How can I find empty git commits?

Question

What command(s) can I use to find empty commits in a git repository, i.e. the commits that would be removed by git filter-branch --prune-empty?

Answer 1

As a first approximation, list all commits in reverse order and log any line that has the same tree hash as the one before:

git log --all --reverse --format='%H %t' | while read h t; do
  if [ "$lt" = "$t" ]; then
    echo "$h"
  fi
  lt="$t"
done

You could improve this by ignoring any commit with multiple parents and confirming that the logged line is actually a child of the one before.

Answer 2

You'd want to exclude parentless commits and merge commits, then see which ones have the same tree as their parent.

for sha in $(git rev-list --min-parents=1 --max-parents=1 --all)
do
   if [ $(git rev-parse ${sha}^{tree}) == $(git rev-parse ${sha}^1^{tree} ) ]
   then
       echo "${sha} will be pruned"
   fi
done