Snapshotting

The Lifecycle of Git files

Understanding the lifecycle of the status of files in Git is crucial for effectively managing changes in your repository. The lifecycle consists of several stages that files can transition through as you work with Git. These stages are reflected in the output of git status and represent the state of files in relation to the repository.

Transition between Stages: Files transition between these stages as you work with Git commands like git add, git commit, and others.
Workflow: The typical workflow involves modifying files in the working directory, staging the changes with git add, and committing the changes with git commit.
Visibility: Use git status to view the current status of files in your repository and identify which stage they are in.

Stage	Description	Representation	Action
Untracked	Files that exist in your working directory but are not yet tracked by Git. These files have not been added to the repository.	Shown in red in the output of `git status`.	Use `git add <file>` to stage untracked files for the next commit.
Tracked (or Modified)	Files that have been modified in the working directory after being staged or committed.	Shown in red in the output of `git status`.	Use `git add <file>` to stage the modified files, then commit the changes with `git commit`.
Deleted	Files that have been deleted from the working directory after being staged or committed.	Shown in red in the output of `git status`.	Use `git add <file>` to stage the deletion, then commit the changes with `git commit`.
Renamed or Moved	Files that have been renamed or moved in the working directory after being staged or committed.	Shown as both deleted and untracked files in the output of `git status`.	Use `git add <file>` to stage the rename or move, then commit the changes with `git commit`.
Staged (or Indexed)	Files that have been added to the staging area. These changes are ready to be included in the next commit.	Shown in green in the output of `git status`.	Use `git commit` to create a commit containing the staged changes.
Committed (or Unmodified)	Files are stored securely in the Git repository. They are unchanged and represent a specific snapshot in the project's history. Files that have not been modified since the last commit.	Typically not displayed in the output of `git status` unless using certain flags.	No action required unless you intend to stage or modify the files.

The working tree is a single checkout of one version of the project. These files are pulled out of the compressed database in the Git directory and placed on disk for you to use or modify.

The staging area is a file, generally contained in your Git directory, that stores information about what will go into your next commit.

The Git directory is where Git stores the metadata and object database for your project. This is the most important part of Git, and it is what is copied when you clone a repository from another computer.

Working with snapshots and the Git staging area

For the basic workflow of staging content and committing it to your history, there are only a few basic commands.

Command	Example
git status	`git status`
git add	`git add myfile.txt`
git diff	`git diff`
git commit	`git commit -m "Add new feature"`

Snapshooting

The difference between Git's snapshooting and delta-based version control systems lies in how git store and track changes within a repository.

Git's Snapshooting Git is a distributed version control system (DVCS) that operates based on snapshots of the entire repository at each point in time. When a commit is made in Git, the entire state of the project at that moment is captured as a snapshot, including all files and their contents. Each commit creates a new snapshot, and Git tracks the changes between snapshots rather than individual file changes.
- Advantages:
  - Offers robustness and data integrity since each commit is self-contained and immutable.
  - Allows for efficient branching and merging operations since entire snapshots are managed independently.
  - Facilitates offline work and collaboration by enabling users to work with their local repositories and synchronize changes later.
Delta-based Version Control (CVS, Subversion, Perforce) Delta-based version control systems track changes by storing the differences (or deltas) between successive versions of a file or repository. Instead of storing complete snapshots of files or repositories, delta-based systems store the changes (insertions, deletions, modifications) made to files over time. Each new version is represented as a set of changes applied to the previous version.
- Advantages:
  - Can be more space-efficient for repositories with large binary files or frequent, small changes since only the differences are stored.
  - Allows for quicker network operations when transmitting changes since only the deltas need to be transferred.
  - May provide more granular control over individual file versions, especially for large files with small modifications.

