-insensitive
Find command is case-sensitive.
A case-insensitive find is more user-friendly because users cannot be
expected to remember the exact case of the keywords.
Let's,
* update the search algorithm to use case-insensitive matching
* add a script to migrate stress tests to the new format
Do some changes to a repo you have.
Commit the changes while writing a full commit message (i.e., subject + body).
When you are ready to commit, use the git commit command (without specifying a commit message).
git commit
This will open your default text editor (like Vim, Nano, or VS Code). Write the commit message inside the editor.
Save and close the editor to create the commit.
You can write your full commit message in the textbox you have been using to write commit messages already.
done!
Following a style guide makes your commit messages more consistent and fit-for-purpose. Many teams adopt established guidelines. These style guides typically contain common conventions that Git users follow when writing commit messages. For example:
- Keep the subject line (the first line) under 50–72 characters.
- Write the subject in the imperative mood (e.g.,
Fix typo in READMErather thanFixed typoorFixes typo). - Leave a blank line between the subject and the body, if you include a body.
- Wrap the body at around 72 characters per line for readability.
PRO-TIP: Configure Git to use your preferred text editor
Git will use the default text editor when it needs you to write a commit message. However, Git can be configured to use a different text editor of your choice.
You can use the following command to set the Git's default text editor:
git config --global core.editor "<editor command>"
Some examples for <editor command>
| Editor | Command to use |
|---|---|
| Vim (default) | vim |
| Nano | nano |
| VS Code | code --wait e.g., git config --global core.editor "code --wait"For this to work, your computer should already be configured to launch VS Code using the code command. See here to find how (refer the 'Launching from command line' section). |
| Sublime Text | subl -n -w |
| Atom | atom --wait |
| Notepad++ | notepad++.exe (Windows only) |
| Notepad | notepad (Windows built-in) |
Why use --wait or -w? Graphical editors (like VS Code or Sublime) start a separate process, which can take a few seconds. Without --wait, Git may think editing is done before you actually write the message. --wait makes Git pause until the editor window is closed.
Reorganising Commits
When the revision history gets 'messy', Git has a way to 'tidy up' the recent commits.
Git has a powerful tool called interactive rebasing which lets you review and reorganise your recent commits. With it, you can reword commit messages, change their order, delete commits, combine several commits into one (squash), or split a commit into smaller pieces. This feature is useful for tidying up a commit history that has become messy — for example, when some commits are out of order, poorly described, or include changes that would be clearer if split up or combined.
Preparation Run the following commands to create a sample repo that we'll be using for this hands-on practical:
mkdir samplerepo-sitcom
cd samplerepo-sitcom
git init
echo "Aspiring actress" >> Penny.txt
git add .
git commit -m "C1: Add Penny.txt"
echo "Scientist" >> Sheldon.txt
git add .
git commit -m "C3: Add Sheldon.txt"
echo "Comic book store owner" >> Stuart.txt
git add .
git commit -m "C2: Add Stuart.txt"
echo "Engineer" >> Stuart.txt
git commit -am "X: Incorrectly update Stuart.txt"
echo "Engineer" >> Howard.txt
git add .
git commit -m "C4: Adddd Howard.txt"
Target Here are the commits that should be in the created repo, and how each commit needs to be 'tidied up'.
C4: Adddd Howard.txt-- Fix typo in the commit messageAdddd→Add.X: Incorrectly update Stuart.txt-- Drop this commit.C2: Add Stuart.txt-- Swap this commit with the one below.C3: Add Sheldon.txt-- Swap this commit with the one above.C1: Add Penny.txt-- No change required.
1 Start the interactive rebasing.
To start the interactive rebase, use the git rebase -i <start-commit> command. -i stands for 'interactive'. In this case, we want to modify the last four commits (hence, HEAD~4).
git rebase -i HEAD~4
pick 97a8c4a C3: Add Sheldon.txt
pick 60bd28d C2: Add Stuart.txt
pick 8b9a36f X: Incorrectly update Stuart.txt
pick 8ab6941 C4: Adddd Howard.txt
# Rebase ee04afe..8ab6941 onto ee04afe (4 commands)
#
# Commands:
# p, pick <commit> = use commit
# r, reword <commit> = use commit, but edit the commit message
# e, edit <commit> = use commit, but stop for amending
# s, squash <commit> = use commit, but meld into previous commit
# f, fixup [-C | -c] <commit> = like "squash" but keep only the previous
# commit's log message, unless -C is used, in which case
# keep only this commit's message; -c is same as -C but
# opens the editor
# x, exec <command> = run command (the rest of the line) using shell
# b, break = stop here (continue rebase later with 'git rebase --continue')
# d, drop <commit> = remove commit
# l, label <label> = label current HEAD with a name
# t, reset <label> = reset HEAD to a label
# m, merge [-C <commit> | -c <commit>] <label> [# <oneline>]
# create a merge commit using the original merge commit's
# message (or the oneline, if no original merge commit was
# specified); use -c <commit> to reword the commit message
# u, update-ref <ref> = track a placeholder for the <ref> to be updated
# to this position in the new commits. The <ref> is
# updated at the end of the rebase
#
# These lines can be re-ordered; they are executed from top to bottom.
#
# If you remove a line here THAT COMMIT WILL BE LOST.
#
# However, if you remove everything, the rebase will be aborted.
#
The command will take you to the text editor, which will present you with a wall of text similar to the above. It has two parts:
- At the top, the list of commits and the action to take on each, oldest commit first, with the action
pickindicated by default (pickmeans 'use this commit in the result') for each. - At the bottom, instructions on how to edit those lines.
2 Edit the commit list to specify the rebase actions, as follows:
pick 60bd28d C2: Add Stuart.txt
pick 97a8c4a C3: Add Sheldon.txt
drop 8b9a36f X: Incorrectly update Stuart.txt
reword 8ab6941 C4: Addddd Howard.txt
4 Once you save edits and exit the text editor, Git will perform the rebase based on the actions you specified, from top to bottom.
At some steps, Git will pause the rebase and ask for your inputs. In this case, it will ask you to specify the new commit message when it is processing the following line.
reword 8ab6941 C4: Addddd Howard.txt
To go to the interactive rebase mode, right-click the parent commit of the earliest commit you want to reorganise (in this case, it is C1: Add Penny.txt) and choose Rebase children of <SHA> interactively...
2 To indicate what action you want to perform on each commit, select the commit in the list and click on the button for the action you want to do on it:
3 To execute the rebase, after indicating the action for all commits (the dialog will look like the below), click OK.
The final result should be something like the following, 'tidied up' exactly as we wanted:
* 727d877 C4: Add Howard.txt
* 764fc29 C3: Add Sheldon.txt
* 08a965a C2: Add Stuart.txt
* 6436598 C1: Add Penny.txt
done!
Rebasing rewrites history. It is not recommended to rebase commits you have already shared with others.
Creating Branches
To work in parallel timelines, you can use Git branches.
Git branches let you develop multiple versions of your work in parallel — effectively creating diverged timelines of your repository’s history. For example, one team member can create a new branch to experiment with a change, while the rest of the team continues working on another branch. Branches can have meaningful names, such as master, release, or draft.
A Git branch is simply a ref (a named label) that points to a commit and automatically moves forward as you add new commits to that branch. As you’ve seen before, the HEAD ref indicates which branch you’re currently working on, by pointing to the corresponding branch ref.
When you add a commit, it goes into the branch you are currently on, and the branch ref (together with the HEAD ref) moves to the new commit.
