Advanced Git Operations and Principles

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This blog post was organized with the assistance of ChatGPT-4o. The keynote can be found at https://github.com/lzwjava/Keynotes.

Undoing Changes

$ git commit --amend 
$ git add * 
$ git status 
$ git reset HEAD CONTRIBUTING.md 
Unstaged changes after reset: 
M CONTRIBUTING.md 
$ git status 
$ git checkout -- CONTRIBUTING.md 

Recommit
Unstage files
Discard changes in working directory

Commands

git revert: Revert, re-commit to counteract a previous commit
git cherry-pick: Select specific commits from multiple commits
git rebase

filter-branch

Remove a file from every commit
You want to delete it, but it’s still in history
How to delete a file from all commits?

$ git filter-branch --tree-filter 'rm -f passwords.txt' HEAD 
Rewrite 6b9b3cf04e7c5686a9cb838c3f36a8cb6a0fc2bd (21/21) 
Ref 'refs/heads/master' was rewritten 

Search

$ git grep -n gmtime_r 
compat/gmtime.c:3:#undef gmtime_r 
compat/gmtime.c:8:return git_gmtime_r(timep, &result); 
compat/gmtime.c:11:struct tm *git_gmtime_r(const time_t *timep, struct tm *result) 
compat/gmtime.c:16:ret = gmtime_r(timep, result); 
compat/mingw.c:606:struct tm *gmtime_r(const time_t *timep, struct tm *result) 
compat/mingw.h:162:struct tm *gmtime_r(const time_t *timep, struct tm *result); 
date.c:429:if (gmtime_r(&now, &now_tm)) 
date.c:492:if (gmtime_r(&time, tm)) { 
git-compat-util.h:721:struct tm *git_gmtime_r(const time_t *, struct tm *); 
git-compat-util.h:723:#define gmtime_r git_gmtime_r 
$ git log -SZLIB_BUF_MAX --oneline 
e01503b zlib: allow feeding more than 4GB in one go 
ef49a7a zlib: zlib can only process 4GB at a time 

.Git Directory

config: Project configuration
info: .gitignore
objects: All content in Git database
refs: Pointers to branches
HEAD: Pointer to the current branch
index: Staging area information

Git Objects

$ git init test 
Initialized empty Git repository in /tmp/test/.git/ 
$ cd test 
$ find .git/objects 
.git/objects 
.git/objects/info 
.git/objects/pack 
$ find .git/objects -type f

$ echo 'test content' | git hash-object -w --stdin 
d670460b4b4aece5915caf5c68d12f560a9fe3e4 
$ find .git/objects -type f 
.git/objects/d6/70460b4b4aece5915caf5c68d12f560a9fe3e4

hash-object: Command to save data to .git directory
-w: Write the object, otherwise just return the key
--stdin: Read from standard input
d670...: 40-character checksum
cat-file: Swiss army knife to view Git Object

$ git cat-file -p d670460b4b4aece5915caf5c68d12f560a9fe3e4 
test content

$ echo 'version 1’ > test.txt
$ git hash-object -w test.txt
83baae61804e65cc73a7201a7252750c76066a30
$ echo 'version 2' > test.txt
$ git hash-object -w test.txt
1f7a7a472abf3dd9643fd615f6da379c4acb3e3a
$ find .git/objects -type f
.git/objects/1f/7a7a472abf3dd9643fd615f6da379c4acb3e3a
.git/objects/83/baae61804e65cc73a7201a7252750c76066a30
.git/objects/d6/70460b4b4aece5915caf5c68d12f560a9fe3e4
$ git cat-file -p 83baae61804e65cc73a7201a7252750c76066a30 > test.txt
$ cat test.txt
version 1
$ git cat-file -p 1f7a7a472abf3dd9643fd615f6da379c4acb3e3a > test.txt
$ cat test.txt
version 2
$ git cat-file -t 1f7a7a472abf3dd9643fd615f6da379c4acb3e3a
blob
$ git cat-file -p master^{tree}
100644 blob a906cb2a4a904a152e80877d4088654daad0c859 README
100644 blob 8f94139338f9404f26296befa88755fc2598c289 Rakefile
040000 tree 99f1a6d12cb4b6f19c8655fca46c3ecf317074e0 lib
$ git cat-file -p 99f1a6d12cb4b6f19c8655fca46c3ecf317074e0
100644 blob 47c6340d6459e05787f644c2447d2595f5d3a54b simplegit.rb
Tree Objects

