Install node dependencies in async way for multiple projects.

build.sh

#!/bin/bash

# install dependencies for all react projects in background (async way)
cd app/frontend/app_a && yarn install &
cd app/frontend/app_b && yarn install &
cd app/frontend/app_c && yarn install &

# wait for all processes to finish before move on (sync step)
wait

# build the react apps (async way)
cd app/frontend/app_a && yarn build &
cd app/frontend/app_b && yarn build &
cd app/frontend/app_c && yarn build &

# wait for all processes to finish before complete the bash script (sync step)
wait