Commit

Commits are snapshots for each phase of the repository, which allowing you to rollback to any instance of that commit. Git is able to compare base on two commits(usually the current and last commit), which is the diff.

• Each commit stores a snapshot of current repository with a unique hash to be identified.
• A commit can have one or more parent from which it was derived.
• Each snapshot file or folder has a unique hash, and can inspect their content in some way.
  • the folder snapshot is called tree
  • the file snapshot is blob
  • a commit always shows the snapshot of root of the repository as entry of a commit

NOTE

Revision is another synonym of Commit in git.

IMPORTANT

Commit is the atomic identifier of how git manage the versions, it's the true and real concept while branch and merge are just fake concepts upon it.

Commit Identifier

Each commit is identified by the uniquely generated SHA hash, you can reference the commit using the full hash or the leading 7 digit hash for short. Each snapshot instance of that particular commit could be found on .git/objects/<first-2digits-of-hash>/<restof-hash>

NOTE

Those identifier of commits are called refs in git, you could find their representations in .git/refs

1. commit hash: the most explicit identification of a commit.
2. branch name: points to latest commit of the branch.
3. tag name: points to the commit of that tag.
4. HEAD or @: points to the current commit you're at.
   • special identifiers like HEAD is independent from any branch, its a primitive and dynamic pointer for your current worktree.
   • for some other special identifier names, see documentation.
5. - or @{-1}: previous commit you're at.
   • - is not a valid identifier in some scenarios, but you can use it in like git checkout - or git merge -.
6. commit expression: see next section.

Commit Expression

Commit expression is special syntax to reference commits backward from a given point. Three special symbols for traversing back in git:

1. @: reference backward through reflog, you can see such symbol like HEAD@{1} in git reflog output.
   • @{-<n>}: go to nth previously checked out commit by git checkout
   • <commit_identifier>@{<n>}: go to previous nth commit
     • @{<n>} is equivalent to <current_branch>@{<n>}, this is only valid when you're on a branch(not detached).
2. ~: trace back to previous commits through the first parent only.
   • <identifier>~3: points to previous 3 commit from current branch.
3. ^: trace back to previous commits through a specified parent by its index.
   • <identifier>^2: points to previous commit through second parent of <identifier>.
   • <identifier>^101: if you're wild enough to merge from 100 branches, you might need to point to previous commit through the 101-th parent.

TIP

~1 and ^1 are always equivalent since they would pick the same first parent of the commit.

NOTE

See git help revisions for more details

NOTE

When saying HEAD we're referring to the head of worktree which might walk across different branches as you checkout. When a branch name is explicitly specified or elided(representing the current branch), the reflog can differ from HEAD's since it's branch specific

Identifier Expansion

You can expand any commit identifier to full commit hash using git rev-parse <identifier>

$ git rev-parse @~1
42804017200559f28d12757e729f95dbd18f4998
$ git rev-parse main
e7bbe687097f56d152c2291190b67d91eed4cd57

Commit Inspection

Commit inspection generally requires git cat-file -p <hash>, see git help cat-file.

A commit can contain elements:

• tree: the structure of current snapshot, including files and folders.
• parent: the parent commit it was derived from, a commit can have multiple parents.
• author: who did the changes.
• committer: who made the commit, may differ from author when rebasing.
• and more...

$ git cat-file -p 5eb5a747cfd9b17603b79d7ab64fc4ecee1751e3

tree e060913d1a04d39082981dc116c5a5cd35ba8e52
parent 02348ec70c2c901e9ecc4be5860330e6dcf911ff
author sharpchen <rui.chen.sharp@gmail.com> 1749697268 +0800
committer sharpchen <rui.chen.sharp@gmail.com> 1749697268 +0800

TIP

It's more general to use git show <commit_identifier> to inspect a commit.

Tree & Blob Inspection

Tree in the inspected commit is a recursive structure and also has a hash that can be inspected by git cat-file -p <hash>. A blob represents a real file, a tree represents a folder as a container. So the tree printed from the commit is actually the root folder of your repository.

$ git cat-file -p e060913d1a04d39082981dc116c5a5cd35ba8e52

100644 blob 397b4a7624e35fa60563a9c03b1213d93f7b6546    .gitignore
100644 blob 67f24d19be001eebf7bcdde488d8ab594bcd969d    PSScriptAnalyzerSettings.psd1
100644 blob 4bd5bb6fcff0b514d0160f3cb8ff01b5de46eeb4    README.md
100644 blob 207846dc4b8f93edd892df9b6058ded48f5355b8    dotfiles.ps1
100755 blob fa9fe71978d17992a3f27eaf78f2619a25a1966d    dotfiles.sh
040000 tree 0199c6476a45a6dadadaf7baf70a136d7f8f217f    dotfiles
100644 blob b6e1531298fed5dc7a019f6c01018b5c1add3c99    flake.lock
100644 blob 92d9549668557adb021f959f8b898ad96ee84efb    flake.nix
100644 blob bb166d071cb8541aac7df35b7c1f5dbf420dd70c    home.nix
100644 blob baa979aff962214ddb5e23502acdb211d8d32064    install.ps1
040000 tree c1aef838a425cd6f209ecc01f28bef2208fd2d26    packages

You can either go deeper to the nested tree or inspect the snapshot content of a blob using the same way.

$ git cat-file -p 67f24d19be001eebf7bcdde488d8ab594bcd969d

@{
    IncludeRules = @('PSAvoidTrailingWhitespace')
}

Checkout Commit