Key Differences

Granularity: Git's snapshot-based approach captures the entire state of the repository at each commit, offering a holistic view of the project's history. Delta-based systems track changes at a finer granularity, focusing on individual file modifications.
Efficiency: Git's approach can be less space-efficient for repositories with frequent changes or large binary files since it stores complete snapshots. Delta-based systems may offer better efficiency in such cases by only storing the differences between versions.
Complexity: Managing deltas and applying them to reconstruct files can introduce complexity and potential performance overhead in delta-based systems. Git's snapshot-based approach simplifies operations like branching and merging since each commit represents a complete, self-contained snapshot.
Network Operations: Git's approach may lead to larger repository sizes due to storing complete snapshots, but it offers efficient network operations since it transfers complete commits during push and pull operations. Delta-based systems may have smaller repository sizes but may require more processing to reconstruct files during network operations.

Overall, Git's snapshot-based approach provides robustness, simplicity, and efficiency for version control, making it a popular choice for software development projects of all sizes. Delta-based version control systems offer advantages in specific use cases where space efficiency or granular control over individual file versions is paramount.

Git doesn’t store data as a series of changesets or differences, but instead as a series of snapshots. When you make a commit, Git stores a commit object that contains a pointer to the snapshot of the content you staged. This object also contains the author’s name and email address, the message that you typed, and pointers to the commit or commits that directly came before this commit (its parent or parents): zero parents for the initial commit, one parent for a normal commit, and multiple parents for a commit that results from a merge of two or more branches.

Let’s assume that you have a directory containing three files, and you stage them all and commit. Staging the files computes a checksum for each one (the SHA-1 hash), stores that version of the file in the Git repository (Git refers to them as blobs), and adds that checksum to the staging area:

$ git add .
$ git commit -m "initial commit"

When you create the commit by running git commit, Git checksums each subdirectory (in this case, just the root project directory) and stores them as a tree object in the Git repository. Git then creates a commit object that has the metadata and a pointer to the root project tree so it can re-create that snapshot when needed.

Your Git repository now contains five objects: three blobs (each representing the contents of one of the three files), one tree that lists the contents of the directory and specifies which file names are stored as which blobs, and one commit with the pointer to that root tree and all the commit metadata.

If you make some changes and commit again, the next commit stores a pointer to the commit that came immediately before it.

A branch in Git is simply a lightweight movable pointer to one commit. The default branch name in Git is master. As you start making commits, you’re given a master branch that points to the last commit you made. Every time you commit, the master branch pointer moves forward automatically. The pointer HEAD tells you where you are in the repository. When you switch branches, HEAD moves to the tip of the new branch.

Fore more details on branches and their history, see Git Branching.

git status

Shows the status of your working directory, indicating which files are modified, untracked, and staged for the next commit.

git status

This command provides a detailed summary of the changes in your working directory. Files listed under "Changes not staged for commit" are modified but not staged, and files under "Changes to be committed" are staged for the next commit.

git add

Adds changes in the specified file to the staging area, preparing them for the next commit.

# Create or modify a file
echo "Hello, Git!" > myfile.txt

# Stage the changes
git add myfile.txt

This sequence of commands creates or modifies myfile.txt and stages the changes for the next commit.

Use git add . to stage all changes in the working directory.

git diff

The git diff command is used when you want to see differences between any two trees. This could be the difference between your working environment and your staging area (git diff by itself), between your staging area and your last commit (git diff --staged), or between two commits (git diff master branchB).

# Make changes to myfile.txt
echo "Updated content" >> myfile.txt

# View the differences
git diff

This command displays the unstaged changes in myfile.txt since the last commit.

git diff --staged

Shows the differences between the staging area and the last commit (changes that are staged but not yet committed).

# Stage the changes in myfile.txt
git add myfile.txt

# View the differences in the staging area
git diff --staged

This command displays the changes that are currently staged for the next commit.

git difftool

The git difftool command simply launches an external tool to show you the difference between two trees in case you want to use something other than the built in git diff command.

P4Merge

For example, P4Merge is a visual diff and merge tool provided by Perforce. It offers a user-friendly interface for comparing and merging files and directories, making it a popular choice among developers and teams working with version control systems like Git.