Git creates a branch named master by default (Git can be configured to use a different name e.g., main).
Given below is an illustration of how branch refs move as branches evolve. Refer to the text below it for explanations of each stage.
- There is only one branch (i.e.,
master) and there is only one commit on it. TheHEADref is pointing to themasterbranch (as we are currently on that branch). - A new commit has been added. The
masterand theHEADrefs have moved to the new commit. - A new branch
fix1has been added. The repo has switched to the new branch too (hence, theHEADref is attached to thefix1branch). - A new commit (
c) has been added. The current branch reffix1moves to the new commit, together with theHEADref. - The repo has switched back to the
masterbranch. Hence, theHEADhas moved back tomasterbranch's .
At this point, the repo's working directory reflects the code at commitb(notc).
- A new commit (
d) has been added. Themasterand theHEADrefs have moved to that commit. - The repo has switched back to the
fix1branch and added a new commiteto it. Note how the branch reffix1(together withHEAD) has moved to the new commitewhile the branch refmasterstill points tod.
Note that appearance of the revision graph (colors, positioning, orientation etc.) varies based on the Git client you use, and might not match the exact diagrams given above.
Preparation Fork the samplerepo-things repo, and clone it onto your computer.
1 Observe that you are on the branch called master.
$ git status
on branch master
2 Start a branch named feature1 and switch to the new branch.
You can use the branch command to create a new branch and the checkout command to switch to a specific branch.
$ git branch feature1
$ git checkout feature1
One-step shortcut to create a branch and switch to it at the same time:
$ git checkout –b feature1
The new switch command
Git recently introduced a switch command that you can use instead of the checkout command given above.
To create a new branch and switch to it:
$ git branch feature1
$ git switch feature1
One-step shortcut (by using -c or --create flag):
$ git switch –c feature1
Click on the Branch button on the main menu. In the next dialog, enter the branch name and click Create Branch.
Note how the feature1 is indicated as the current branch (reason: Sourcetree automatically switches to the new branch when you create a new branch, if the Checkout New Branch was selected in the previous dialog).
3 Create some commits in the new branch, as follows.
- Add a file named
numbers.txt, stage it, commit it. - Observe how commits you add while on
featurebranch will becomes part of that branch.
Observe how themasterref and theHEADref move to the new commit.
As before, you can use the git log --oneline --decorate command for this.
At times, the
HEADref of the local repo is represented as in Sourcetree, as illustrated in the screenshot below
.The
HEADref is not shown in the UI if it is already pointing at the active branch.
- Add some texts to
numbers.txt, stage the changes, and commit it. This commit too will be added to thefeature1branch.
4 Switch to the master branch. Note how the changes you made in the feature1 branch are no longer in the working directory.
$ git switch master
Double-click the master branch.
Revisiting master vs origin/master
In the screenshot above, you see a master ref and a origin/master ref for the same commit. The former identifies the of the local master branch while the latter identifies the tip of the master branch at the remote repo named origin. The fact that both refs point to the same commit means the local master branch and its remote counterpart are with each other.
Similarly, origin/HEAD ref appearing against the same commit indicates that of the remote repo is pointing to this commit as well.
5 Add a commit to the master branch. Let’s imagine it’s a bug fix.
To keep things simple for the time being, this commit should not involve the numbers.txt file that you changed in the feature1 branch. Of course, this is easily done, as the numbers.txt file you added in the feature branch is not even visible when you are in the master branch.
gitGraph BT:
%%{init: { 'theme': 'default', 'gitGraph': {'mainBranchName': 'master'}} }%%
commit id: "m1"
commit id: "m2"
branch feature1
commit id: "f1"
commit id: "[feature1] f2"
checkout master
commit id: "[HEAD → master] m3"
checkout feature1
6 Switch between the two branches and see how the working directory changes accordingly. That is, now you have two parallel timelines that you can freely switch between.
done!
You can also start a branch from an earlier commit, instead of the latest commit in the current branch. For that, simply check out the commit you wish to start from.
In the samplerepo-things repo that you used above, let's create a new branch that starts from the same commit the feature1 branch started from. Let's pretend this branch will contain an alternative version of the content we added in the feature1 branch.
gitGraph BT:
%%{init: { 'theme': 'default', 'gitGraph': {'mainBranchName': 'master'}} }%%
commit id: "m1"
commit id: "m2"
branch feature1
branch feature1-alt
checkout feature1
commit id: "f1"
commit id: "[feature1] f2"
checkout master
commit id: "[HEAD → master] m3"
checkout feature1-alt
commit id: "[HEAD → feature1-alt] a1"
Avoid this rookie mistake!
Always remember to switch back to the master branch before creating a new branch. If not, your new branch will be created on top of the current branch.
- Switch to the
masterbranch. - Checkout the commit that is at which the
feature1branch diverged from themasterbranch (e.g.git checkout HEAD~1). This will create a detachedHEAD. - Create a new branch called
feature1-alt. TheHEADwill now point to this new branch (i.e., no longer 'detached'). - Add a commit on the new branch.
PRO-TIP: Creating a branch based on another branch in one shot
Suppose you are currently on branch b2 and you want to create a new branch b3 that starts from b1. Normally, you can do that in two steps:
git switch b1 # switch to the intended base branch first
git switch -c b3 # create the new branch and switch to it
This can be done in one shot using the git switch -c <new-branch> <base-branch> command:
git switch -c b3 b1
done!
Merging Branches
Most work done in branches eventually gets merged together.
Merging combines the changes from one branch into another, bringing their diverged timelines back together.
When you merge, Git looks at the two branches and figures out how their histories have diverged since their merge base (i.e., the most recent common ancestor commit of two branches). It then applies the changes from the other branch onto your current branch, creating a new commit. The new commit created when merging is called a merge commit — it records the result of combining both sets of changes.
Given below is an illustration of how such a merge looks like in the revision graph:
- We are on the
fix1branch (as indicated byHEAD). - We have switched to the
masterbranch (thus,HEADis now pointing tomasterref). - The
fix1branch has been merged into themasterbranch, creating a merge commitf. The repo is still on themasterbranch.
A merge commit has two parent commits e.g., in the above example, the merge commit f has both d and e as parent commits. The parent commit on the receiving branch is considered the first parent and the other is considered the second parent e.g., in the example above, fix1 branch is being merged into the master branch (i.e., the receiving branch) -- accordingly, d is the first parent and e is the second parent.
Preparation We continue with the samplerepo-things repo from earlier, which should look like the following. Note that we are ignoring the feature1-alt branch, for simplicity.
gitGraph BT:
%%{init: { 'theme': 'default', 'gitGraph': {'mainBranchName': 'master'}} }%%
commit id: "m1"
commit id: "m2"
branch feature1
commit id: "f1"
commit id: "[feature1] f2"
checkout master
commit id: "[HEAD → master] m3"
checkout feature1
1 Switch back to the feature1 branch.
gitGraph BT:
%%{init: { 'theme': 'default', 'gitGraph': {'mainBranchName': 'master'}} }%%
commit id: "m1"
commit id: "m2"
branch feature1
commit id: "f1"
commit id: "[HEAD → feature1] f2"
checkout master
commit id: "[master] m3"
checkout feature1
2 Merge the master branch to the feature1 branch, giving an end-result like the following. Also note how Git has created a merge commit (shown as mc1 in the diagram below).
gitGraph BT:
%%{init: { 'theme': 'default', 'gitGraph': {'mainBranchName': 'master'}} }%%
commit id: "m1"
commit id: "m2"
branch feature1
commit id: "f1"
commit id: "f2"
checkout master
commit id: "[master] m3"
checkout feature1
merge master id: "[HEAD → feature1] mc1"
$ git merge master
Right-click on the master branch and choose merge master into the current branch. Click OK in the next dialog.
