Git Committing Changes and Inspecting Differences

index file, but against the tree we just wrote. It just so happens that those two are obviously the same, so we get the same result. Again, because this is a common operation, you can also just shorthand it with ---------------- $ git diff HEAD ---------------- which ends up doing the above for you. In other words, 'git diff-index' normally compares a tree against the working tree, but when given the `--cached` flag, it is told to instead compare against just the index cache contents, and ignore the current working tree state entirely. Since we just wrote the index file to HEAD, doing `git diff-index --cached -p HEAD` should thus return an empty set of differences, and that's exactly what it does. [NOTE] ================ 'git diff-index' really always uses the index for its comparisons, and saying that it compares a tree against the working tree is thus not strictly accurate. In particular, the list of files to compare (the "meta-data") *always* comes from the index file, regardless of whether the `--cached` flag is used or not. The `--cached` flag really only determines whether the file *contents* to be compared come from the working tree or not. This is not hard to understand, as soon as you realize that Git simply never knows (or cares) about files that it is not told about explicitly. Git will never go *looking* for files to compare, it expects you to tell it what the files are, and that's what the index is there for. ================ However, our next step is to commit the *change* we did, and again, to understand what's going on, keep in mind the difference between "working tree contents", "index file" and "committed tree". We have changes in the working tree that we want to commit, and we always have to work through the index file, so the first thing we need to do is to update the index cache: ------------------------------------------------ $ git update-index hello ------------------------------------------------ (note how we didn't need the `--add` flag this time, since Git knew about the file already). Note what happens to the different 'git diff-{asterisk}' versions here. After we've updated `hello` in the index, `git diff-files -p` now shows no differences, but `git diff-index -p HEAD` still *does* show that the current state is different from the state we committed. In fact, now 'git diff-index' shows the same difference whether we use the `--cached` flag or not, since now the index is coherent with the working tree. Now, since we've updated `hello` in the index, we can commit the new version. We could do it by writing the tree by hand again, and committing the tree (this time we'd have to use the `-p HEAD` flag to tell commit that the HEAD was the *parent* of the new commit, and that this wasn't an initial commit any more), but you've done that once already, so let's just use the helpful script this time: ------------------------------------------------ $ git commit ------------------------------------------------ which starts an editor for you to write the commit message and tells you a bit about what you have done. Write whatever message you want, and all the lines that start with '#' will be pruned out, and the rest will be used as the commit message for the change. If you decide you don't want to commit anything after all at this point (you can continue to edit things and update the index), you can just leave an empty message. Otherwise `git commit` will commit the change for you. You've now made your first real Git commit. And if you're interested in looking at what `git commit` really does, feel free to investigate: it's a few very simple shell scripts to generate the helpful (?) commit message headers, and a few one-liners that actually do the commit itself ('git commit'). Inspecting Changes ------------------ While creating changes is useful, it's even more useful if you can tell later what changed. The most useful command for this is another of the 'diff' family, namely 'git diff-tree'.

This section explains how to commit changes to a Git repository, including updating the index cache and using `git commit` to create a new commit, and how to use various `git diff` commands to inspect differences between the working tree, index, and committed trees, including `git diff-index` and `git diff-tree`.