Here are some key features of P4Merge:

Visual Diffing: P4Merge provides a graphical representation of the differences between files, allowing users to easily identify changes with color-coded highlights and line-by-line comparisons.
Three-Way Merge: It supports three-way merging, which is essential for resolving conflicts that arise during collaborative development. P4Merge intelligently combines changes from two different branches or versions with a common ancestor, helping users reconcile differences efficiently.
Image Comparison: In addition to text-based files, P4Merge can compare and merge images, making it suitable for projects involving graphical assets or multimedia content.
Folder Diff: P4Merge can compare entire directories, displaying side-by-side comparisons of files within the folders. This feature is useful for understanding the overall differences between two directory structures and identifying missing or added files.
Customizable Settings: Users can customize various aspects of P4Merge, such as colors, fonts, and keyboard shortcuts, to tailor the tool to their preferences and workflow.

VS Code

The git difftool command can be used with VS Code's diff and merge capabilities. The Diff editor has a separate gutter in the middle, which enables you to Stage or Revert changes code blocks. If you select a block of text, you can revert or stage the changes that are included in the selection.

You can diff any two files by first right clicking on a file in the Explorer or OPEN EDITORS list and selecting Select for Compare and then right-click on the second file to compare with and select Compare with 'file_name_you_chose'. Alternatively from the keyboard hit Ctrl+Shift+P and select File: Compare Active File With and you will be presented with a list of recent files.

There is an Accessible Diff Viewer in the Diff editor that presents changes in a unified patch format. You can navigate between changes with Go to Next Difference (F7) and Go to Previous Difference (Shift+F7). Lines can be navigated with arrow keys and pressing Enter will jump back in the Diff editor and the selected line. This experience is especially helpful for screen reader users.

git commit

The git commit command in Git is a powerful tool for creating and committing changes. It has several options, each affecting different parts of the Git repository.

git commit

When you run git commit without any arguments, it opens your editor of choice to create a commit message.

You can see that the default commit message contains the latest output of the git status command commented out and one empty line on top. You can remove these comments and type your commit message, or you can leave them there to help you remember what you’re committing. When you exit the editor, Git creates your commit with that commit message (with the comments and diff stripped out).

Alternatively, you can type your commit message inline with the commit command by specifying it after a -m flag. This commits the changes that are staged in the snapshot, with a descriptive commit message.

# Commit the changes with a message
git commit -m "Add new feature"

This command creates a new commit with the staged changes and the specified commit message.

Combine git add and git commit in one step using git commit -am "[descriptive message]" to stage and commit changes.

For an even more explicit reminder of what you’ve modified, you can pass the -v option to git commit. Doing so also puts the diff of your change in the editor so you can see exactly what changes you’re committing.

Committing Specific Files

# Stage changes in specific files
git add file1.txt file2.txt

# Commit the changes with a message
git commit -m "Fix issues in file1.txt and file2.txt"

Here, only specific files (file1.txt and file2.txt) are staged and then committed with a message describing the fix.

Amending the Last Commit

# Make additional changes
echo "Additional content" >> myfile.txt

# Amend the last commit with new changes
git add myfile.txt
git commit --amend -m "Update myfile.txt with additional content"

This example adds new content to myfile.txt and then amends the last commit with the additional changes.

Interactive Commit

# Interactively stage changes
git add -i

# Follow the interactive prompts to stage changes and commit

Running git add -i opens an interactive mode where you can choose which changes to stage, unstage, or commit. This provides a more granular control over the commit process.

Committing with Multiple Messages

# Stage changes in files
git add file1.txt file2.txt

# Commit changes with multiple messages
git commit -m "Fix issues in file1.txt" -m "Add new feature in file2.txt"

You can use the -m option multiple times to provide multiple commit messages. Each -m adds a new paragraph to the commit message.

Committing with a Future Date

# Stage changes
git add .

# Commit with a future date
git commit --date="2023-01-01T12:00:00" -m "Commit with a future date"

This example commits changes with a specified future date using the --date option.

Committing Only Tracked Changes

# Stage changes in tracked files
git add -u

# Commit only tracked changes
git commit -m "Commit tracked changes"

The git add -u command stages only tracked files, and then a commit is made with a message indicating the commit of tracked changes.