The revision graph should look like this now (colours and line alignment might vary but the graph structure should be the same):
Observe how the changes you made in the master branch (i.e., the imaginary bug fix in m3) is now available even when you are in the feature1 branch.
Furthermore, observe (e.g., git show HEAD) how the merge commit contains the sum of changes done in commits m3, f1, and f2.
3 Add another commit to the feature1 branch, in which you do some further changes to the numbers.txt.
Switch to the master branch and add one more commit.
gitGraph BT:
%%{init: { 'theme': 'default', 'gitGraph': {'mainBranchName': 'master'}} }%%
commit id: "m1"
commit id: "m2"
branch feature1
commit id: "f1"
commit id: "f2"
checkout master
commit id: "m3"
checkout feature1
merge master id: "mc1"
commit id: "[feature1] f3"
checkout master
commit id: "[HEAD → master] m4"
4 Merge feature1 to the master branch, giving an end-result like this:
gitGraph BT:
%%{init: { 'theme': 'default', 'gitGraph': {'mainBranchName': 'master'}} }%%
commit id: "m1"
commit id: "m2"
branch feature1
commit id: "f1"
commit id: "f2"
checkout master
commit id: "m3"
checkout feature1
merge master id: "mc1"
commit id: "[feature1] f3"
checkout master
commit id: "m4"
merge feature1 id: "[HEAD → master] mc2"
git merge feature1
Right-click on the feature1 branch and choose Merge.... The resulting revision graph should look like this:
Now, any changes you made in feature1 branch are available in the master branch.
done!
When the branch you're merging into hasn't diverged — meaning it hasn't had any new commits since the merge base — Git simply moves the branch pointer forward to include all the new commits, keeping the history clean and linear. This is called a fast-forward merge because Git simply "fast-forwards" the branch pointer to the tip of the other branch. The result looks as if all the changes had been made directly on one branch, without any branching at all.
gitGraph BT:
%%{init: { 'theme': 'default', 'gitGraph': {'mainBranchName': 'master'}} }%%
commit id: "m1"
commit id: "[HEAD → master] m2"
branch bug-fix
commit id: "b1"
commit id: "[bug-fix] b2"
checkout master
→
[merge bug-fix]
gitGraph BT:
%%{init: { 'theme': 'default', 'gitGraph': {'mainBranchName': 'master'}} }%%
commit id: "m1"
commit id: "m2"
commit id: "b1"
commit id: "[HEAD → master][bug-fix] b2"
checkout master
In the example above, the master branch has not changed since the merge base (i.e., m2). Hence, merging the branch bug-fix onto master can be done by fast-forwarding the master branch ref to the tip of the bug-fix branch (i.e., b2).
Preparation Let's continue with the same samplerepo-things repo we used above, and do a fast-forward merge this time.
1 Create a new branch called add-countries, and some commits to it as follows:
Switch to the new branch, add a file named countries.txt, stage it, and commit it.
Do some changes to countries.txt, and commit those changes.
You should have something like this now:
gitGraph BT:
%%{init: { 'theme': 'default', 'gitGraph': {'mainBranchName': 'master'}} }%%
commit id: "[master] mc2"
branch add-countries
commit id: "a1"
commit id: "[HEAD → add-countries] a2"
2 Go back to the master branch.
gitGraph BT:
%%{init: { 'theme': 'default', 'gitGraph': {'mainBranchName': 'master'}} }%%
commit id: "[HEAD → master] mc2"
branch add-countries
commit id: "a1"
commit id: "add-countries] a2"
3 Merge the add-countries branch onto the master branch. Observe that there is no merge commit. The master branch ref (and the HEAD ref along with it) moved to the tip of the add-countries branch (i.e., a2) and both branches now point to a2.
gitGraph BT:
%%{init: { 'theme': 'default', 'gitGraph': {'mainBranchName': 'master (and add-countries)'}} }%%
commit id: "mc2"
commit id: "a1"
commit id: "[HEAD → master][add-countries] a2"
done!
It is possible to force Git to create a merge commit even if fast forwarding is possible. This is useful if you prefer the revision graph to visually show when each branch was merged to the main timeline.
To prevent Git from fast-forwarding, use the --no-ff switch when merging. Example:
git merge --no-ff add-countries
Windows: Tick the box shown below when you merge a branch:
Mac:
Trigger the branch operation using the following menu button:
In the next dialog, tick the following option:
To permanently prevent fast-forwarding:
- Go to Sourcetree
Settings. - Navigate to the
Gitsection. - Tick the box
Do not fast-forward when merging, always create commit.
A squash merge combines all the changes from a branch into a single commit on the receiving branch, without preserving the full commit history of the branch being merged. This is especially useful when the feature branch contains many small or experimental commits that would clutter the main branch’s history. By squashing, you retain the final state of the changes while presenting them as one cohesive unit, making the project history easier to read and manage. It also helps maintain a linear, simplified commit log on the main branch.
gitGraph BT:
%%{init: { 'theme': 'default', 'gitGraph': {'mainBranchName': 'master'}} }%%
commit id: "[HEAD → master] m1"
branch feature
checkout feature
commit id: "f1"
commit id: "[feature] f2"
→
[squash merge...]
gitGraph BT:
%%{init: { 'theme': 'default', 'gitGraph': {'mainBranchName': 'master'}} }%%
commit id: "m1"
branch feature
checkout feature
commit id: "f1"
commit id: "[feature] f2"
checkout master
commit id: "[HEAD → master] s1 (same as f1+f2)" type: HIGHLIGHT
In the example above, the branch feature has been squash merged onto the master branch, creating a single 'squashed' commit s1 that combines all the commits in feature branch.
Resolving Merge Conflicts
When merging branches, you need to guide Git on how to resolve conflicting changes in different branches.
A merge conflict happens when Git can't automatically combine changes from two branches because the same parts of a file were modified differently in each branch. When this happens, Git pauses the merge and marks the conflicting sections in the affected files so you can resolve them yourself. Once you've reviewed and fixed the conflicts, you can tell Git they're resolved and complete the merge.
More generally, a conflict occurs when Git cannot automatically reconcile different changes made to the same part of a file -- branch merge conflicts is just one example.
Target To simulate a merge conflict and use it to learn how to resolve merge conflicts.
Preparation You can use any repo with at least one commit in the master branch.
1 Start a branch named fix1 in the repo. Create a commit that adds a line with some text to one of the files.
2 Switch back to master branch. Create a commit with a conflicting change i.e., it adds a line with some different text in the exact location the previous line was added.
3 Try to merge the fix1 branch onto the master branch. Git will pause mid-way during the merge and report a merge conflict. If you open the conflicted file, you will see something like this:
COLORS
------
blue
<<<<<< HEAD
black
=======
green
>>>>>> fix1
red
white
4 Observe how the conflicted part is marked between a line starting with <<<<<< and a line starting with >>>>>>, separated by another line starting with =======.
Highlighted below is the conflicting part that is coming from the master branch:
blue
<<<<<< HEAD
black
=======
green
>>>>>> fix1
red
This is the conflicting part that is coming from the fix1 branch:
blue
<<<<<< HEAD
black
=======
green
>>>>>> fix1
red
5 Resolve the conflict by editing the file. Let us assume you want to keep both lines in the merged version. You can modify the file to be like this:
COLORS
------
blue
black
green
red
white
6 Stage the changes, and commit. You have now successfully resolved the merge conflict.
done!
