View commit history (git log)

Viewing Commit History (git log)

Git's commit history records all changes to a project from its creation to its current state. By viewing the commit history, you can understand the evolution of the code, identify the source of issues, or revert to specific versions. The git log command is the primary tool for viewing this history, offering various parameters to customize the output format and content.

Basic Usage

The simplest git log command lists all commits in reverse chronological order:

git log

Example output:

commit a1b2c3d4e5f6g7h8i9j0k1l2m3n4o5p6q7r8s9t0
Author: John Doe <john@example.com>
Date:   Mon Oct 2 14:30:22 2023 +0800

    Fix login page styling issues

commit b2c3d4e5f6g7h8i9j0k1l2m3n4o5p6q7r8s9t0u1
Author: Jane Smith <jane@example.com>
Date:   Sun Oct 1 09:15:10 2023 +0800

    Add user authentication middleware

Common Parameters

Limiting Output Quantity

Use the -n parameter to limit the number of displayed commits:

git log -n 3

Single-Line Display

The --oneline parameter compresses each commit into a single line:

git log --oneline

Output:

a1b2c3d Fix login page styling issues
b2c3d4e Add user authentication middleware

Graphical Branch Display

The --graph parameter displays an ASCII graph representing branch and merge history:

git log --graph --oneline

Output:

* a1b2c3d Fix login page styling issues
* b2c3d4e Add user authentication middleware
| * c3d4e5f Update README (feature-branch)
|/
* d4e5f6g Initial commit

Filtering Commits

By Author

The --author parameter filters commits by a specific author:

git log --author="John"

By Time Range

The --since and --until parameters specify a time range:

git log --since="2023-09-01" --until="2023-09-30"

By File Changes

View commits affecting a specific file:

git log -- path/to/file.js

Viewing Change Details

The -p or --patch parameter shows the specific changes introduced by each commit:

git log -p

Example output:

commit a1b2c3d4e5f6g7h8i9j0k1l2m3n4o5p6q7r8s9t0
Author: John Doe <john@example.com>
Date:   Mon Oct 2 14:30:22 2023 +0800

    Fix login page styling issues

diff --git a/src/components/Login.js b/src/components/Login.js
index abc1234..def5678 100644
--- a/src/components/Login.js
+++ b/src/components/Login.js
@@ -15,7 +15,7 @@ function Login() {
       <form>
         <div className="form-group">
-          <label style={{color: 'red'}}>Username</label>
+          <label className="text-primary">Username</label>
          <input type="text" />
        </div>

Customizing Output Format

The --pretty parameter allows customizing the display format of commit information:

git log --pretty=format:"%h - %an, %ar : %s"

Output:

a1b2c3d - John Doe, 2 hours ago : Fix login page styling issues
b2c3d4e - Jane Smith, 3 days ago : Add user authentication middleware

Common format placeholders:

• %h: Abbreviated commit hash
• %an: Author name
• %ar: Relative time (e.g., "2 hours ago")
• %s: Commit message
• %ad: Author date
• %d: Reference names (e.g., branches, tags)

Viewing File Change Statistics

The --stat parameter shows statistics on files and line numbers changed in each commit:

git log --stat

Example output:

commit a1b2c3d4e5f6g7h8i9j0k1l2m3n4o5p6q7r8s9t0
Author: John Doe <john@example.com>
Date:   Mon Oct 2 14:30:22 2023 +0800

    Fix login page styling issues

 src/components/Login.js | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Advanced Usage

Finding Commits Containing Specific Text

The -S parameter searches for commits that add or remove specific text:

git log -S "useState"

Viewing Differences Between Two Commits

Compare the history between two commits:

git log commit1..commit2

Viewing Branch Differences

View commits in the feature branch but not in the main branch:

git log main..feature

Integration with GUI Tools

Many Git GUI tools provide more intuitive ways to view history. For example, in VS Code:

1. Open the Source Control view (Ctrl+Shift+G)
2. Click the "..." menu in the top-right corner
3. Select "View Commit History"

This displays a graphical commit history where you can click each commit for details.

Practical Examples

Suppose we have a React project and want to view the development history of the user authentication feature:

git log --grep="auth" --oneline -p src/components/Auth/

This lists all commits with messages containing "auth" that affect files in the Auth directory and shows the specific changes.

Another example: view all commits from last week that modified API-related files:

git log --since="last week" --name-only -- src/api/

Performance Optimization

For large repositories, git log may become slow. Here are optimization methods:

1. Limit the scope:

git log -n 50

2. Disable paging:

git --no-pager log

3. Use a simple format:

git log --oneline

Combining with Other Commands

git log is often used with other commands. For example, after identifying the commit that introduced a bug, you can use git show to view details:

git show a1b2c3d

Or use git diff to compare two commits:

git diff b2c3d4e..a1b2c3d

Using in Scripts

git log output can be used in automation scripts. For example, to generate a changelog:

git log --pretty=format:"- %s (%h)" v1.0.0..HEAD > CHANGELOG.md

Or to process commit history in Node.js:

const { execSync } = require('child_process');

function getRecentCommits(n = 5) {
  const output = execSync(`git log -n ${n} --pretty=format:"%h|%an|%s"`).toString();
  return output.split('\n').map(line => {
    const [hash, author, message] = line.split('|');
    return { hash, author, message };
  });
}

console.log(getRecentCommits());