secret empty tree object reliable, and why is there not a symbolic name for it?
Git has a well-known, or at least sort-of-well-known, empty tree whose SHA1 is:
4b825dc642cb6eb9a060e54bf8d69288fbee4904
(you can see this in any repo, even a newly created one, with git cat-file -t and git cat-file -p ).
If you work hard and are very careful you can sort of use this empty tree to store a directory that has no files (see answer to How do I add an empty directory to a git repository), although it's not really a great idea.
It's more useful as one argument to git diff-tree , which one of the sample hooks does.
What I'm wondering is,
4b825dc642cb6eb9a060e54bf8d69288fbee4904 ? (A quick and dirty way to create a symbolic name is to put the SHA1 in, eg, .git/Nulltree . Unfortunately you have to do this for every repo. Seems better to just put the magic number in scripts, etc. I just have a general aversion to magic numbers.)
This thread mentions:
If you don't remember the empty tree sha1, you can always derive it with:
git hash-object -t tree /dev/null
Or, as Ciro Santilli proposes in the comments:
printf '' | git hash-object --stdin -t tree
So I guess it is safer to define a variable with the result of that command as your empty sha1 tree (instead of relying of a "well known value").
Note, you will see that SHA1 pop up on some GitHub repo when the author wants its first commit to be empty (see blog post "How I initialize my Git repositories"):
$ GIT_AUTHOR_DATE="Thu, 01 Jan 1970 00:00:00 +0000" GIT_COMMITTER_DATE="Thu, 01 Jan 1970 00:00:00 +0000" git commit --allow-empty -m 'Initial commit'
Will give you:
(See the tree SHA1?)
You can even rebase your existing history on top of that empty commit (see "git: how to insert a commit as the first, shifting all the others?")
In both cases, you don't rely on the exact SHA1 value of that empty tree.
You simply follow a best practice, initializing your repo with a first empty commit .
To do that:
git init my_new_repo
cd my_new_repo
git config user.name username
git config user.email email@com
git commit --allow-empty -m "initial empty commit"
That will generate a commit with a SHA1 specific to your repo, username, email, date of creation (meaning the SHA1 of the commit itself will be different every time).
But the tree referenced by that commit will be 4b825dc642cb6eb9a060e54bf8d69288fbee4904 , the empty tree SHA1.
git log --pretty=raw
commit 9ed4ff9ac204f20f826ddacc3f85ef7186d6cc14
tree 4b825dc642cb6eb9a060e54bf8d69288fbee4904 <====
author VonC <vonc@laposte.net> 1381232247 +0200
committer VonC <vonc@laposte.net> 1381232247 +0200
initial empty commit
To show just the tree of a commit (display the commit tree SHA1):
git show --pretty=format:%T 9ed4ff9ac204f20f826ddacc3f85ef7186d6cc14
4b825dc642cb6eb9a060e54bf8d69288fbee4904
If that commit, referencing an empty tree, is indeed your first commit, you can show that empty tree SHA1 with:
git log --pretty=format:%h --reverse | head -1 | xargs git show --pretty=format:%T
4b825dc642cb6eb9a060e54bf8d69288fbee4904
(and that even works on Windows, with Gnu On Windows commands)
As commented below, using git diff <commit> HEAD , this will show all your file in the current branch HEAD:
git diff --name-only 4b825dc642cb6eb9a060e54bf8d69288fbee4904 HEAD
Note: that empty tree value is formally defined in cache.h .
#define EMPTY_TREE_SHA1_HEX
"4b825dc642cb6eb9a060e54bf8d69288fbee4904"
It is now (Git 2.16, Q1 2018), use in a structure which is no longer tied to (only) SHA1, as seen in commit eb0ccfd:
Switch empty tree and blob lookups to use hash abstraction
Switch the uses of empty_tree_oid and empty_blob_oid to use the current_hash abstraction that represents the current hash algorithm in use.
See more at "Why doesn't Git use more modern SHA?"
I wrote up a blog post with two different ways of finding the hash: http://colinschimmelfing.com/blog/gits-empty-tree/
If it were to ever change for some reason, you could use the two ways below to find it. However, I would feel pretty confident using the hash in .bashrc aliases, etc., and I don't think it will change anytime soon. At the very least it would probably be a major release of git.
The two ways are:
git hash-object -t tree --stdin < /dev/null git write-tree in that new repo - the hash will be output by git write-tree.