Renaming Branches
Branches can be renamed, for example, to fix a mistake in the branch name.
Local branches can be renamed easily. Renaming a branch simply changes the branch reference (i.e., the name used to identify the branch) — it is just a cosmetic change.
Preparation First, create the repo samplerepo-books for this hands-on practical, by running the following commands in your terminal.
mkdir samplerepo-books
cd samplerepo-books
git init
echo "Horror Stories" >> horror.txt
git add .
git commit -m "Add horror.txt"
git switch -c textbooks
echo "Textbooks" >> textbooks.txt
git add .
git commit -m "Add textbooks.txt"
git switch master
git switch -c fantasy
echo "Fantasy Books" >> fantasy.txt
git add .
git commit -m "Add fantasy.txt"
git switch master
git merge --no-ff -m "Merge branch textbooks" textbooks
The above should give you a repo similar to the revision graph given below, on the left.
gitGraph BT:
%%{init: { 'theme': 'default', 'gitGraph': {'mainBranchName': 'master'}} }%%
commit id: "m1"
branch textbooks
checkout textbooks
commit id: "[textbooks] t1"
checkout master
branch fantasy
checkout fantasy
commit id: "[fantasy] f1"
checkout master
merge textbooks id: "[HEAD → master] mc1"
→
[rename branches]
gitGraph BT:
%%{init: { 'theme': 'default', 'gitGraph': {'mainBranchName': 'master'}} }%%
commit id: "m1"
branch study-books
checkout study-books
commit id: "[study-books] t1"
checkout master
branch fantasy-books
checkout fantasy-books
commit id: "[fantasy-books] f1"
checkout master
merge study-books id: "[HEAD → master] mc1"
Target Rename the fantasy branch to fantasy-books. Similarly, rename textbooks branch to study-books. The outcome should be similar to the revision graph above, on the right.
steps:
To rename a branch, use the git branch -m <current-name> <new-name> command (-m stands for 'move'):
git branch -m fantasy fantasy-books
git branch -m textbooks study-books
git log --oneline --decorate --graph --all # verify the changes
* 443132a (HEAD -> master) Merge branch textbooks
|\
| * 4969163 (study-books) Add textbooks.txt
|/
| * 0586ee1 (fantasy-books) Add fantasy.txt
|/
* 7f28f0e Add horror.txt
Note these additional switches to the log command:
--all: Shows all branches, not just the current branch.--graph: Shows a graph-like visualisation (notice how*is used to indicate a commit, and branches are indicated using vertical lines).
Right-click on the branch name and choose Rename.... Provide the new branch name in the next dialog.
done!
SIDEBAR: Branch naming conventions
Branch names can contain lowercase letters, numbers, /, dashes (-), underscores (_), and dots (.).
You can use uppercase letters too, but many teams avoid them for consistency.
A common branch naming convention is to prefix it with <category>/. Some examples:
feature/login-form— for new features (origin/feature/login-formcould be the matching remote-tracking branch)bugfix/profile-photo— for fixing bugshotfix/payment-crash— for urgent production fixesrelease/2.0— for prepping a releaseexperiment/ai-chatbot— for “just trying stuff”
Although forward-slash (/) in the prefix doesn't mean folders, some tools treat it kind of like a path so you can group related branches when you run git branch. Shown below is an example of how Sourcetree groups branches with the same prefix.
Deleting Branches
Branches can be deleted to get rid of them when they are no longer needed.
Deleting a branch deletes the corresponding branch ref from the revision history (it does not delete any commits). The impact of the loss of the branch ref depends on whether the branch has been merged.
When you delete a branch that has been merged, the commits of the branch will still exist in the history and will be safe. Only the branch ref is lost.
gitGraph BT:
%%{init: { 'theme': 'default', 'gitGraph': {'mainBranchName': 'master'}} }%%
commit id: "m1"
branch bug-fix
checkout bug-fix
commit id: "[bug-fix] b1"
checkout master
merge bug-fix id: "[HEAD → master] mc1"
→
[delete branch bug-fix]
gitGraph BT:
%%{init: { 'theme': 'default', 'gitGraph': {'mainBranchName': 'master'}} }%%
commit id: "m1"
branch _
checkout _
commit id: "b1"
checkout master
merge _ id: "[HEAD → master] mc1"
In the above example, the only impact of the deletion is the loss of the branch ref bug-fix. All commits remain reachable (via the master branch), and there is no other impact on the revision history.
In fact, some prefer to delete the branch soon after merging it, to reduce branch references cluttering up the revision history.
When you delete a branch that has not been merged, the loss of the branch ref can render some commits unreachable (unless you know their commit IDs or they are reachable through other refs), putting them at risk of being lost eventually.
gitGraph BT:
%%{init: { 'theme': 'default', 'gitGraph': {'mainBranchName': 'master'}} }%%
commit id: "[HEAD → master] m1"
branch bug-fix
checkout bug-fix
commit id: "[bug-fix] b1"
checkout master
→
[delete branch bug-fix]
gitGraph BT:
%%{init: { 'theme': 'default', 'gitGraph': {'mainBranchName': 'master'}} }%%
commit id: "[HEAD → master] m1"
branch _
checkout _
commit id: "b1"
checkout master
In the above example, the commit b1 is no longer reachable, unless we know its commit ID (i.e., the SHA).
SIDEBAR: What makes a commit 'unreachable'?
Recall that a commit only has a pointer to its parent commit (not its descendent commits).
A commit is considered reachable if you can get to it by starting at a branch, tag, or other ref and walking backward through its parent commits. This is the normal state for commits — they are part of the visible history of a branch or tag.
When no branch, tag, or ref points to a commit (directly or indirectly), it becomes unreachable. This often happens when you delete a branch or rewrite history (e.g., with reset or rebase), leaving some commits "orphaned" (or "dangling") without a ref pointing to them.
In the example below, C4 is unreachable (i.e., cannot be reached by starting at any of the three refs: v1.0 or master or ←HEAD), but the other three are all reachable.
Unreachable commits are not deleted immediately — Git keeps them for a while before cleaning them up. By default, Git retains unreachable commits for at least 30 days, during which they can still be recovered if you know their SHA. After that, they will be garbage-collected, and will be lost for good.
Preparation First, create the repo samplerepo-books-2 for this hands-on practical, by running the following commands in your terminal.
mkdir samplerepo-books-2
cd samplerepo-books-2
git init
echo "Horror Stories" >> horror.txt
git add .
git commit -m "Add horror.txt"
git switch -c textbooks
echo "Textbooks" >> textbooks.txt
git add .
git commit -m "Add textbooks.txt"
git switch master
git switch -c fantasy
echo "Fantasy Books" >> fantasy.txt
git add .
git commit -m "Add fantasy.txt"
git switch master
git merge --no-ff -m "Merge branch textbooks" textbooks
1 Delete the (the merged) textbooks branch.
Use the git branch -d <branch> command to delete a local branch 'safely' -- this command will fail if the branch has unmerged changes.
git branch -d textbooks
git log --oneline --decorate --graph --all # check the current revision graph
* 443132a (HEAD -> master) Merge branch textbooks
|\
| * 4969163 Add textbooks.txt
|/
| * 0586ee1 (fantasy) Add fantasy.txt
|/
* 7f28f0e Add horror.txt
Right-click on the branch name and choose Delete <branch>:
In the next dialog, click OK:
Observe that all commits remain. The only missing thing is the textbook ref.
