- The history items are stored as commit objects that are identified as a SHA-1 sum of the contents (including meta-data like Authored By, Committed By, etc).
- One of those meta-data items is "Parent Commit," so if you change one item in history, it changes the SHA-1 sum of all subsequent items (because at the very least they all need to be re-parented).
- All of the commit objects are stored under .git/objects.
- Branches are just files under .git/refs/ that contain the SHA-1 sum of the most recent commit on that branch. This is why they are called 'branch pointers.' That's basically all they are.
- If you have a history of 5 commits, and make a change to the initial commit, you now have 10 commits in your .git/ directory. Your (e.g.) 'master' branch will point to the most recent 'tree' of 5 commits. The other commits will still exist in .git/objects, but there will be no branches pointing them. You can use 'git reflog' to find them, or access them by their SHA-1 sum.
- Eventually 'git gc' (gc = garbage collect) will clean out the unreferenced commits, but this happens rarely if you don't explicitly run the command.
- When you 'git push,' you are only pushing branches to the remote repo, so commits that are stored locally, which are not referenced by one of those branches you are pushing, will not be pushed out. If you have commits that you don't want to end up in limbo like this, you should 'git tag' them or create a branch (e.g. 'archive/master-2013-12' that points to them).
It looks like .git/logs contains the history. It looks like the file format is a space-separated list, with the format "$parentcommitsha1 $newcommitsha1 ... $commitmessage". That's fairly comprehensible. What are the SHA-1 sums of? Are they of the entire snapshot, or the delta? I went into objects/ and ran `sha1sum $objfile`, and the sum did not match the file name. So that remains obscure. `file $objfile` could not identify the format; it gave nonsense.
Thanks for the help.
>One of those meta-data items is "Parent Commit," so if you change one item in history, it changes the SHA-1 sum of all subsequent items (because at the very least they all need to be re-parented).
What sequence of operations would change a history item in that way?
> It looks like .git/logs contains the history. It looks like the file format is a space-separated list, with the format "$parentcommitsha1 $newcommitsha1 ... $commitmessage". That's fairly comprehensible.
I've never looked at .git/logs, but it looks like that is used by the `git reflog` command. It's basically a history (or log) of every commit that a particular reference has pointed to[1]. For example, I cloned the git source code:
user@host ~/src/git % cat .git/logs/HEAD
0000000000000000000000000000000000000000 d7aced95cd681b761468635f8d2a8b82d7ed26fd First Last <user@example.com> 1387237920 -0500 clone: from https://github.com/git/git.git
user@host ~/src/git % git reflog
d7aced9 HEAD@{0}: clone: from https://github.com/git/git.git
Note: `HEAD` is a reference to the current branch. E.g.:
It's also of note that branches are referred to as 'references' too, hence storing them under `.git/refs/`.
> What are the SHA-1 sums of? Are they of the entire snapshot, or the delta? I went into objects/ and ran `sha1sum $objfile`, and the sum did not match the file name. So that remains obscure.
>I've never looked at .git/logs, but it looks like that is used by the `git reflog` command. It's basically a history (or log) of every commit that a particular reference has pointed to[1]. For example, I cloned the git source code:
I think it's more or less the DAG represented as an adjacency list. I'd have to think a bit about why there is a separate log file for each branch. It seems that there's some redundancy in doing that, and I'm wondering what the advantages and disadvantages are of splitting the history up in that way.
>It's also of note that branches are referred to as 'references' too, hence storing them under `.git/refs/`.
I've developed a loathing of excessive hierarchies/trees, so I'd rather see them flattened in a single directory. But that makes sense.
I'm guessing that the reason each branch has its own history is probably related to the goal of only appending new entries at the end of things. Since any branch can be under development, they need their own files. It sort of makes sense.