How to stage only part of a new file with git?

Question

I love git add --interactive. It is now part of my daily workflow.

The problem seems that it does not work with untracked files. What I wan

Answer 1

git update-index --add --cacheinfo 100644 $(git hash-object -w /dev/null) newfile
git add --interactive newfile

Simple demo:

mkdir /tmp/demo
cd /tmp/demo
git init .

echo hello > newfile
git update-index --add --cacheinfo 100644 $(git hash-object -w /dev/null) newfile

• _{Hint If you're sure the 'empty' blob already exists in your git object database, you could hardcode the hash e69de29bb2d1d6434b8b29ae775ad8c2e48c5391 instead. I don't recommend doing that_}
• _{Hint If you're on Windows, you probably can just use NUL: instead of /dev/null. Otherwise, use something like echo -n '' | git hash-object --stdin -w}

Now the index will contain newfile as the empty blob, and the empty blob has been entered into the object database if it didn't exist yet:

$ find .git/objects/ -type f 
.git/objects/e6/9de29bb2d1d6434b8b29ae775ad8c2e48c5391

$ git status
# On branch master
#
# Initial commit
#
# Changes to be committed:
#   (use "git rm --cached ..." to unstage)
#
#   new file:   newfile
#
# Changed but not updated:
#   (use "git add ..." to update what will be committed)
#   (use "git checkout -- ..." to discard changes in working directory)
#
#   modified:   newfile
#

$ git diff
diff --git a/newfile b/newfile
index e69de29..ce01362 100644
--- a/newfile
+++ b/newfile
@@ -0,0 +1 @@
+hello

This should be precisely what you want. May I also recommend the vim fugitive plugin for very intelligent index management (see Better git add -p?)