2 Make a copy of the SHA of the tip of the (unmerged) fantasy branch.
3 Delete the fantasy branch.
Attempt to delete the branch. It should fail, as shown below:
git branch -d fantasy
error: the branch 'fantasy' is not fully merged
hint: If you are sure you want to delete it, run 'git branch -D fantasy'
As also hinted by the error message, you can replace the -d with -D to 'force' the deletion.
git branch -D fantasy
Now, check the revision graph:
git log --oneline --decorate --graph --all
* 443132a (HEAD -> master) Merge branch textbooks
|\
| * 4969163 Add textbooks.txt
|/
* 7f28f0e Add horror.txt
Attempt to delete the branch as you did before. It will fail because the branch has unmerged commits.
Try again but this time, tick the Force delete option, which will force Git to delete the unmerged branch:
Observe how the branch ref fantasy is gone, together with any unmerged commits on it.
4 Attempt to view the 'unreachable' commit whose SHA you noted in step 2.
e.g., git show 32b34fb (use the SHA you copied earlier)
Observe how the commit still exists and still is reachable using the commit ID, although it is not reachable by other means, and not visible in the revision graph.
done!
Merging to Sync Branches
Merging is one way to keep one branch synchronised with another.
When working in parallel branches, you’ll often need to sync (short for synchronise) one branch with another. For example, while developing a feature in one branch, you might want to bring in a recent bug fix from another branch that your branch doesn’t yet have.
The simplest way to sync branches is to merge — that is, to sync a branch b1 with changes from another branch b2, you merge b2 into b1. In fact, you can merge them periodically to keep one branch up to date with the other.
gitGraph
%%{init: { 'theme': 'default', 'gitGraph': {'mainBranchName': 'master'}} }%%
commit id: "m1"
branch bug-fix
branch feature
commit id: "f1"
checkout master
checkout bug-fix
commit id: "b1"
checkout master
merge bug-fix
checkout feature
merge master id: "mc1"
commit id: "f2"
checkout master
commit id: "m2"
checkout feature
merge master id: "mc2"
checkout master
commit id: "m3"
checkout feature
commit id: "[feature] f3"
checkout master
commit id: "[HEAD → master] m4"
In the example above, you can see how the feature branch is merging the master branch periodically to keep itself in sync with the changes being introduced to the master branch.
Rebasing to Sync Branches
Rebasing is another way to synchronise one branch with another.
Rebasing is another way to synchronise one branch with another, while keeping the history cleaner and more linear. Instead of creating a merge commit to combine the branches, rebasing moves the entire sequence of commits from your branch and "replays" them on top of another branch. This effectively moves the base of your branch to the tip of the other branch (i.e., it 're-bases' it — hence the name), as if you had started your work from there in the first place.
Rebasing is especially useful when you want to update your branch with the latest changes from a main branch, but you prefer an uncluttered history with fewer merge commits.
Suppose we have the following revision graph, and we want to sync the feature branch with master, so that changes in commit m2 become visible to the feature branch.
gitGraph
%%{init: { 'theme': 'default', 'gitGraph': {'mainBranchName': 'master'}} }%%
commit id: "m1"
branch feature
checkout feature
commit id: "f1"
checkout master
commit id: "[master] m2"
checkout feature
commit id: "[HEAD → feature] f2"
If we merge the master branch to the feature branch as given below, m2 becomes visible to the feature branch. However, it creates a merge commit.
gitGraph
%%{init: { 'theme': 'default', 'gitGraph': {'mainBranchName': 'master'}} }%%
commit id: "m1"
branch feature
checkout feature
commit id: "f1"
checkout master
commit id: "[master] m2"
checkout feature
commit id: "f2"
merge master id: "[HEAD → feature] mc1"
Instead of merging, if we rebased the feature branch on the master branch, we would get the following.
gitGraph
%%{init: { 'theme': 'default', 'gitGraph': {'mainBranchName': 'master'}} }%%
commit id: "m1"
checkout master
commit id: "[branch: master] m2"
branch feature
checkout feature
commit id: "f1a"
commit id: "[HEAD → feature] f2a"
Note how the rebasing changed the base of the feature branch from m1 to m2. As a result, changes done in m2 are now visible to the feature branch. But there is no merge commit, and the revision graph is simpler.
Also note how the first commit in the feature branch, previously shown as f1, is now shown as f1a after the rebase. Although both commits contain the same changes, other details -- such as the parent commit -- are different, making them two distinct Git objects with different SHA values. Similarly, f2 and f2a are also different. Thus, the history of the entire feature branch has changed after the rebase.
Because rebasing rewrites the commit history of your branch, you should avoid rebasing branches that you’ve already published, and are potentially used by others -- rewriting published history can cause confusion and conflicts for those using the previous version of the commits.
Copying Specific Commits
Cherry-picking is a Git operation that copies over a specific commit from one branch to another.
Cherry-picking is another way to synchronise branches, by applying specific commits from one branch onto another.
Unlike merging or rebasing — which bring over all changes since the branches diverged — cherry-picking lets you choose individual commits and apply just those, one at a time, to your current branch. This is useful when you want to bring over a bug fix or a small feature from another branch without merging the entire branch history.
Because cherry-picking copies only the chosen commits, it creates new commits on your branch with the same changes but different SHA values.
Suppose we have the following revision graph, and we want to bring over the changes introduced in m3 (in the master branch) onto the feature branch.
gitGraph
%%{init: { 'theme': 'default', 'gitGraph': {'mainBranchName': 'master'}} }%%
commit id: "m1"
branch feature
checkout feature
commit id: "f1"
checkout master
commit id: "m2"
commit id: "m3" type: HIGHLIGHT
commit id: "[master] m4"
checkout feature
commit id: "[HEAD → feature] f2"
After cherry-picking m3 onto the feature branch, the revision graph should look like the following:
gitGraph
%%{init: { 'theme': 'default', 'gitGraph': {'mainBranchName': 'master'}} }%%
commit id: "m1"
branch feature
checkout feature
commit id: "f1"
checkout master
commit id: "m2"
commit id: "m3" type: HIGHLIGHT
commit id: "[master] m4"
checkout feature
commit id: "f3"
commit id: "[HEAD → feature] m3a" type: HIGHLIGHT
Note how it makes the changes done in m3 available (from now on) in the feature branch, with minimal changes to the revision graph. Also note that the new commit m3a contains the same changes as m3, but it will be a different Git object with a different SHA value.
Cherry-picking is another Git operation that can result in conflicts i.e., if the changes in the cherry-picked commit conflict with the changes in the receiving branch.
Pushing Branches to a Remote
Local branches can be replicated in a remote.
Pushing a copy of local branches to the corresponding remote repo makes those branches available remotely.
In a previous lesson, we saw how to push the default branch to a remote repository and have Git set up tracking between the local and remote branches using a remote-tracking reference. Pushing any other local branch to a remote works the same way as pushing the default branch — you simply specify the target branch instead of the default branch. Pushing any new commits in any local branch to a corresponding remote branch is done similarly as well.
gitGraph BT:
%%{init: { 'theme': 'default', 'gitGraph': {'mainBranchName': 'master'}} }%%
commit id: "m1"
branch bug-fix
checkout master
commit id: "[origin/master][HEAD → master] m2"
checkout bug-fix
commit id: "[bug-fix] b1"
checkout master
[bug-fix branch does not exist in the remote origin]
→
gitGraph BT:
%%{init: { 'theme': 'default', 'gitGraph': {'mainBranchName': 'master'}} }%%
commit id: "m1"
branch bug-fix
checkout master
commit id: "[origin/master][HEAD → master] m2"
checkout bug-fix
commit id: "[origin/bug-fix][bug-fix] b1"
checkout master
[after pushing bug-fix branch to origin,
and setting up a remote-tracking branch]
Preparation Fork the samplerepo-company to your GitHub account. When doing so, un-tick the Copy the master branch only option.
