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Git Theory

The revision Graph

Revisions form a GRAPH

In [1]:
import os
top_dir = os.getcwd()
git_dir = os.path.join(top_dir, 'learning_git')
working_dir=os.path.join(git_dir, 'git_example')
os.chdir(working_dir)
In [2]:
%%bash
git log --graph --oneline
*   4a35412 Merge branch 'master' of https://github.com/alan-turing-institute/github-example
|\  
| * 22098c9 Add another Beacon
* | 0c25997 Add Glyder
|/  
*   1bc5264 Merge branch 'master' of https://github.com/alan-turing-institute/github-example
|\  
| * 28411f9 Add a beacon
* | 75469d1 Translating from the Welsh
|/  
*   138c6d9 Merge branch 'master' of https://github.com/alan-turing-institute/github-example
|\  
| * 6eecc82 Add Scotland
* | d2ca8d2 Add wales
|/  
* 563f95e Add Helvellyn
* b5f225b Include lakes in the scope
* 86cf371 Add lakeland
* 03e1d59 Revert "Add a lie about a mountain"
* d4a9132 Change title
* cb67f8a Add a lie about a mountain
* a2f6abf First commit of discourse on UK topography

Git concepts

  • Each revision has a parent that it is based on
  • These revisions form a graph
  • Each revision has a unique hash code
    • In Sue's copy, revision 43 is ab3578d6
    • Jim might think that is revision 38, but it's still ab3579d6
  • Branches, tags, and HEAD are labels pointing at revisions
  • Some operations (like fast forward merges) just move labels.

The levels of Git

There are four Separate levels a change can reach in git:

  • The Working Copy
  • The index (aka staging area)
  • The local repository
  • The remote repository

Understanding all the things git reset can do requires a good grasp of git theory.

  • git reset <commit> <filename> : Reset index and working version of that file to the version in a given commit
  • git reset --soft <commit>: Move local repository branch label to that commit, leave working dir and index unchanged
  • git reset <commit>: Move local repository and index to commit ("--mixed")
  • git reset --hard <commit>: Move local repostiory, index, and working directory copy to that state