視覚的なGit内部入門

Git内部の働きを視覚的に説明する。
Gitを使う時に内部で何が起きるかを理解することで、日常的な利用を促進するのがこのプレゼンの目的です。

前職で発表したLTの資料です。当時はSVNからGitへの移動を始めたところで、Gitのコマンド一覧だけで苦労していた社員が各コマンドの理解を高めるつもりで発表した。

英語版は https://speakerdeck.com/desseim/visual-introduction-to-git-internals へ。

Transcript

1. Gitを理解しよう Bare repository !? squash !? rebase !? refs !? detached head !? ハッ！？ ファーストフォウウァ アァド！？ HEAD !? Legrand Guillaume 2012/02/22
2. Gitを理解してみよう Bare repository !? refs !? detached head !? ハッ！？ HEAD !? Legrand Guillaume 2012/02/22
3. $ Working directory src_dir src_file.pl print “txet”;
4. $ git init Working directory .git/ objects/ refs/ untracked src_dir src_file.pl print “txet”;
5. $ git add ./src_dir Working directory src_dir src_file.pl print “txet”; .git/ objects/ refs/ Blobs Trees Commits 0f4de32 print “txet”; staged
6. $ git commit -m “First commit” Working directory src_dir src_file.pl print “txet”; .git/ objects/ refs/ Blobs Trees Commits 0f4de32 print “txet”; 5fs342w 0f4de32 scr_file .pl c92a25a e6fae02 User.name 2012-02-22 First commit e6fae02 5fs342w scr_dir HEAD heads/master heads/master c92a25a
7. $ Working directory src_dir src_file.pl print “txet”; .git/ objects/ refs/ Blobs Trees Commits 0f4de32 print “txet”; 5fs342w c92a25a First commit e6fae02 HEAD heads/master heads/master c92a25a
8. $ vim ./src_dir/src_file.pl Working directory src_dir src_file.pl print “text”; .git/ objects/ refs/ Blobs Trees Commits 0f4de32 print “txet”; c92a25a First commit modified 5fs342w e6fae02 HEAD heads/master heads/master c92a25a
9. $ git add ./src_dir Working directory src_dir src_file.pl print “text”; .git/ objects/ refs/ Blobs Trees Commits 0f4de32 print “txet”; c92a25a First commit staged 8e27be7 print “text”; 5fs342w e6fae02 HEAD heads/master heads/master c92a25a
10. $ git reset HEAD ./src_dir #unstage Working directory src_dir src_file.pl print “text”; .git/ objects/ refs/ Blobs Trees Commits 0f4de32 print “txet”; c92a25a First commit 8e27be7 print “text”; 5fs342w e6fae02 modified HEAD heads/master heads/master c92a25a
11. $ git checkout -- ./src_dir Working directory .git/ objects/ refs/ Blobs Trees Commits 0f4de32 print “txet”; c92a25a First commit 8e27be7 print “text”; 5fs342w e6fae02 src_dir src_file.pl print “txet”; HEAD heads/master heads/master c92a25a
12. $ git cat-file -p 8e27be7 print “text”; $ Working directory .git/ objects/ refs/ Blobs Trees Commits 0f4de32 print “txet”; c92a25a First commit 8e27be7 print “text”; 5fs342w e6fae02 src_dir src_file.pl print “txet”; HEAD heads/master heads/master c92a25a
13. $ vim ./src_dir/src_file.pl Working directory src_dir src_file.pl print “text”; .git/ objects/ refs/ Blobs Trees Commits 0f4de32 print “txet”; c92a25a First commit modified 5fs342w e6fae02 8e27be7 print “text”; HEAD heads/master heads/master c92a25a
14. $ git add ./src_dir Working directory src_dir src_file.pl print “text”; .git/ objects/ refs/ Blobs Trees Commits 0f4de32 print “txet”; c92a25a First commit staged 8e27be7 print “text”; 5fs342w e6fae02 HEAD heads/master heads/master c92a25a