After forking, go to the fork and ensure both branches (master, and track-sales) are in there.
Clone the fork to your computer. It should look something like this:
gitGraph BT:
%%{init: { 'theme': 'default', 'gitGraph': {'mainBranchName': 'master'}} }%%
commit id: "m1"
commit id: "m2"
branch track-sales
checkout track-sales
commit id: "[origin/track-sales] s1"
checkout master
commit id: "[origin/master][origin/HEAD][HEAD → master] m3"
The origin/HEAD remote-tracking ref indicates where the HEAD ref is in the remote origin.
1 Create a new branch called hiring, and add a commit to that branch. The commit can contain any changes you want.
Here are the commands you can run in the terminal to do this step in one shot:
git switch -c hiring
echo "Receptionist: Pam" >> employees.txt
git commit -am "Add Pam to employees.txt"
gitGraph BT:
%%{init: { 'theme': 'default', 'gitGraph': {'mainBranchName': 'master'}} }%%
commit id: "m1"
commit id: "m2"
branch track-sales
checkout track-sales
commit id: "[origin/track-sales] s1"
checkout master
commit id: "[origin/master][origin/HEAD][master] m3"
branch hiring
checkout hiring
commit id: "[HEAD → hiring] h1"
The resulting revision graph should look like the one above.
2 Push the hiring branch to the remote.
You can use the usual git push <remote> -u <branch> command to push the branch to the remote, and set up a remote-tracking branch at the same time.
git push origin -u hiring
3 Verify that the branch has been pushed to the remote by visiting the fork on GitHub, and looking for the origin/hiring remote-tracking ref in the local repo.
done!
Pulling Branches from a Remote
Branches in a remote can be replicated in the local repo, and maintained in sync with each other.
Sometimes we need to create a local copy of a branch from a remote repository, make further changes to it, and keep it synchronised with the remote branch. Let's explore how to handle this in a few common use cases:
Use case 1: Working with branches that already existed in the remote repo when you cloned it to your computer.
When you clone a repository,
- Git checks out the default branch. You can start working on this branch immediately. This branch is tracking the default branch in the remote, which means you can easily synchronise changes in this branch with the remote by pulling and pushing.
- Git also fetches all the other branches from the remote. These other branches are not immediately available as local branches, but they are visible as remote-tracking branches.
You can think of remote-tracking branches as read-only references to the state of those branches in the remote repository at the time of cloning. They allow you to see what work has been done on those branches without yet making local copies of them.
To work on one of these branches, you can create a new local branch based on the remote-tracking branch. Once you do this, your local branch will usually be configured to track the corresponding branch on the remote, so you can easily synchronise your work later.
Preparation Use the same samplerepo-company repo you used in Lesson T8L1. Pushing Branches to a Remote. Fork and clone it if you haven't done that already.
1 Verify that the remote-tracking branch origin/track-sales exists in the local repo, but there is no local copy of it.
You can use the git branch -a command to list all local and tracking branches.
git branch -a
* hiring
master
remotes/origin/HEAD -> origin/master
remotes/origin/hiring
remotes/origin/master
remotes/origin/track-sales
The * in the output above indicates the currently active branch.
Note how there is no track-sales in the list of branches (i.e., no local branch named track-sales), but there is a remotes/origin/track-sales (i.e., the remote-tracking branch)
Observe how the branch track-sales appear under REMOTES → origin but not under BRANCHES.
2 Create a local copy of the remote branch origin/track-sales.
You can use the git switch -c <branch> <remote-branch> command for this e.g.,
git switch -c track-sales origin/track-sales
Locate the track-sales remote-tracking branch (look under REMOTES → origin), right-click, and choose Checkout....
In the next dialog, choose as follows:
The above command/action does several things:
- Creates a new branch
track-sales. - Sets the new branch to track the remote branch
origin/track-sales, which means the local branch reftrack-saleswill also move to where theorigin/track-salesis. - Switch to the newly-created branch i.e., makes it the current branch.
3 Add a commit to the track-sales branch and push to the remote, to verify that the local branch is tracking the remote branch.
Commands to perform this step in one shot:
echo "5 reams of paper" >> sales.txt
git commit -am "Update sales.txt"
git push origin track-sales
done!
Use case 2: Working with branches that were added to the remote repository after you cloned it e.g., a branch someone else pushed to the remote after you cloned.
Simply fetch to update your local repository with information about the new branch. After that, you can create a local copy of it and work with it just as you did in Use Case 1.
Fetching was covered in Lesson T3L3. Downloading Data Into a Local Repo.
Deleting Branches from a Remote
Often, you'll need to delete a branch in a remote repo after it has served its purpose.
To delete a branch in a remote repository, you simply tell Git to remove the reference to that branch from the remote. This does not delete the branch from your local repository — it only removes it from the remote, so others won’t see it anymore. This is useful for cleaning up clutter in the remote repo e.g., delete old or merged branches that are no longer needed on the remote.
Preparation Fork the samplerepo-books to your GitHub account. When doing so, un-tick the Copy the master branch only option.
After forking, go to the fork and ensure all three branches are in there.
Clone the fork to your computer.
1 Create a local copy of the fantasy branch in your clone.
Follow instructions in Lesson T8L2. Pulling Branches from a Remote.
2 Delete the remote branch fantasy.
You can use the git push <remote> --delete <branch> command to delete a branch in a remote. This is like pushing changes in a branch to a remote, except we request the branch to be deleted instead, by adding the --delete switch.
git push origin --delete fantasy
Locate the remote branch under REMOTES → origin, right-click on the branch name, and choose Delete...:
3 Verify that the branch was deleted from the remote, by going to the fork on GitHub and checking the branches page https://github.com/{YOUR_USERNAME}/samplerepo-books/branches
e.g., https://github.com/johndoe/samplerepo-books/branches.
Also verify that the local copy has not been deleted.
4 Restore the remote branch from the local copy.
Push the local branch to the remote, while enabling the tracking option (as if pushing the branch to the remote for the first time), as covered in Lesson T8L1. Pushing Branches to a Remote.
In the above steps, we first created a local copy of the branch before deleting it in the remote repo. Doing so is optional. You can delete a remote branch without ever checking it out locally — you just need to know its name on the remote. Deleting the remote branch directly without creating a local copy is recommended if you simply want to clean up a remote branch you no longer need.
done!
Renaming Branches in a Remote
Occasionally, you might need to rename a branch in a remote repo.
Git does not have a way to rename remote branches in place. Instead, you create a new branch with the desired name and delete the old one. This involves renaming your local branch to the new name, pushing it to the remote (which effectively creates a new remote branch), and then removing the old branch from the remote. This ensures the remote reflects the updated name while preserving the commit history and any work already done on the branch.
While Git cannot rename a remote branch in place, GitHub allows you to rename a branch in a remote repo. If you use this approach, the local repo still needs to be updated to reflect the change.
Preparation You can use the fork and the clone of the samplerepo-books that you created in Lesson T8L3. Deleting Branches from a Remote.
Target Rename the branch fantasy in the remote (i.e., your fork) to fantasy-books.
