Understanding Git Objects and Index

entry with the `--add` flag (or removing an entry with the `--remove`) flag. So to populate the index with the two files you just created, you can do ------------------------------------------------ $ git update-index --add hello example ------------------------------------------------ and you have now told Git to track those two files. In fact, as you did that, if you now look into your object directory, you'll notice that Git will have added two new objects to the object database. If you did exactly the steps above, you should now be able to do ---------------- $ ls .git/objects/??/* ---------------- and see two files: ---------------- .git/objects/55/7db03de997c86a4a028e1ebd3a1ceb225be238 .git/objects/f2/4c74a2e500f5ee1332c86b94199f52b1d1d962 ---------------- which correspond with the objects with names of `557db...` and `f24c7...` respectively. If you want to, you can use 'git cat-file' to look at those objects, but you'll have to use the object name, not the filename of the object: ---------------- $ git cat-file -t 557db03de997c86a4a028e1ebd3a1ceb225be238 ---------------- where the `-t` tells 'git cat-file' to tell you what the "type" of the object is. Git will tell you that you have a "blob" object (i.e., just a regular file), and you can see the contents with ---------------- $ git cat-file blob 557db03 ---------------- which will print out "Hello World". The object `557db03` is nothing more than the contents of your file `hello`. [NOTE] Don't confuse that object with the file `hello` itself. The object is literally just those specific *contents* of the file, and however much you later change the contents in file `hello`, the object we just looked at will never change. Objects are immutable. [NOTE] The second example demonstrates that you can abbreviate the object name to only the first several hexadecimal digits in most places. Anyway, as we mentioned previously, you normally never actually take a look at the objects themselves, and typing long 40-character hex names is not something you'd normally want to do. The above digression was just to show that 'git update-index' did something magical, and actually saved away the contents of your files into the Git object database. Updating the index did something else too: it created a `.git/index` file. This is the index that describes your current working tree, and something you should be very aware of. Again, you normally never worry about the index file itself, but you should be aware of the fact that you have not actually really "checked in" your files into Git so far, you've only *told* Git about them. However, since Git knows about them, you can now start using some of the most basic Git commands to manipulate the files or look at their status. In particular, let's not even check in the two files into Git yet, we'll start off by adding another line to `hello` first: ------------------------------------------------ $ echo "It's a new day for git" >>hello ------------------------------------------------ and you can now, since you told Git about the previous state of `hello`, ask Git what has changed in the tree compared to your old index, using the 'git diff-files' command: ------------ $ git diff-files ------------ Oops. That wasn't very readable. It just spit out its own internal version of a 'diff', but that internal version really just tells you that it has noticed that "hello" has been modified, and that the old object contents it had have been replaced with something else. To make it readable, we can tell 'git diff-files' to output the differences as a patch, using the `-p` flag: ------------ $ git diff-files -p diff --git a/hello b/hello index 557db03..263414f 100644 --- a/hello +++ b/hello @@ -1 +1,2 @@ Hello World +It's a new day for git ------------ i.e. the diff of the change we caused by adding another line to `hello`. In other words, 'git diff-files' always shows us the difference between what is recorded in the index, and what is

This section explains how Git stores file contents as immutable objects, and how the index is used to track changes to files, including using `git update-index` to add files to the index, `git cat-file` to view object contents, and `git diff-files` to show differences between the index and working tree.