Suppose that I made some changes and did this:
git add -A
git commit -m "comment commit_1"
Now I made more changes and did this again:
git add -A
git commit -m "comment commit_2"
Now, I basically want to discard commit_2 and start modifying my project again at the point of commit_1.
How do I do this?
If you don't mind losing the revisions at all (as if they never happened at all), you can do:
git reset --hard HEAD~
If you want to keep the current history and get a new version on top of what you have where you get rid of the changes of the revision, you can do:
git checkout HEAD~
git reset --soft the-branch
git commit -m "Taking back changes from the last revision"
# And if you like the result of this:
git branch -f the-branch
git checkout the-branch
There you go.
Now, I basically want to discard commit_2 and start modifying my project again at the point of commit_1.
If you literally want to discard commit_2, then it may be that you can use
git reset --hard HEAD^
(Note that at the time I wrote this, another post incorrectly told you to use HEAD~2; that would discard both commits, which is not what was asked.)
There are caveats to using commands that remove commits from a branch.
The first is that you could lose the changes you made in affected commits. That may seem a strange warning - you asked how to discard the changes - but realizing that mistakes happen, it's cause to be careful with these commands. (Just to clarify - if you did something like this accidentally, the changes would not be immediately lost forever; the reflogs offer a little safety net.)
Other issues arise if your repo has remotes and you've pushed the commit(s) in question to any of the remotes. It doesn't sound like that's an issue here, but if it is then there is more you should consider before removing commits from a branch.
But if you've looked it over and determined that you really do just want to discard the commits, the above should work. Do note, HEAD^ is just one of the possible expressions that refers to the second-to-last commit. At the moment it may be the simplest thing to use, but it won't always refer to that particular commit. HEAD^ really means "the commit which is the parent of the currently-checked-out commit".
If you're on the master branch, then master^ would also work - and would not depend on what's currently checked out. Even that changes whenever the master branch moves; if for some reason you need an expression that would continue to refer to the particular commit, you could create a tag
git tag mytag master^
and then mytag would work; or you could look up the commit ID of the commit in question (though it won't be a user-friendly name)
But I digress...
This is different from - and more drastic than - merely loading the 2nd-to-last commit as the question title suggests. To simply update your working tree with the 2nd-to-last commit's state, you could
git checkout HEAD^
This moves the HEAD (i.e. changes what's checked out) and updates the index and work tree, but it still keeps the commit on master in case you need it again later. However, it also puts you in a state called "detached head", meaning that git isn't really expecting you to create new commits from here.
If you want to both keep the old commit around, and be in a state where you can add commits on top of commit_1, then you need to create a new branch. If you did the previous checkout command, you could then
git checkout -b mybranch
and new changes could be committed to the new branch while leaving the original commit_2 on master. Or, you could put a branch (or tag) on commit_2 and then move master back to commit_1.
git checkout master
git branch mybranch
git reset --hard HEAD^
That may seem like a confusing number of options; but really that's because each one serves a somewhat different purpose, and only someone involved in your project is likely to have all the context to decide which makes sense.
So tl;dr - it sounds like you might be looking for git reset, but just doing that alone is only suitable if you really intend to permanently discard the changes from commit_2. If you really would want to preserve them for future reference (But maybe just weren't sure if that's an option) then there are ways to do that, too.
