Undo git pull, how to bring repos to old state

Question

Is there any way to revert or undo git pull so that my source/repos will come to old state that was before doing git pull ? I want to do this because it merged some files which I didn't want to do so, but only merge other remaining files. So, I want to get those files back, is that possible?

EDIT: I want to undo git merge for clarification. After seeing some answers, I did this

git reflog bb3139b... HEAD@{0}: pull : Fast forward 01b34fa... HEAD@{1}: clone: from ...name... 

Now, what should I do ? Doing git reset --hard is OK ? I don't want to screw it again, so asking for detailed steps ?

Answer 1

Running git pull performs the following tasks, in order:

1. git fetch
2. git merge

The merge step combines branches that have been setup to be merged in your config. You want to undo the merge step, but probably not the fetch (doesn't make a lot of sense and shouldn't be necessary).

To undo the merge, use git reset --hard to reset the local repository to a previous state; use git-reflog to find the SHA-1 of the previous state and then reset to it.

Warning

The commands listed in this section remove all uncommitted changes, potentially leading to a loss of work:

git reset --hard 

Alternatively, reset to a particular point in time, such as:

git reset --hard master@{"10 minutes ago"} 

Answer 2

Same as jkp's answer, but here's the full command:

git reset --hard a0d3fe6 

where a0d3fe6 is found by doing

git reflog 

and looking at the point at which you want to undo to.