$ git update-index --add --cacheinfo 100644 \
83baae61804e65cc73a7201a7252750c76066a30 test.txt
update-index

update-index: Command to create a tree
--add: Create index
--cacheinfo: Read from Git database
100644: File mode, normal file; 100755 for executable files; 120000 for symlinks
83baae: Previous content, version 1
\: Split a command into two lines

write-tree

$ git write-tree
d8329fc1cc938780ffdd9f94e0d364e0ea74f579
$ git cat-file -p d8329fc1cc938780ffdd9f94e0d364e0ea74f579
100644 blob 83baae61804e65cc73a7201a7252750c76066a30 test.txt
$ git cat-file -t d8329fc1cc938780ffdd9f94e0d364e0ea74f579
tree

write-tree: Write the content of the staging area to Tree Object

read-tree

$ echo 'new file' > new.txt 
$ git update-index test.txt 
$ git update-index --add new.txt 
$ git write-tree 
0155eb4229851634a0f03eb265b69f5a2d56f341 
$ git cat-file -p 0155eb4229851634a0f03eb265b69f5a2d56f341 
100644 blob fa49b077972391ad58037050f2a75f74e3671e92 new.txt 
100644 blob 1f7a7a472abf3dd9643fd615f6da379c4acb3e3a test.txt 
$ git read-tree --prefix=bak d8329fc1cc938780ffdd9f94e0d364e0ea74f579 
$ git write-tree 
3c4e9cd789d88d8d89c1073707c3585e41b0e614 
$ git cat-file -p 3c4e9cd789d88d8d89c1073707c3585e41b0e614 
040000 tree d8329fc1cc938780ffdd9f94e0d364e0ea74f579 bak 
100644 blob fa49b077972391ad58037050f2a75f74e3671e92 new.txt 
100644 blob 1f7a7a472abf3dd9643fd615f6da379c4acb3e3a test.txt

Commit Objects

$ echo 'first commit' | git commit-tree d8329f 
d5ffe1aa4b7b089eee03dccea5e0439ad6d72037 
$ git cat-file -p d5ffe1aa4b7b089eee03dccea5e0439ad6d72037 
tree d8329fc1cc938780ffdd9f94e0d364e0ea74f579 
author lzwjava <lzwjava@gmail.com> 1462090215 +0800 
committer lzwjava <lzwjava@gmail.com>

 1462090215 +0800 
first commit

Hash values differ because creation time and author are different

$ echo 'second commit' | git commit-tree 0155eb -p d5ffe1aa4 
e946d6367f07de45cac242dca7cd002f5eaa72b1 
$ echo 'third commit' | git commit-tree 3c4e9c -p e946d6 
09490bf051c34b3693dfd5c7fb63dfe27b295904 
$ git log --stat 09490 
commit 09490bf051c34b3693dfd5c7fb63dfe27b295904 
Author: lzwjava <lzwjava@gmail.com> 
Date: Sun May 1 16:38:55 2016 +0800 
third commit 
bak/test.txt | 1 + 
1 file changed, 1 insertion(+) 
commit e946d6367f07de45cac242dca7cd002f5eaa72b1 
Author: lzwjava <lzwjava@gmail.com> 
Date: Sun May 1 16:37:01 2016 +0800 
second commit 
new.txt | 1 + 
test.txt | 2 +- 
2 files changed, 2 insertions(+), 1 deletion(-) 
commit d5ffe1aa4b7b089eee03dccea5e0439ad6d72037 
Author: lzwjava <lzwjava@gmail.com> 
Date: Sun May 1 16:10:15 2016 +0800 
first commit 
test.txt | 1 + 
1 file changed, 1 insertion(+)

commit-tree: First parameter points to blob or tree
-p: Points to parent node

third commit 
second commit 
first commit 
tree
tree
tree
“version 2”
“new file”
“version 1”
09490b
e946d6
d5ffe1 d8329f
0155eb
3c4e9c
83baae
fa49b0
1f7a7a
test.txt
new.txt 
test.txt 
new.txt 
test.txt 
bak

Branch Principles

$ find .git/refs  
.git/refs 
.git/refs/heads 
.git/refs/tags 
$ find .git/refs -type f  
$ echo "09490bf051c34b3693dfd5c7fb63dfe27b295904" > .git/refs/heads/master 
$ git log --pretty=oneline master 
09490bf051c34b3693dfd5c7fb63dfe27b295904 third commit 
e946d6367f07de45cac242dca7cd002f5eaa72b1 second commit 
d5ffe1aa4b7b089eee03dccea5e0439ad6d72037 first commit 
$ git update-ref refs/heads/master 09490bf051c34b3693dfd5c7fb63dfe27b295904 
$ git update-ref refs/heads/test e946d6367f07de45cac242dca7cd002f5eaa72b1 
$ git branch 
* master 
test