Steps
- Ensure you are in the
masterbranch. - Create a local copy of the remote-tracking branch
origin/fantasy. - Rename the local copy of the branch to
fantasy-books. - Push the renamed local branch to the remote, while setting up tracking for the branch as well.
- Delete the remote branch.
git switch master # ensure you are on the master branch
git switch -c fantasy origin/fantasy # create a local copy, tracking the remote branch
git branch -m fantasy fantasy-books # rename local branch
git push -u origin fantasy-books # push the new branch to remote, and set it to track
git push origin --delete fantasy # delete the old branch
You can run the git log --oneline --decorate --graph --all to check the revision graph after each step. The final outcome should be something like the below:
* 355915c (HEAD -> fantasy-books, origin/fantasy-books) Add fantasy.txt
| * 027b2b0 (origin/master, origin/HEAD, master) Merge branch textbooks
|/|
| * a6ebaec (origin/textbooks) Add textbooks.txt
|/
* d462638 Add horror.txt
Perform the above steps (each step was covered in a previous lesson).
done!
Creating PRs
A pull request (PR for short) is a mechanism for contributing code to a remote repo i.e., "I'm requesting you to pull my proposed changes to your repo". It's feature provided by RCS platforms such as GitHub. For this to work, the two repos must have a shared history. The most common case is sending PRs from a fork to its repo.
Suppose you want to propose some changes to a GitHub repo (e.g., samplerepo-pr-practice) as a pull request (PR).
samplerepo-pr-practice is an unmonitored repo you can use to practice working with PRs. Feel free to send PRs to it.
Given below is a scenario you can try in order to learn how to create PRs:
1. Fork the repo onto your GitHub account.
2. Clone it onto your computer.
3. Commit your changes e.g., add a new file with some contents and commit it.
- Option A - Commit changes to the
masterbranch - Option B - Commit to a new branch e.g., create a branch named
add-intro(remember to switch to themasterbranch before creating a new branch) and add your commit to it.
4. Push the branch you updated (i.e., master branch or the new branch) to your fork, as explained here.
5. Initiate the PR creation:
Go to your fork.
Click on the Pull requests tab followed by the New pull request button. This will bring you to the
Compare changespage.Set the appropriate target repo and the branch that should receive your PR, using the
base repositoryandbasedropdowns. e.g.,
base repository: se-edu/samplerepo-pr-practice base: masterNormally, the default value shown in the dropdown is what you want but in case your fork has , the default may not be what you want.
Indicate which repo:branch contains your proposed code, using the
head repositoryandcomparedropdowns. e.g.,
head repository: myrepo/samplerepo-pr-practice compare: master
6. Verify the proposed code: Verify that the diff view in the page shows the exact change you intend to propose. If it doesn't, as necessary.
7. Submit the PR:
Click the Create pull request button.
Fill in the PR name and description e.g.,
Name:Add an introduction to the README.md
Description:Add some paragraph to the README.md to explain ... Also add a heading ...If you want to indicate that the PR you are about to create is 'still work in progress, not yet ready', click on the dropdown arrow in the Create pull request button and choose
Create draft pull requestoption.Click the Create pull request button to create the PR.
Go to the receiving repo to verify that your PR appears there in the
Pull requeststab.
The next step of the PR lifecycle is the PR review. The members of the repo that received your PR can now review your proposed changes.
- If they like the changes, they can merge the changes to their repo, which also closes the PR automatically.
- If they don't like it at all, they can simply close the PR too i.e., they reject your proposed change.
- In most cases, they will add comments to the PR to suggest further changes. When that happens, GitHub will notify you.
You can update the PR along the way too. Suppose PR reviewers suggested a certain improvement to your proposed code. To update your PR as per the suggestion, you can simply modify the code in your local repo, commit the updated code to the same branch as before, and push to your fork as you did earlier. The PR will auto-update accordingly.
Sending PRs using the master branch is less common than sending PRs using separate branches. For example, suppose you wanted to propose two bug fixes that are not related to each other. In that case, it is more appropriate to send two separate PRs so that each fix can be reviewed, refined, and merged independently. But if you send PRs using the master branch only, both fixes (and any other change you do in the master branch) will appear in the PRs you create from it.
To create another PR while the current PR is still under review, create a new branch (remember to switch back to the master branch first), add your new proposed change in that branch, and create a new PR following the steps given above.
It is possible to create PRs within the same repo e.g., you can create a PR from branch feature-x to the master branch, within the same repo. Doing so will allow the code to be reviewed by other developers (using PR review mechanism) before it is merged.
Problem: merge conflicts in ongoing PRs, indicated by the message This branch has conflicts that must be resolved. That means the upstream repo's master branch has been updated in a way that the PR code conflicts with that master branch. Here is the standard way to fix this problem:
- Pull the
masterbranch from the upstream repo to your local repo.git checkout master git pull upstream master - In the local repo, attempt to merge the
masterbranch (that you updated in the previous step) onto the PR branch, in order to bring over the new code in themasterbranch to your PR branch.git checkout pr-branch # assuming pr-branch is the name of branch in the PR git merge master - The merge you are attempting will run into a merge conflict, due to the aforementioned conflicting code in the
masterbranch. Resolve the conflict manually (this topic is covered elsewhere), and complete the merge. - Push the PR branch to your fork. As the updated code in that branch no longer is conflicting with the
masterbranch, the merge conflict alert in the PR will go away automatically.
Reviewing PRs
The PR review stage is a dialog between the PR author and members of the repo that received the PR, in order to refine and eventually merge the PR.
Given below are some steps you can follow when reviewing a PR.
1. Locate the PR:
- Go to the GitHub page of the repo.
- Click on the Pull requests tab.
- Click on the PR you want to review.
2. Read the PR description. It might contain information relevant to reviewing the PR.
3. Click on the Files changed tab to see the diff view.
You can use the following setting to try the two different views available and pick the one you like.
4. Add review comments:
- Hover over the line you want to comment on and click on the icon that appears on the left margin. That should create a text box for you to enter your comment.
- To give a comment related to multiple lines, click-and-drag the icon. The result will look like this:
- To give a comment related to multiple lines, click-and-drag the icon. The result will look like this:
- Enter your comment.
- This page @SE-EDU/guides has some best practices PR reviewers can follow.
- To suggest an in-line code change, click on this icon:
After that, you can proceed to edit thesuggestioncode block generated by GitHub (as seen in the screenshot above).
The comment will look like this to the viewers:
- After typing in the comment, click on the Start a review button (not the Add single comment button. This way, your comment is saved but not visible to others yet. It will be visible to others only when you have finished the entire review.
- Repeat the above steps to add more comments.
5. Submit the review:
- When there are no more comments to add, click on the Review changes button (on the top right of the diff page).
- Type in an overall comment about the PR, if any. e.g.,
Overall, I found your code easy to read for the most part except a few places where the nesting was too deep. I noted a few minor coding standard violations too. Some of the classes are getting quite long. Consider splitting into smaller classes if that makes sense.LGTMis often used in such overall comments, to indicateLooks good to me(orLooks good to merge).
nit(as in nit-picking) is another such term, used to indicate minor flaws e.g.,LGTM. Just a few nits to fix.. - Choose
Approve,Comment, orRequest changesoption as appropriate and click on the Submit review button.
Merging PRs
Let's look at the steps involved in merging a PR, assuming the PR has been reviewed, refined, and approved for merging already.
Preparation: If you would like to try merging a PR yourself, you can create a dummy PR in the following manner.
