rajeshkumar created the topic: What does commit object contain and What is SHA1?
The SHA
All the information needed to represent the history of a project is stored in files referenced by a 40-digit “object name” that looks something like this:
6ff87c4664981e4397625791c8ea3bbb5f2279a3
You will see these 40-character strings all over the place in Git. In each case the name is calculated by taking the SHA1 hash of the contents of the object. The SHA1 hash is a cryptographic hash function. What that means to us is that it is virtually impossible to find two different objects with the same name. This has a number of advantages; among others:
Git can quickly determine whether two objects are identical or not, just by comparing names. Since object names are computed the same way in every repository, the same content stored in two repositories will always be stored under the same name. Git can detect errors when it reads an object, by checking that the object’s name is still the SHA1 hash of its contents.
The Objects
Every object consists of three things – a type, a size and content. The size is simply the size of the contents, the contents depend on what type of object it is, and there are four different types of objects: “blob”, “tree”, “commit”, and “tag”.
A “blob” is used to store file data – it is generally a file.
A “tree” is basically like a directory – it references a bunch of other trees and/or blobs (i.e. files and sub-directories)
A “commit” points to a single tree, marking it as what the project looked like at a certain point in time. It contains meta-information about that point in time, such as a timestamp, the author of the changes since the last commit, a pointer to the previous commit(s), etc.
A “tag” is a way to mark a specific commit as special in some way. It is normally used to tag certain commits as specific releases or something along those lines.
Almost all of Git is built around manipulating this simple structure of four different object types. It is sort of its own little filesystem that sits on top of your machine’s filesystem.
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Rajesh Kumar
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