title = Adding changes step by step
cards = add commit

[description]

The index is really useful, because it allows us to be precise about which changes we want to include in each commit!

[setup]

echo "A small, but heavy glass ball. It is not touching the book." > ball
echo "A thin book, that's standing upright." > book
echo "A candle, burning with a blue flame." > candle

git add .
git commit -m "The beginning"

[win]

# Make changes to all three objects!
test "$(git diff --name-only | wc -l)" -eq 3 || file -f .git/candle-changed && touch .git/candle-changed

# Only add one of these changes!
test "$(git diff --cached --name-only | wc -l)" -eq 1 || file -f .git/candle-added && touch .git/candle-added

# And make a commit.
COUNT=0
for commit in $(git cat-file --batch-check='%(objectname) %(objecttype)' --batch-all-objects | grep 'commit$' | cut -f1 -d' '); do
    if test "$(git diff --name-only $commit $commit^ | wc -l)" -eq 1; then
        COUNT=$((COUNT+1))
    fi
done

test "$COUNT" -ge 1

# Make a second commit that only records a single change.
COUNT=0
for commit in $(git cat-file --batch-check='%(objectname) %(objecttype)' --batch-all-objects | grep 'commit$' | cut -f1 -d' '); do
    if test "$(git diff --name-only $commit $commit^ | wc -l)" -eq 1; then
        COUNT=$((COUNT+1))
    fi
done

test "$COUNT" -ge 2

# And a third one.
COUNT=0
for commit in $(git cat-file --batch-check='%(objectname) %(objecttype)' --batch-all-objects | grep 'commit$' | cut -f1 -d' '); do
    if test "$(git diff --name-only $commit $commit^ | wc -l)" -eq 1; then
        COUNT=$((COUNT+1))
    fi
done

test "$COUNT" -ge 3