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\author "" 
\end_header

\begin_body

\begin_layout Title
Git repository internals
\end_layout

\begin_layout Part
What's inside
\end_layout

\begin_layout Standard
Git repository consists of the following objects:
\end_layout

\begin_layout Itemize
Object database
\end_layout

\begin_deeper
\begin_layout Itemize
Blobs
\end_layout

\begin_layout Itemize
Trees
\end_layout

\begin_layout Itemize
Commits
\end_layout

\begin_layout Itemize
Annotated tags
\end_layout

\end_deeper
\begin_layout Itemize
Refs
\end_layout

\begin_deeper
\begin_layout Itemize
Your branches and tags
\end_layout

\begin_layout Itemize
Remote branches
\end_layout

\begin_layout Itemize
Special (HEAD etc.)
\end_layout

\end_deeper
\begin_layout Itemize
Config/misc
\end_layout

\begin_deeper
\begin_layout Itemize
.git/config
\end_layout

\begin_layout Itemize
hooks
\end_layout

\begin_layout Itemize
reflogs
\end_layout

\end_deeper
\begin_layout Standard
.
\end_layout

\begin_layout Standard
Blob is a piece of content without metadata
\end_layout

\begin_layout Standard
Tree is a list of file/directory names pointing at blobs or other trees
 
\size tiny
/* not considering subprojects */
\size default
.
\end_layout

\begin_layout Standard
Commit is an object that 1.
 has pointer to other commit[s], 2.
 has pointer to tree (project root), 3.
 contains metadata (commit message, dates, authors, etc.)
\end_layout

\begin_layout Standard
Each ref points to (usually last) commit in some branch (either your or
 remote)
\end_layout

\begin_layout Part
A short story
\end_layout

\begin_layout Section
Adding files
\end_layout

\begin_layout Standard
Let's create a simple project:
\end_layout

\begin_layout Standard
\begin_inset listings
inline false
status open

\begin_layout Plain Layout

mkdir qqq && cd qqq
\end_layout

\begin_layout Plain Layout

mkdir src
\end_layout

\begin_layout Plain Layout

echo 'int main(){printf("Helo wrold
\backslash
r");}' > src/main.c
\end_layout

\begin_layout Plain Layout

cat > Makefile << 
\backslash
EOF
\end_layout

\begin_layout Plain Layout

all:
\end_layout

\begin_layout Plain Layout

        gcc src/*.c -o hello
\end_layout

\begin_layout Plain Layout

EOF
\end_layout

\begin_layout Plain Layout

\end_layout

\begin_layout Plain Layout

git init
\end_layout

\begin_layout Plain Layout

git add .
\end_layout

\end_inset


\end_layout

\begin_layout Standard
\begin_inset Quotes eld
\end_inset

git add .
\begin_inset Quotes erd
\end_inset

 addded src/main.c and Makefile.
 It created two blobs (main.c and Makefile) and two trees (root tree and
 
\begin_inset Quotes eld
\end_inset

src
\begin_inset Quotes erd
\end_inset

 tree).
\end_layout

\begin_layout Standard
\begin_inset Graphics
	filename git1.svg
	scale 50

\end_inset


\end_layout

\begin_layout Section
Committing
\end_layout

\begin_layout Standard
\begin_inset listings
inline false
status open

\begin_layout Plain Layout

git commit -m "Add files to project"
\end_layout

\end_inset

Now we have just created a commit.
 Commit is pointed by ref (name of branch), tree is pointed by commit, blob
 is pointed by tree.
\end_layout

\begin_layout Standard
\begin_inset Graphics
	filename git2.svg
	scale 50

\end_inset


\end_layout

\begin_layout Section
More commits
\end_layout

\begin_layout Standard
\begin_inset listings
inline false
status open

\begin_layout Plain Layout

echo 'int main(){ printf("%s", "Hello world
\backslash
n"); return 0;}' > src/main.c
\end_layout

\begin_layout Plain Layout

git commit -am "Typo fix"
\end_layout

\end_inset


\end_layout

\begin_layout Standard
We changed main.c.
 It gets new blob.
 So we need to have new 
\begin_inset Quotes eld
\end_inset

src
\begin_inset Quotes erd
\end_inset

 tree (with updated blob pointer).
 So we need to have the new root tree (with updated 
\begin_inset Quotes eld
\end_inset

src
\begin_inset Quotes erd
\end_inset

 tree pointer).
 New commit points to the new tree.
\end_layout

\begin_layout Standard
\begin_inset Graphics
	filename git3.svg
	scale 50

\end_inset


\end_layout

\begin_layout Standard
\begin_inset listings
inline false
status open

\begin_layout Plain Layout

mkdir src/plugins
\end_layout

\begin_layout Plain Layout

echo 'void func(){}' > src/plugins/extra.c
\end_layout

\begin_layout Plain Layout

git add src/plugins
\end_layout

\begin_layout Plain Layout

git commit -m "Add a plugin"
\end_layout

\end_inset


\end_layout

\begin_layout Standard
\begin_inset Graphics
	filename git4.svg
	scale 50

\end_inset


\end_layout

\begin_layout Standard
\begin_inset listings
inline false
status open

\begin_layout Plain Layout

cat > Makefile << 
\backslash
EOF
\end_layout

\begin_layout Plain Layout

all:
\end_layout

\begin_layout Plain Layout

        gcc src/*.c src/plugins/*.c -o hello
\end_layout

\begin_layout Plain Layout

EOF
\end_layout

\begin_layout Plain Layout

git add Makefile
\end_layout

\begin_layout Plain Layout

git commit -m "Oh, forgot about Makefile"
\end_layout

\end_inset


\end_layout

\begin_layout Standard
\begin_inset Graphics
	filename git5.svg
	scale 50

\end_inset


\end_layout

\begin_layout Standard
Note that now 
\begin_inset Quotes eld
\end_inset

src
\begin_inset Quotes erd
\end_inset

 tree is reused from previous commit (as nothing have changed there)
\end_layout

\begin_layout Part
Rewriting history
\end_layout

\begin_layout Standard
Commit 
\begin_inset Quotes eld
\end_inset

c54c
\begin_inset Quotes erd
\end_inset

 is not useful: the very next commit fixes obvious thing.
 We want to merge that two commits into one, eliminating the 
\begin_inset Quotes eld
\end_inset

