https://github.com/firebase/firebase-tools/issues/1167#issuecomment-640294000

seed-storage.sh

#!/bin/bash

# seed-storage
# ============
# Seed an existing firebase cloud storage emulator instance. Run it inside your
# project's firebase folder, ie: ./seed-storage.sh <project-name> <dest-folder>

name=firebase_export                # firebase export dir name
project=$1                          # project name
root=$2                             # export root path
uri=gs://$project.appspot.com/$name # cloud storage URI

trap "kill 0" EXIT
echo '[1/5] Preparing local/remote filesystems for seeding…'

# check if a remote file exists and wipe parent dir if needed
gsutil -q stat $uri/$name.overall_export_metadata
[ $? -eq 0 ] && gsutil -q rm -f ${uri}**

# create root path if needed
mkdir -p $root

echo '[2/5] Starting emulators in the background…'

# start emulator if port isn't open
lsof -i TCP:4000 &> /dev/null
[ $? -eq 0 ] || screen -S $name -d -m firebase emulators:start &

# wait until server is detected
lsof -i TCP:4000 &> /dev/null
while [ $? -ne 0 ]; do sleep 5; done

# exit on any error from now on
trap "exit" INT ERR

# create importable file/dir structure
firebase --non-interactive --project $project emulators:export --force $root &> /dev/null

echo "[3/5] Exporting firestore snapshot to $uri"
gcloud --verbosity=none firestore export $uri &> /dev/null

echo "[4/5] Downloading exported data to $root"
gsutil -q cp -r $uri $root

echo '[5/5] Cleaning up…'
screen -S $name -X quit

echo '\nDone! Use the following command to run the emulator:'
echo "firebase emulators:start --import=$root"