Introduction

The merge base of two commits is their best common ancestor: the foundation Git uses for three-way merges. Choosing the right base is what makes "automatic" merges actually work.

Introduction

When Git cannot automatically combine two changes to the same region of a file, it leaves conflict markers in the working copy and lets you resolve manually. The markers are not magic; they record exactly what each side did relative to a common base.

Introduction

git push uploads commits to a remote and updates that remote's refs. Unlike fetch, it modifies state on the server, so safety mechanisms exist to prevent overwriting other people's work.

Introduction

git pull is sugar for git fetch followed by integration of the upstream branch into the current branch. The integration step is either a merge (default) or a rebase.

The two steps

1. git fetch <remote> <refspec>: download and update tracking refs.
2. If pull.rebase is false: git merge FETCH_HEAD.
3. If pull.rebase is true: git rebase FETCH_HEAD.

Equivalent manual sequence:

Introduction

git fetch is the network operation that downloads new objects and updates remote-tracking refs. It does not change your branches or working tree, which makes it the safest "what's new?" command.

Introduction

git clone looks like one operation but actually composes several. Understanding the steps helps when troubleshooting partial clones, slow clones, or surprising defaults.

Introduction

Git supports two kinds of tags: lightweight, which are just refs pointing at commits, and annotated, which are full Git objects with metadata. Use annotated tags for releases.

Introduction

Git keeps several special "auxiliary" refs in .git/ to record state during multi-step operations. Knowing them turns scary recoveries into one-liners.

ORIG_HEAD

ORIG_HEAD is set whenever a "dangerous" operation moves HEAD by a lot: merge, rebase, reset, am. It captures the previous tip so you can undo:

git merge feature
# decide it was a mistake
git reset --hard ORIG_HEAD

Same trick after a bad rebase or reset:

Introduction

Git stores objects in two formats: loose (one file per object under .git/objects/xx/yyyy...) and packed (many objects in a single .pack file with a companion .idx). Packs are far more efficient on disk and on the wire.

Introduction

A Git commit references zero, one, or several parent commits. The whole history is a directed acyclic graph (DAG) of these parent links. Branches and tags are simply labels on nodes of this graph.

Anatomy of a parent link

git cat-file -p HEAD
# tree 9f1a...
# parent b2c3...
# parent d4e5...        (only on merge commits)
# author Ada ...
# committer Ada ...

One parent: linear history. Two parents: a merge. Zero parents: a root commit. Three or more: an octopus merge.