git-main-branch

#!/bin/sh
set -eu

# Bail early with git's own error message if not in a repo
git is-repo

probe_branch_name() {
    local full_ref="$(git symbolic-ref refs/remotes/origin/HEAD 2>/dev/null)"
    local branch_name="${full_ref#refs/remotes/origin/}"
    if [ -n "$branch_name" ]; then
        echo "$branch_name"
        exit 0
    fi
}

# Step 1: Check what the origin's HEAD points to (fast, local-only)
probe_branch_name

# Step 2: If no origin exists (e.g. newly initialized repo), try common branch names
for probe in main mainline master production; do
    if git local-branch-exists "$probe"; then
        echo "$probe"
        exit 0
    fi
done

# Step 3: Ask the remote for its HEAD (expensive network call, but only needed once)
if git remote set-head origin --auto 2>/dev/null; then
    probe_branch_name
fi

echo "No main branch found" >&2
exit 2