Unstage a deleted file in git

Assuming you're wanting to undo the effects of git rm <file> or rm <file> followed by git add -A or something similar:

# this restores the file status in the index
git reset -- <file>
# then check out a copy from the index
git checkout -- <file>

To undo git add <file>, the first line above suffices, assuming you haven't committed yet.

The answers to your two questions are related. I'll start with the second:

Once you have staged a file (often with git add, though some other commands implicitly stage the changes as well, like git rm) you can back out that change with git reset -- <file>.

In your case you must have used git rm to remove the file, which is equivalent to simply removing it with rm and then staging that change. If you first unstage it with git reset -- <file> you can then recover it with git checkout -- <file>.

If it has been staged and committed, then the following will reset the file:

git reset COMMIT_HASH file_path
git checkout COMMIT_HASH file_path
git add file_path

This will work for a deletion that occurred several commits previous.

Both questions are answered in git status.

To unstage adding a new file use git rm --cached filename.ext

# Changes to be committed:
#   (use "git rm --cached <file>..." to unstage)
#
#   new file:   test

To unstage deleting a file use git reset HEAD filename.ext

# Changes to be committed:
#   (use "git reset HEAD <file>..." to unstage)
#
#   deleted:    test

In the other hand, git checkout -- never unstage, it just discards non-staged changes.