When a commit occurs, Git records a snapshot of the {{c1::index}} and places that snapshot in the object store. There is a one-to-one correspondence between a commit and a set of changes. A {{c1::commit}} is the only method of introducing changes to a Git repository. How you decide when to commit is up to you and your preferences or development style. Git is well-suited to frequent commits.

The most rigorous name for a commit is the hash ID which is an absolute name that can only refer to one commit, since it is a globally unique ID.

A {{c2::ref}} is a {{c1::SHA1}} hash ID that refers to an object within the Git object store. A {{c2::ref}} may refer to any Git object, but it is usually a {{c1::commit}}. A symbolic reference, or {{c2::symref}}, is just a {{c1::ref}} but it indirectly points to a Git object.

Git maintains several special symrefs automatically. {{c1::HEAD}} is a {{c2::symref}} that always refers to the most recent commit on the current branch. HEAD updates automatically to refer to the latest commit when you change branches. {{c1::ORIG_HEAD}} is the previous version of HEAD recorded after certain operations like merge and reset. FETCH_HEAD is a shorthand for the head of the last branch fetched and is valid only immediately after a fetch operation. {{c1::MERGE_HEAD}} is the tip of the other branch being merged when a merge is in progress. All these special symrefs can be used anywhere a commit can be used.

All commits except for the root commit derive from one or more parent commits. Given a commit with three parents, the three parent commits are C^1, C^2, and C^3. C~1, C~2, and C~3 refer to the first parent, the first grandparent, and the first great-grandparent commits.

The primary command to show the history of commits is git log.

In CS, a {{c1::graph}} is a collection of nodes and a set of edges between the nodes. There are several types of graphs with different properties. Git makes use of a special graph called a {{c1::directed acyclic graph}} to implement the history of commits. There are two important properties of a DAG. First, the edges within the graph are all directed from one node to another. Second, starting at any node, there is no path along the directed edges that leads back to the starting node. Git implements the history of commits as a DAG. In the commit graph, each node is a commit, and all edges are directed from descendent nodes to parent nodes.

The thing to understand is that normal commits have exactly one parent, that there is usually only one commit with zero parents, and that merge commits have more than one parent. A commit with more than one child is where the history diverged into two branches.

git bisect can be used to isolate and determine that commit that introduced a problem systematically.

git blame tells you who last modified each line of a file and which commit introduced the change.

The -S option to git log searches through the history of a file's diffs for the given string (git log -Sstring). The -S option is called pickaxe.