wrong
\begin_inset Quotes erd
\end_inset

 intermediate c54c.
\end_layout

\begin_layout Standard
\begin_inset listings
inline false
status open

\begin_layout Plain Layout

git rebase -i HEAD~2  # ~2 means we need to process 2 last commits
\end_layout

\begin_layout Plain Layout

# in editor:
\end_layout

\begin_layout Plain Layout

pick c54c
\end_layout

\begin_layout Plain Layout

squash ff5f
\end_layout

\end_inset


\end_layout

\begin_layout Standard
This is convert commits c54c and ff5f to patches, rewind to commit 58cd
 then apply that two patches, but producing only one commit (you can also
 split and edit them).
 
\size tiny
/* Note: this case can be done simpler: 
\family typewriter
git reset --soft HEAD~2 && git commit -m 
\begin_inset Quotes eld
\end_inset

Add a plugin
\begin_inset Quotes erd
\end_inset


\family default
*/
\end_layout

\begin_layout Standard
Stale commits 
\begin_inset Quotes eld
\end_inset

ff5f
\begin_inset Quotes erd
\end_inset

 and 
\begin_inset Quotes eld
\end_inset

c54c
\begin_inset Quotes erd
\end_inset

 still remain in repository and can be used for recovery (unless finally
 cleaned by 
\begin_inset Quotes eld
\end_inset

git gc
\begin_inset Quotes erd
\end_inset

)
\end_layout

\begin_layout Standard
\begin_inset Graphics
	filename git6.svg
	scale 50

\end_inset


\end_layout

\begin_layout Standard
If/when stale commits get cleaned (they will be there for a certain time),
 it will be like this:
\end_layout

\begin_layout Standard
\begin_inset Graphics
	filename git7.svg
	scale 50

\end_inset


\end_layout

\begin_layout Standard
\begin_inset Quotes eld
\end_inset

ff5f
\begin_inset Quotes erd
\end_inset

 and 
\begin_inset Quotes eld
\end_inset

c54c
\begin_inset Quotes erd
\end_inset

 are gone, tree 
\begin_inset Quotes eld
\end_inset

aa54
\begin_inset Quotes erd
\end_inset

 is gone too (as it is not referenced by anything anymore).
\end_layout

\begin_layout Part
Interaction with other repositories
\end_layout

\begin_layout Standard
We can fetch or push commits to other repositories.
 Obtaining (or publishing) a commit implies transferring it's trees and
 blobs as well.
 For example,
\end_layout

\begin_layout Standard
\begin_inset listings
inline false
status open

\begin_layout Plain Layout

git remote add origin git://github.com/hello/exaple.git
\end_layout

\begin_layout Plain Layout

git fetch origin +refs/heads/master:refs/remotes/origin/master
\end_layout

\end_inset


\end_layout

\begin_layout Standard
Registers URL of remote repository under the name 
\begin_inset Quotes eld
\end_inset

origin
\begin_inset Quotes erd
\end_inset

 and then fetches origin's ref named 
\begin_inset Quotes eld
\end_inset

refs/heads/master
\begin_inset Quotes erd
\end_inset

 (or just 
\begin_inset Quotes eld
\end_inset

master
\begin_inset Quotes erd
\end_inset

) to our local ref 
\begin_inset Quotes eld
\end_inset

refs/remotes/origin/master
\begin_inset Quotes erd
\end_inset

, replacing old 
\begin_inset Quotes eld
\end_inset

refs/remotes/origin/master
\begin_inset Quotes erd
\end_inset

 even if it is not direct child 
\size footnotesize
(this will occur if, for example, somebody have fetched our commit ff5f
 or c54c and now we rewritten it to new one 7747)
\size default
.
\end_layout

\begin_layout Standard
Note: This was a full form of fetch command, usually 
\begin_inset Quotes eld
\end_inset

git fetch origin
\begin_inset Quotes erd
\end_inset

 or 
\begin_inset Quotes eld
\end_inset

git fetch
\begin_inset Quotes erd
\end_inset

 does the required thing.
\end_layout

\begin_layout Standard
Pushing command example is 
\begin_inset Quotes eld
\end_inset

git push origin 4112:refs/heads/master
\begin_inset Quotes erd
\end_inset

.
 It also can have forcing 
\begin_inset Quotes eld
\end_inset

+
\begin_inset Quotes erd
\end_inset

, source ref can be 
\begin_inset Quotes eld
\end_inset

master
\begin_inset Quotes erd
\end_inset

 or 
\begin_inset Quotes eld
\end_inset

HEAD
\begin_inset Quotes erd
\end_inset

 
\size footnotesize
(or even nothing - means 
\begin_inset Quotes eld
\end_inset

delete it
\begin_inset Quotes erd
\end_inset

)
\size default
 instead of direct commit number 
\begin_inset Quotes eld
\end_inset

4112
\begin_inset Quotes erd
\end_inset

 or be configured for just 
\begin_inset Quotes eld
\end_inset

git push
\begin_inset Quotes erd
\end_inset

 to do what you expect.
\end_layout

\end_body
\end_document