Postgresql: How to determine if database docker container running?

On an docker image's entrypoint shell script I want to determine if a postgresql container can listen to connections. For mysql I used the following snippet:

while ! mysqladmin ping -h"$MOODLE_DB_HOST" -P $MOODLE_DB_PORT --silent; do
  echo "Connecting to ${MOODLE_DB_HOST} Failed"
  sleep 1

How can I achieve a similar using postgresql?

On the dockerfile add:

RUN apt-get update && apt-get install -f -y postgresql-client

Then on the entrypoint script use:

  while ! pg_isready -h ${MOODLE_DB_HOST} -p ${MOODLE_DB_PORT} > /dev/null 2> /dev/null; do
    echo "Connecting to ${MOODLE_DB_HOST} Failed"
    sleep 1

Another approach

An another approach is by using netcat:

     for count in {1..100}; do
          echo "Pinging mysql database attempt "${count}
          if  $(nc -z ${DB_HOST} ${DB_PORT}) ; then
            echo "Can connect into database"
          sleep 5

Where variable ${DB_HOST} contains the database host whilst ${DB_PORT} contains the database port. Thas works on most databases (except if you want to detect its type as well where a custom script is needed).