unstage files with git

gistfile1.txt

#   new file:   to-be-added
#   modified:   to-be-modified
#   deleted:    to-be-removed
#

me$ git reset -q HEAD to-be-added

    # ok

me$ git reset -q HEAD to-be-modified

    # ok

me$ git reset -q HEAD to-be-removed

    # ok

# or alternatively:

me$ git reset -q HEAD to-be-added to-be-removed to-be-modified

    # ok

me$ git status
# On branch master
# Changes not staged for commit:
#   (use "git add/rm <file>..." to update what will be committed)
#   (use "git checkout -- <file>..." to discard changes in working directory)
#
#   modified:   to-be-modified
#   deleted:    to-be-removed
#
# Untracked files:
#   (use "git add <file>..." to include in what will be committed)
#
#   to-be-added
no changes added to commit (use "git add" and/or "git commit -a")
git reset HEAD (without -q) gives a warning about the modified file and its exit code is 1 which will be considered as an error in a script.

Edit: git checkout HEAD to-be-modified to-be-removed also works for unstaging, but removes the change completely from the workspace