Rename the default branch on all your GitHub repos. Warning: this script deletes the old default branch, which makes GitHub *automatically close* open PRs to that branch.

rename-github-repos.sh

#! /usr/bin/env bash
set -e

oldBranch="master"
newBranch="main"

# {{{ Colors, logging boilerplate
readonly PROG_NAME="$0"

function RESET            { echo -e "\e[0m";  }
function BOLD             { echo -e "\e[1m";  }
function RESET_BOLD       { echo -e "\e[21m"; }
function DIM              { echo -e "\e[2m";  }
function RESET_DIM        { echo -e "\e[22m"; }
function UNDERLINED       { echo -e "\e[4m";  }
function RESET_UNDERLINED { echo -e "\e[24m"; }
function BRRED            { echo -e "\e[31m"; }
function RED              { echo -e "\e[91m"; }
function BRGREEN          { echo -e "\e[32m"; }
function GREEN            { echo -e "\e[92m"; }
function BRYELLOW         { echo -e "\e[33m"; }
function YELLOW           { echo -e "\e[93m"; }
function BRBLUE           { echo -e "\e[34m"; }
function BLUE             { echo -e "\e[94m"; }
function BRPURPLE         { echo -e "\e[35m"; }
function PURPLE           { echo -e "\e[95m"; }
function BRCYAN           { echo -e "\e[36m"; }
function CYAN             { echo -e "\e[96m"; }
function BRGRAY           { echo -e "\e[37m"; }
function GRAY             { echo -e "\e[97m"; }
function RESET_FG         { echo -e "\e[39m"; }

function now { date +%FT%T; }

# _log COLORS LABEL [MESSAGE [...]]
function _log {
    color="$1"
    shift
    level="$1"
    shift
    echo -n "$color$level $PROG_NAME ${color}[$(now)]:" "$@"
    RESET
}
# }}}

function dbg   { _log "$(GRAY)"         "[debug]" "$@"; }
function info  { _log "$(BRGREEN)"      "[info] " "$@"; }
function warn  { _log "$(BRYELLOW)"     "[warn] " "$@"; }
function error { _log "$(BRRED)"        "[error]" "$@"; }
function fatal { _log "$(BOLD)$(BRRED)" "[FATAL]" "$@"; exit 1; }
function cmd   { _log "$(CYAN)"         "[run]  " "\$ $(BOLD)$(UNDERLINED)$*"; }

function rename {
    repo="$1"
    clone_url="https://github.com/$repo.git"
    oldRef=$(git ls-remote "$clone_url" "refs/heads/$oldBranch")
    if [[ -z "$oldRef" ]]; then
        refs=$(git ls-remote "$clone_url")
        if [[ -z "$refs" ]]; then
            warn "$repo: no branches/refs found; probably empty"
        else
            headRef=$(echo "$refs" | grep -E $'\t''HEAD$' | cut -f1)
            defaultBranch=$(echo "$refs" \
                | grep -E "^$headRef"$'\t' \
                | cut -f2 \
                | grep -vE '^HEAD$' \
                | sed 's|^refs/heads/||')
            if [[ "$defaultBranch" = "$newBranch" ]]; then
                info "$repo: no $oldBranch branch: default branch is already $defaultBranch"
            else
                warn "$repo: no $oldBranch branch: default branch is currently $defaultBranch"
            fi
        fi
        return
    fi

    newRef=$(git ls-remote "$clone_url" "refs/heads/$newBranch")

    tmp=$(mktemp -d)
    cmd "git clone $clone_url $tmp"
    git clone -q "$clone_url" "$tmp"
    pushd "$tmp" >/dev/null || return

    if [[ -z "$newRef" ]]; then
        # no $newBranch branch; create
        info "$repo: creating $newBranch branch"
        cmd "git branch -m $oldBranch $newBranch"
        git branch -m "$oldBranch" "$newBranch"
        cmd "git push -u origin $newBranch"
        git push -q -u origin "$newBranch" || return
    fi

    git switch -q "$newBranch"
    info "$repo: setting default branch to $newBranch"
    hub api --obey-ratelimit -XPATCH "repos/$repo" --field "default_branch=$newBranch" >/dev/null

    info "$repo: removing $oldBranch branch"
    cmd "git push origin --delete $oldBranch"
    git push -q origin --delete "$oldBranch"
    cmd "git remote set-head origin -a"
    git remote set-head origin -a

    popd >/dev/null || return
    info "$repo: deleting tmpdir"
    rm -rf "$tmp"
}

if ! hub --version >/dev/null; then
    fatal "hub not found; see $(UNDERLINED)https://github.com/github/hub"
fi

if ! jq --version >/dev/null; then
    fatal "jq not found; see $(UNDERLINED)https://stedolan.github.io/jq/"
fi

info "Listing user repositories; this requires a GitHub API call so it'll list about 50 repos/second"
repos=$(hub api \
        --method GET \
        --paginate \
        --obey-ratelimit \
        user/repos \
        --field 'type=owner' \
    | jq --raw-output \
        --arg branch "$newBranch" \
        '.[]
        | select(.fork or .archived or .default_branch == $branch | not)
        | .full_name')
reposCount=$(echo "$repos" | wc -l)
dbg "Found $reposCount repos: $repos"

i=0  # repo counter just for progress
while IFS="" read -r repo
do
    (( i += 1 ))
    info "$i / $reposCount"
    rename "$repo"
done < <(echo "$repos")