How does git assure that commit SHA keys for identical operations/data are still unique?

Question

If I create a file foo with touch foo and then run shasum foo it will print out

da39a3ee5e6b4b0d3255bfef95601890afd80709

Answer 1

The only things which are SHA-1'd to give the commit object its reference are what is shown by git show .

commit e6e53f5256c47b039ed19e95a073484dbb97cbf7
tree 543b9bebdc6bd5c4b22136034a95dd097a57d3dd
author Alex Balhatchet  1406774132 -0700
committer Alex Balhatchet  1406774132 -0700

    foo

That is:

1. The tree's id
2. The author name, email
3. The author commit timestamp
4. The committer name, email
5. The committer timestamp

The reason the examples with --date from other answers haven't worked is because you need to override both the committer timestamp and the author timestamp.

For example the following is completely repeatable:

alex@yuzu:~$ ( mkdir foo ; cd foo ; git init ; export GIT_AUTHOR_DATE='Wed Jul 30 19:35:32 2014 -0700'; export GIT_COMMITTER_DATE=$GIT_AUTHOR_DATE; touch README; git add README; git commit README --message 'foo' --author 'Foo Bar '; git show HEAD --format=raw ; cd .. ; rm -rf foo ) 2>&1 | grep '^commit '
commit 7438e0a18888854650e6a53a9a5d823d6382de45

If you run it on your machine you should get exactly the same output.

Update

If you get different output it should at least be repeatable. For example I get different output for different versions of git; 1.7.10.4 reports a new empty README file as 0 files changed whereas 1.9.1 reports it as 1 file changed, 0 insertions(+), 0 deletions(-) which changes the commit object's contents.