Save a pointer reference
update-ref: Modify or add a reference

third commit 
second commit 
first commit 
tree
tree
tree
“version 2”
“new file”
“version 1”
09490b
e946d6
d5ffe1 d8329f
0155eb
3c4e9c
83baae
fa49b0
1f7a7a
test.txt
new.txt 
test.txt 
new.txt 
test.txt 
bak
refs/heads/master
refs/heads/test

$ cat .git/HEAD 
ref: refs/heads/master 
$ git symbolic-ref HEAD 
refs/heads/master 
$ git symbolic-ref HEAD refs/heads/test 
$ cat .git/HEAD 
ref: refs/heads/test

symbolic-ref

Tag Principles

$ git update-ref refs/tags/v1.0 e946d6367f07de45cac242dca7cd002f5eaa72b1 
$ git tag 
v1.0 
$ git tag -a v1.1 e946d6367f07de45cac242dca7cd002f5eaa72b1 -m 'test tag’ 
$ cat .git/refs/tags/v1.1 
2766532f03289bc5e158629a8b3faffa5f80b8b6 
$ git cat-file -p 276653 
object e946d6367f07de45cac242dca7cd002f5eaa72b1 
type commit 
tag v1.1 
tagger lzwjava <lzwjava@gmail.com> 1462103203 +0800 
test tag

Remotes

$ git remote add origin git@github.com:schacon/simplegit-progit.git 
$ git push origin master 
Counting objects: 11, done. 
Compressing objects: 100% (5/5), done. 
Writing objects: 100% (7/7), 716 bytes, done. 
Total 7 (delta 2), reused 4 (delta 1) 
To git@github.com:schacon/simplegit-progit.git 
a11bef0..ca82a6d master -> master 
$ cat .git/refs/remotes/origin/master 
ca82a6dff817ec66f44342007202690a93763949

Stored in refs/remotes directory
Difference between remotes and branches is that remotes are read-only
Can git checkout, but HEAD does not change, so commit cannot change remote references

$ curl https://raw.githubusercontent.com/mojombo/grit/master/lib/grit/repo.rb > repo.rb 
$ git checkout master 
$ git add repo.rb 
$ git commit -m 'added repo.rb' 
[master 484a592] added repo.rb 
3 files changed, 709 insertions(+), 2 deletions(-) 
delete mode 100644 bak/test.txt 
create mode 100644 repo.rb 
rewrite test.txt (100%) 
$ git cat-file -p master^{tree} 
100644 blob fa49b077972391ad58037050f2a75f74e3671e92 new.txt 
100644 blob 033b4468fa6b2a9547a70d88d1bbe8bf3f9ed0d5 repo.rb 
100644 blob e3f094f522629ae358806b17daf78246c27c007b test.txt 
$ git cat-file -s 033b4468fa6b2a9547a70d88d1bbe8bf3f9ed0d5 
22044 
$ echo '# testing' >> repo.rb 
$ git commit -am 'modified repo a bit' 
[master 2431da6] modified repo.rb a bit 
1 file changed, 1 insertion(+) 
$ git cat-file -p master^{tree} 
100644 blob fa49b077972391ad58037050f2a75f74e3671e92 new.txt 
100644 blob b042a60ef7dff760008df33cee372b945b6e884e repo.rb 
100644 blob e3f094f522629ae358806b17daf78246c27c007b test.txt 
$ git cat-file -s b042a60ef7dff760008df33cee372b945b6e884e 
22054 
$ git gc 
Counting objects: 18, done. 
Delta compression using up to 8 threads. 
Compressing objects: 100% (14/14), done. 
Writing objects: 100% (18/18), done. 
Total 18 (delta 3), reused 0 (delta 0) 
$ find .git/objects -type f 
$ git verify-pack -v .git/objects/pack/pack-978e03944f5c581011e6998cd0e9e30000905586.idx

Refspec

$ git remote add origin https://github.com/schacon/simplegit-progit 
[remote "origin"] 
url = https://github.com/schacon/simplegit-progit 
fetch = +refs/heads/*:refs/remotes/origin/* 
$ git log origin/master 
$ git log remotes/origin/master 
$ git log refs/remotes/origin/master 
fetch = +refs/heads/master:refs/remotes/origin/master 
[remote "origin"] 
url = https://github.com/schacon/simplegit-progit 
fetch = +refs/heads/master:refs/remotes/origin/master 
fetch = +refs/heads/qa/*:refs/remotes/origin/qa/* 
push = refs/heads/master:refs/heads/qa/master

<src>:<dst>
refs/remotes: Local position

https://github.com/schacon/simplegit-progit
https://github.com/schacon/simplegit-progit

References

Pro Git

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