I have a similar problem and I figured out it.
I want to detect forced update within hook script in remote (bare) repository, So my answer might not be suitable for original question, but I hope to be useful my answer for future visitors.
How to detect forced update or not from in Git hooks script
https://github.com/kyanny/git-hooks-detect-force-update
This is a sample git pre-receive hook script for learning about how to detect forced update.
Conclusion
$ git rev-list oldrev ^newrev
How to test
$ rake -T
rake forced_push # git hooks test detect forced update
rake normal_push # git hooks test
Step-by-Step introduction
Firstly, I describe a syntax of git-rev-list(1).
In this case, we assume within a Git working repository that has this straight history.
1 --- 2 --- O --- X --- 3 --- 4 --- N
General usage of git-rev-list is below.
$ git rev-list N
This command will show all of commits reachable from commit N (note: git-rev-list shows commits reverse chronological order)
git-rev-list accepts multiple arguments.
$ git rev-list N O
This command will show same output as git rev-list N, because commit O is an ancestor of commit N.
Then, git-rev-list allows you to exclude commits from output.
$ git rev-list N ^O
^O means that to exclude commits reachable from O, so this command will show N, 4, 3, X (note: O is excluded)
Since we are learned about git-rev-list, I describe a case about forced update occured.
In this case, we assume within a Git working repository that has this complex history.
* --- B --- * --- O ($oldrev)
\
* --- X --- * --- N ($newrev)
- In old tree, we had 4 commits (*, B, *, O) and pushed them to remote.
- We checkout a new branch from commit B, it’s new tree.
- In new tree, we had 4 commits (*, X, *, N) and pushed them to remote with –force option!
When pushed, hook pre-receive script invoked with standard input. The format of stdin parameter is described at githooks(5).
Typically, we extract two commit object sha1 from stdin – oldrev and newrev. oldrev is HEAD of old tree, newrev is HEAD of new tree.
In this situation, we can detect forced push by git-rev-list output.
git rev-list oldrev ^newrev shows the commits reachable from oldrev but not reachable from newrev. This shows the commits existed only old tree.
If this command show any commits, old tree was replaced with new tree, so forced update was occured. This is what we want!
If this command show none of commits, new tree was normally updated, so forced update was not occured. Simply.