Git Glossary
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Working Directory and Staging Area
The working directory is the actual files on disk that you edit directly, while the staging area (also called the index) is a separate holding zone where you place exactly the changes you want in your next commit. Running git add moves a change from the working directory into the staging area, and only staged changes get included when you run git commit.
Example: Working Directory and Staging Area
git add file.txt
git statusHEAD and Detached HEAD
HEAD is a pointer to whichever commit you currently have checked out, and it normally points to a branch name rather than a commit directly, so committing moves both HEAD and the branch forward together. Checking out a specific commit hash instead of a branch puts you in detached HEAD state, where new commits aren't attached to any branch and can be lost once you switch away unless you create a branch to save them.
Example: HEAD and Detached HEAD
git checkout a1b2c3d
# HEAD is now detached, pointing directly at a1b2c3dUpstream and Tracking Branches
An upstream branch is the remote branch a local branch is configured to push to and pull from by default, set either at clone time or explicitly with git branch --set-upstream-to. Once set, plain git push and git pull with no arguments know exactly which remote branch to talk to, and Git can tell you how many commits your local branch is ahead or behind it.
Example: Upstream and Tracking Branches
git branch --set-upstream-to=origin/main main
git push
git pullFast-Forward vs Merge Commit
A fast-forward merge happens when the target branch has no new commits since it diverged, so Git simply moves the branch pointer forward to match the incoming branch with no new commit created. A merge commit happens when both branches have diverged with their own new commits, so Git creates a new commit with two parents to tie the histories back together, visible as a fork-and-rejoin shape in the log graph.
Example: Fast-Forward vs Merge Commit
git merge feature-x
# fast-forward if main has no new commits, else a merge commitCommit Hash and Ancestors
Every commit is identified by a SHA-1 hash computed from its content, parent commit, author, and message, which is why changing anything about a commit produces a different hash. An ancestor commit is any commit reachable by following parent links backward from a given commit, and Git's relative references like HEAD~1 or HEAD~3 walk that ancestor chain a fixed number of steps.
Example: Commit Hash and Ancestors
git log --oneline
git log a1b2c3d --onelineChapter Quiz — Complete all 8 topics to unlock
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