git-rebase-head

#!/usr/bin/env bash
#
# While a rebase or cherry-pick is paused due to conflicts, this helper prints the SHA of the
# commit being rebased or cherry-picked.
#
# In such a conflicted state, HEAD will typically point to the most recently applied commit,
# which does not exist in the original branch, and corresponds to the parent of the commit
# currently being applied.

set -eo pipefail

git_dir="$(git dir)"
rebase_head="$git_dir/REBASE_HEAD"
if [ -f "$rebase_head" ]; then
    cat "$rebase_head"
    exit 0
fi

cherry_pick_head="$git_dir/CHERRY_PICK_HEAD"
if [ -f "$cherry_pick_head" ]; then
    cat "$cherry_pick_head"
    exit 0
fi

# During an `exec` phase (e.g. via `git rebase -x <cmd> …`), REBASE_HEAD is not present, but we can
# parse the last `pick`ed SHA from `.git/rebase-merge-done`.
rebase_done="$git_dir/rebase-merge/done"
if ! [ -f "$rebase_done" ]; then
    echo "No REBASE_HEAD, CHERRY_PICK_HEAD, or rebase-merge-done found in Git dir $git_dir" >&2
    exit 11
fi

# Extract the SHA from the last `pick`, `edit`, or `merge` line.
# For merge lines, the format is "merge -C <sha> <label>"
last_line="$(cat "$rebase_done" | grep -E '^(pick|edit|merge)' | tail -n1)"
if echo "$last_line" | grep -q '^merge'; then
    # Extract SHA from merge line (format: "merge -C SHA label")
    sha="$(echo "$last_line" | awk '{print $3}')"
else
    # Extract SHA from pick/edit line (format: "pick SHA")
    sha="$(echo "$last_line" | awk '{print $2}')"
fi
echo "$sha"