git command to recover the modification

without using git add

for single revised file:

git checkout -- filename

for all the revised files:

git checkout .

with using git add

for single added file:

git reset HEAD filename

for all the added files:

git reset HEAD 

with using git commit

return to the last commit state:

git reset --hard HEAD^

return to the specific commit state:

# git log: query the commit ids
git reset --hard commit_id