15. $ git commit -m “2nd commit” Working directory .git/ objects/ refs/ Blobs Trees Commits 0f4de32 print “txet”; c92a25a First commit 8e27be7 print “text”; 5fs342w e6fae02 136b9d6 2nd commit parent 8e27be7 699022a src_dir src_file.pl print “text”; HEAD heads/master heads/master 136b9d6
16. $ git branch first c92a25a Working directory .git/ objects/ refs/ Blobs Trees Commits 0f4de32 print “txet”; c92a25a First commit 8e27be7 print “text”; 5fs342w e6fae02 136b9d6 2nd commit parent 8e27be7 699022a src_dir src_file.pl print “text”; HEAD heads/master heads/master 136b9d6 heads/first c92a25a
17. $ git checkout first Working directory .git/ objects/ refs/ Blobs Trees Commits 0f4de32 print “txet”; c92a25a First commit 8e27be7 print “text”; 5fs342w e6fae02 136b9d6 2nd commit parent 8e27be7 699022a src_dir src_file.pl print “txet”; HEAD heads/first heads/master 136b9d6 heads/first c92a25a
18. $ git branch -D master Working directory .git/ objects/ refs/ Blobs Trees Commits 0f4de32 print “txet”; c92a25a First commit 8e27be7 print “text”; 5fs342w e6fae02 136b9d6 2nd commit parent 8e27be7 699022a src_dir src_file.pl print “txet”; HEAD heads/first heads/first c92a25a
19. $ git checkout 136b9d6 Working directory .git/ objects/ refs/ Blobs Trees Commits 0f4de32 print “txet”; c92a25a First commit 8e27be7 print “text”; 5fs342w e6fae02 136b9d6 2nd commit parent 8e27be7 699022a src_dir src_file.pl print “text”; HEAD 136b9d6 heads/first c92a25a detached head
20. $ git fetch origin #originにpushしてあった 前提 Working directory .git/ objects/ refs/ Blobs Trees Commits 0f4de32 print “txet”; c92a25a First commit 8e27be7 print “text”; 5fs342w e6fae02 136b9d6 2nd commit parent 8e27be7 699022a src_dir src_file.pl print “text”; heads/first c92a25a remotes/origin /master 136b9d6 HEAD 136b9d6 detached head
21. $ git checkout master Working directory .git/ objects/ refs/ Blobs Trees Commits 0f4de32 print “txet”; c92a25a First commit 8e27be7 print “text”; 5fs342w e6fae02 136b9d6 2nd commit parent 8e27be7 699022a src_dir src_file.pl print “text”; heads/first c92a25a remotes/origin /master 136b9d6 HEAD Heads/master heads/master 136b9d6
22. $ echo 'c92a25a' > ./.git/refs/heads/master #やるな！ @see git-update-ref Working directory .git/ objects/ refs/ Blobs Trees Commits 0f4de32 print “txet”; c92a25a First commit 8e27be7 print “text”; 5fs342w e6fae02 136b9d6 2nd commit parent 8e27be7 699022a src_dir src_file.pl print “text”; heads/first c92a25a remotes/origin /master 136b9d6 HEAD Heads/master heads/master c92a25a
23. まとめ • Gitリポ == .git/ディレクトリ – cp -R でバックアップ – これがあればどんな過去な状況が復活可能 • Gitは情報をなくさない – COWで、変更は複製 – 何失敗しても、git addさえやった事があればなくなって ない – 差分を保存するのではない • すべてzlibで圧縮
24. まとめ • 履歴管理付きのファイルシステムみたいな • Content ⇔ SHA-1 hash ⇔ 履歴 – 例えばrebaseすると、commitのparentが変わる ⇒ commitの内容が変わる ⇒ commitのhashが変わる ⇒ 子commitのparentが変わる ⇒ … ＝＝ 履歴が変わ る – SHA-1 == unique ID • バックアップツールとしてGitは◎ – ソースコードではなくても