- Fork any repo (e.g., samplerepo-pr-practice).
- Clone in to your computer.
- Create a new branch e.g., (
feature1) and add some commits to it. - Push the new branch to the fork.
- Create a PR from that branch to the
masterbranch in your fork. Yes, it is possible to create a PR within the same repo.
1. Locate the PR to be merged in your repo's GitHub page.
2. Click on the Conversation tab and scroll to the bottom. You'll see a panel containing the PR status summary.
3. If the PR is not merge-able in the current state, the Merge pull request will not be green. Here are the possible reasons and remedies:
- Problem: The PR code is out-of-date, indicated by the message This branch is out-of-date with the base branch. That means the repo's
masterbranch has been updated since the PR code was last updated.- If the PR author has allowed you to update the PR and you have sufficient permissions, GitHub will allow you to update the PR simply by clicking the Update branch on the right side of the 'out-of-date' error message. If that option is not available, post a message in the PR requesting the PR author to update the PR.
- Problem: There are merge conflicts, indicated by the message This branch has conflicts that must be resolved. That means the repo's
masterbranch has been updated since the PR code was last updated, in a way that the PR code conflicts with the currentmasterbranch. Those conflicts must be resolved before the PR can be merged.- If the conflicts are simple, GitHub might allow you to resolve them using the Web interface.
- If that option is not available, post a message in the PR requesting the PR author to update the PR.
4. Merge the PR by clicking on the Merge pull request button, followed by the Confirm merge button. You should see a Pull request successfully merged and closed message after the PR is merged.
- You can choose between three merging options by clicking on the down-arrow in the Merge pull request button. If you are new to Git and GitHub, the
Create merge commitoption is recommended.
Next, sync your local repos (and forks). Merging a PR simply merges the code in the upstream remote repository in which it was merged. The PR author (and other members of the repo) needs to pull the merged code from the upstream repo to their local repos and push the new code to their respective forks to sync the fork with the upstream repo.
DRCS vs CRCS
RCS can be done in two ways: the centralized way and the distributed way.
Centralized RCS (CRCS for short) uses a central remote repo that is shared by the team. Team members interact directly with this central repository. Older RCS tools such as CVS and SVN support only this model.
The centralized RCS approach without any local repos (e.g., CVS, SVN)
Distributed RCS (DRCS for short, also known as Decentralized RCS) allows multiple remote/local repos working together. The workflow can vary from team to team. For example, every team member can have his/her own remote repository in addition to their own local repository, as shown in the diagram below. Git and Mercurial are some prominent RCS tools that support the distributed approach.
The decentralized RCS approach
Forking Workflow
In the forking workflow, the 'official' version of the software is kept in a remote repo designated as the 'main repo'. All team members fork the main repo and create pull requests from their fork to the main repo.
To illustrate how the workflow goes, let’s assume Jean wants to fix a bug in the code. Here are the steps:
- Jean creates a separate branch in her local repo and fixes the bug in that branch.
Common mistake: Doing the proposed changes in themasterbranch -- if Jean does that, she will not be able to have more than one PR open at any time because any changes to themasterbranch will be reflected in all open PRs. - Jean pushes the branch to her fork.
- Jean creates a pull request from that branch in her fork to the main repo.
- Other members review Jean’s pull request.
- If reviewers suggested any changes, Jean updates the PR accordingly.
- When reviewers are satisfied with the PR, one of the members (usually the team lead or a designated 'maintainer' of the main repo) merges the PR, which brings Jean’s code to the main repo.
- Other members, realizing there is new code in the upstream repo, sync their forks with the new upstream repo (i.e., the main repo). This is done by pulling the new code to their own local repo and pushing the updated code to their own fork. If there are unmerged branches in the local repo, they can be updated too e.g., by merging the new
masterbranch to each of them.
Possible mistake: Creating another 'reverse' PR from the team repo to the team member's fork to sync the member's fork with the merged code. PRs are meant to go from downstream repos to upstream repos, not in the other direction.
One main benefit of this workflow is that it does not require most contributors to have write permissions to the main repository. Only those who are merging PRs need write permissions. The main drawback of this workflow is the extra overhead of sending everything through forks.
You can follow the steps in the simulation of a forking workflow given below to learn how to follow such a workflow.
This activity is best done as a team.
Step 1. One member: set up the team org and the team repo.
Create a GitHub organization for your team, if you don't have one already. The org name is up to you. We'll refer to this organization as team org from now on.
Add a team called
developersto your team org.Add team members to the
developersteam.Fork se-edu/samplerepo-workflow-practice to your team org. We'll refer to this as the team repo.
Add the forked repo to the
developersteam. Give write access.
Step 2. Each team member: create PRs via own fork.
Fork that repo from your team org to your own GitHub account.
Create a branch named
add-{your name}-info(e.g.add-johnTan-info) in the local repo.Add a file
yourName.mdinto themembersdirectory (e.g.,members/johnTan.md) containing some info about you into that branch.Push that branch to your fork.
Create a PR from that branch to the
masterbranch of the team repo.
Step 3. For each PR: review, update, and merge.
[A team member (not the PR author)] Review the PR by adding comments (can be just dummy comments).
[PR author] Update the PR by pushing more commits to it, to simulate updating the PR based on review comments.
[Another team member] Approve and merge the PR using the GitHub interface.
[All members] Sync your local repo (and your fork) with upstream repo. In this case, your upstream repo is the repo in your team org.
- The basic mechanism for this has two steps (which you can do using Git CLI or any Git GUI):
(1) First, pull from the upstream repo -- this will update your clone with the latest code from the upstream repo.
If there are any unmerged branches in your local repo, you can update them too e.g., you can merge the newmasterbranch to each of them.
(2) Then, push the updated branches to your fork. This will also update any PRs from your fork to the upstream repo. - Some alternatives mechanisms to achieve the same can be found in this GitHub help page.
If you are new to Git, we recommend that you use the above two-step mechanism instead, so that you get a better view of what's actually happening behind the scene.
- The basic mechanism for this has two steps (which you can do using Git CLI or any Git GUI):
Step 4. Create conflicting PRs.
[One member]: Update README: In the
masterbranch, remove John Doe and Jane Doe from theREADME.md, commit, and push to the main repo.[Each team member] Create a PR to add yourself under the
Team Memberssection in theREADME.md. Use a new branch for the PR e.g.,add-johnTan-name.
Step 5. Merge conflicting PRs one at a time. Before merging a PR, you’ll have to resolve conflicts.
[Optional] A member can inform the PR author (by posting a comment) that there is a conflict in the PR.
[PR author] Resolve the conflict locally:
- Pull the
masterbranch from the repo in your team org. - Merge the pulled
masterbranch to your PR branch. - Resolve the merge conflict that crops up during the merge.
- Push the updated PR branch to your fork.
- Pull the
[Another member or the PR author]: Merge the de-conflicted PR: When GitHub does not indicate a conflict anymore, you can go ahead and merge the PR.
Feature Branch flow
Feature branch workflow is similar to forking workflow except there are no forks. Everyone is pushing/pulling from the same remote repo. The phrase feature branch is used because each new feature (or bug fix, or any other modification) is done in a separate branch and merged to the master branch when ready. Pull requests can still be created within the central repository, from the feature branch to the main branch.
As this workflow require all team members to have write access to the repository,
- it is better to protect the main branch using some mechanism, to reduce the risk of accidental undesirable changes to it.
- it is not suitable for situations where the code contributors are not 'trusted' enough to be given write permission.
