In the world of Git, every commit generates a unique identifier called the commit hash. It’s like an “ID number” for each commit. Although it looks complex, understanding its role is crucial for using Git effectively.

一、What is a Commit Hash?¶

When you run the git commit command to save changes, Git generates a 40-character hexadecimal string as the commit hash. This hash is calculated using a “hashing algorithm” based on the content of the commit (such as modified files, commit message, author, timestamp, etc.).

In simple terms: The commit hash is a unique “fingerprint” that Git generates for each commit. If the content (including modified files, commit message, etc.) remains unchanged, Git will always produce the same commit hash. Any change in content will result in a different commit hash.

二、What Does a Commit Hash Look Like?¶

It typically appears like this (example):
a1b2c3d4e5f67890abcdef1234567890abcdef12
(In reality, it’s 40 hexadecimal characters composed of 0-9 and a-f).

While it seems long, you rarely need to remember the entire hash. Remembering the first 7 characters (e.g., a1b2c3d) is usually sufficient to distinguish different commits.

三、Why Are Commit Hashes Important?¶

Their importance lies in several key aspects:

1. Unique Identification of History & Version Tracking¶

Each commit has a unique commit hash, acting like a “tag” for every version. With the commit hash, you can precisely locate any past commit in Git’s history.

For example, when you run git log to view commit history, each entry shows its commit hash, allowing you to quickly identify which commit introduced a specific change.

2. Core for Version Rollbacks & Branch Management¶

When reverting to a historical version (e.g., fixing a bug in the latest commits), the commit hash serves as a “navigation marker.” You can use git checkout <commit-hash> to switch to that version or git revert <commit-hash> to create a new commit that undoes changes.

During branch merging (e.g., merging dev into master), commit hashes help Git identify the order of commits across branches, preventing conflicts.

3. “Authentication” in Distributed Collaboration¶

In multi-person collaboration, each developer’s commits are pushed to a remote repository (e.g., GitHub, GitLab). Commit hashes ensure clarity: even if two developers modify the same file, their commit hashes differ if the content is unique, avoiding confusion.

4. Immutable “Historical Anchor”¶

A commit hash is generated based on the commit content and cannot be modified once created. If someone tries to alter a past commit, Git detects the content change, generating a new hash while preserving the original hash in the history. This guarantees the authenticity and traceability of version history.

四、How to Use Commit Hashes?¶

• View History: Use git log or git log --oneline (simplified display showing only the first 7 characters) to find the target commit hash.
• Switch Versions: Run git checkout <commit-hash> to temporarily switch to that commit (entering a “detached HEAD” state, where changes won’t affect branches).
• Create Branches: Use git branch <new-branch> <commit-hash> to create a new branch from a historical commit.
• Revert Commits: Use git revert <commit-hash> to create a new commit that undoes changes from the target commit, maintaining continuous history.

五、Summary¶

Commit hashes are the “cornerstone” of Git version control—they act as a key to tracking history, rolling back versions, and managing collaboration. Though they appear as random strings, they are essentially Git’s “unique signature” for each commit.

Key Takeaways:
- A commit hash is a unique 40-character hexadecimal identifier for each commit.
- It is generated via hashing the commit content (files, messages, timestamps, etc.), and the hash remains identical if the content is unchanged.
- Critical for version rollbacks, branch management, and historical tracking.

Now, the next time you see that long string in git log, you’ll know exactly what it represents!