Container Remain Running for postgres and redis when no process running on it

Docker
#1

As taught in docker lecture when we run a container example- using image postgres it will pull the image from docker hub and run container and exited if no process running on it. But in lecture of docker compose–>Demo-Example voting application when redis and postgres container run using docker run command they remain in running state also no process is running on that?

#2

Those containers have applications running. For example when you start a redis container, redis will run.

#3

so if i run a container of mysql image it will also be running?

#4

Yes when you run the mysql container with the right options then mysql will be running.

I see in your first post you refer to postgres. This needs env vars set - it is always best to run docker containers in the foreground first to see output until you are comfortable in how they work:

$ docker run postgres
Error: Database is uninitialized and superuser password is not specified.
       You must specify POSTGRES_PASSWORD to a non-empty value for the
       superuser. For example, "-e POSTGRES_PASSWORD=password" on "docker run".

       You may also use "POSTGRES_HOST_AUTH_METHOD=trust" to allow all
       connections without a password. This is *not* recommended.

       See PostgreSQL documentation about "trust":
       https://www.postgresql.org/docs/current/auth-trust.html