Dive into .git

This slide was made for 10 minutes demo. in a tech meeting of my company.

Transcript

1. Dive into .git 2012-05-24 Dr. NISHIO Hirokazu http://www.nishiohirokazu.org/
2. about • This slide was made for 10 minutes demo. in a tech meeting of my company. • When you print it, put 16 slides per a page will be good enough. • This slide was published under CC-BY 3.0 License. You don’t need my permission to share it.
3. “Git is difficult!” • “A lot of difficult concepts!” • OK, put aside those concepts and look actually what is going on with Git! • I hope this demonstration helps you draw picture of Git in your mind.
4. $ mkdir test $ cd test $ ls -a . .. $ git init Initialized empty Git repository in .../test/.git/ “$ git init test” works same No tricks, no gimmicks! Just an empty directory. Make a repository ←You got it!
5. $ ls -a . .. .git Here is your repository!
6. Let’s look into it! $ cd .git $ tree . |-- HEAD |-- config |-- description |-- hooks |-- info | `-- exclude |-- objects | |-- info | `-- pack `-- refs |-- heads `-- tags (hooks is omitted)
7. $ cd .. $ touch README $ git add README $ git commit -m “initial commit” [master (root-commit) 4dd66d3] initial commit What is changed when you committed?
8. You got 3 objects! $ tree .git/objects .git/objects |-- 4d | `-- d66d3a32a66f3578317717ccfb18 |-- 54 | `-- 3b9bebdc6bd5c4b22136034a95dd |-- e6 | `-- 9de29bb2d1d6434b8b29ae775ad8 |-- info `-- pack Last of filename is omitted. Some changes outside of objects are omitted
9. $ cat > show.py #!/usr/bin/env python import sys import zlib data = file(sys.argv[1], "rb").read() data = zlib.decompress(data) print repr(data) Look into the objects! Make show.py: Don’t forget chmod +x
10. In commit obj $ ./show.py .git/objects/4d/d6... 'commit 201\x00 tree 543b...\n author NISHIO Hirokazu <...> 1337655529 +0900\n committer NISHIO Hirokazu <...> 1337655529 +0900\n \n initial commit\n' I broke lines for readability. Notice on “tree 543b” Its filename was shown when you committed.
11. In tree obj $ ./show.py 54/3b... 'tree 34\x00 100644 README\x00 \xe6\x9d\xe2\x9b...' I broke lines. Notice on e69b
12. In blob obj $ ./show.py e6/9d... 'blob 0\x00' It is content of README. It is empty now, thus size=0 and nothing are after \x00
13. Filename of objects $ python -c “import hashlib; hashlib.sha1('blob 0\x00').hexdigest()” 'e69de29b...' It is sha1 hash of its content!
14. Conclusion • Repository is in .git • There are many objects in .git/objects/ • Their contents are compressed with zlib and their filenames are sha1 hash of uncompressed contents. • They are commit obj, tree obj and blob obj. • Today I omitted on tags and refs (next time?)
15. Let’s try! • Edit README and look changes! • New commit obj has “parent <hash>” line • New blob has new content of README • Add new files and look changes in tree obj • When you add lines on README, does blob have whether diff or total content?
17. Appendix • Q: Why don’t you use “git show --format=raw” • A: Because it doesn’t show important information.
18. In tree obj... $ git show --format=raw 543b tree 543b README Oh, how can I know its contents is in e69d? That’s why I need to make show.py
19. Appendix • Q: Why don’t you use gunzip to extract it? • A: It is compressed with zlib, however it is now a valid zip-file (it doesn’t have headers) • If you know easier way, please tell me!