In-Memory Merging and Merge Queues — Quick Reference

Core Command Syntax

git merge-tree --write-tree <branch1> <branch2>

Flag	Description
--write-tree	Executes 3-way merge in memory and returns root tree SHA
--merge-base=<commit>	Overrides the common merge ancestor commit
--name-only	Lists only the names of conflicting files
--no-messages	Suppresses informational merge messages
--messages	Includes informational messages (default)
--allow-unrelated-histories	Allows merging branches that share no common ancestor
--trivial-merge	Disallows 3-way file content merges

---

Common Patterns

1. Mergeability check in CI

if git merge-tree --write-tree --name-only origin/main HEAD >/dev/null 2>&1; then
  echo "Clean merge"
else
  echo "Conflicts detected"
fi

2. Synthesize a merge commit

TREE=$(git merge-tree --write-tree --no-messages origin/main feature/branch)
P1=$(git rev-parse origin/main)
P2=$(git rev-parse feature/branch)
COMMIT=$(git commit-tree "$TREE" -p "$P1" -p "$P2" -m "Merge feature/branch into main")
echo "Synthetic commit SHA: $COMMIT"

3. Extract conflicting file paths

# Output list of conflicted files
git merge-tree --write-tree --name-only origin/main feature/branch | tail -n +2

4. Rebase simulation

BASE=$(git merge-base origin/main feature/branch)
if git merge-tree --write-tree --merge-base="$BASE" origin/main feature/branch >/dev/null 2>&1; then
  echo "Clean rebase possible"
else
  echo "Rebase conflict detected"
fi

---

Exit Code Matrix

Exit code	Meaning	Action
0	Clean merge	Safe to create commit or advance queue
1	Merge conflicts	Reject pull request or eject from queue
>1	Fatal error	Check branch names and repository state