Getting the name of the parent origin branch in git?

Question

I've cloned some git repository, and did "git branch abcd", whcih switched me to a branch derived from origin/abcd. abcd is not the default origin branch. Then, I have created a feature branch "my_feature" out of abcd. I want to do "git merge origin/abcd" into my_feature in a script that would be applicable regardless of the name of the origin branch used (or at least applicable to easy cases when there are no complex branch structures described in some other answers about git).

How do I find what is the "closest"/"parent" origin branch off which the current branch was created?

Answer 1

This is hard to do well. In git, a branch is just an auto-advancing pointer to a commit, and a commit can have any number of branch names on it. Consider this case:

your master:      y1---y2---y3
                 /
master: a---b---c---d---e
                 \
feature:          f1---f2---f3---f4

You checked out branch "master" at c, and committed y1, y2, and y3. Your history thus looks like a b c y1 y2 y3. Meanwhile master has advanced to d and e, but someone created a feature branch and committed f1 through f4 based on c. Git has no way to determine that your branch came from master instead of feature, so at best you will have a choice of branches to merge.

Were you to do this automatically you would have to apply a heuristic about picking the shortest branch, or the longest branch, or the one with the most/least commits, or something else like that. Naturally, since there are so many options, it's not really a good choice for a git built-in function. However, using git's "plumbing" functions, you can write your own:

#!/bin/bash

# Tries to determine a good merge base from among all local branches.
# Here used, a "good" merge base is the one sharing the most recent commit
# on this branch. This function will exit 1 if no branch is found,
# or exit 2 in a tie.
#
# Untested - use at your own risk.

MAX_SEARCH=20   # only search the last 20 commits on this branch
FOUND=0
LAST_BRANCH=

# iterate through the commits, most recent first
for COMMIT in $(git rev-list --max-count=$MAX_SEARCH HEAD); do
  # check every local branch
  for BRANCH in $(git for-each-ref --format="%(refname)" refs/heads); do
    # look for the commit in that branch's history
    if (git rev-list $BRANCH | fgrep -q COMMIT); then
      echo $BRANCH
      FOUND=$((FOUND + 1))
      LAST_BRANCH="$BRANCH"
    fi
  done
  if [ $FOUND -gt 1 ]; then
    # more than one choice; exit
    exit 2
  elif [ $FOUND -eq 1 ]; then
    git merge $LAST_BRANCH
    exit 0
  fi
done
exit 1   # could not find a parent