Docker Run Command

The docker run command is used to create a container from an image. It can be executed in a few different ways to provide images in different formats.

Create a Persistent Container:

• docker container run --name my-postgres -e POSTGRES_PASSWORD=password -d postgres
  • Here I am creating a container named 'my-postgres'. This container will exist on my local file system and can be started/stopped at will.
  • Because I have used the -d flag the container will be started in the background.
• You can see the container by running:
  • docker container ls | grep my-postgres.

  1234567890 postgres "docker-entrypoint.s…" 27 seconds ago Up 26 seconds 5432/tcp my-postgres
  

• Now we can attach to it with a bash shell:
  • docker exec -it my-postgres bash.
    • From inside this shell we can run a program (like psql in this case).
• Start a container with the psql shell:
  • docker exec -it my-postgres psql -U postgres
    • I should be in an interactive psql session within my container.

Create a Temporary Container:

Each time we execute the run command and specify a --name flag, we create a container by that name on our host. Therefore, if we run our docker run command again, we will get an error to the effect of: 'this container already exists'. We can circumvent this issue by specifying a different value for the --name flag. Alternatively, we can also use the --rm flag.

• docker container run --name my-postgres -it --rm -e POSTGRES_PASSWORD=password postgres bash
  • Here I get an interactive temporary container that will not be saved to my file system upon exit.
• docker container run --name my-postgres --rm -e POSTGRES_PASSWORD=password -d postgres
  • Here I am starting my new container in the background. It will exist until it is stopped. At which point it will be deleted due to the --rm flag.

Interactive Session:

• As noted above, using -it and passing a shell to the container like: bash will allow me to connect to a shell instance inside my running container.
• docker container run --name my-postgres -it --rm -e POSTGRES_PASSWORD=password postgres bash
• If you are debugging an application like rails for instance and want to use a debugger, make sure you specify the -it. Otherwise you may spend a lot of time trying to determine why your server sessions is running but not hitting any of your breakpoints. I speak from experience :)

Connect to Your Running Container from your Host:

• If I expose the container's listen port to my host machine via the -p flag, then I can connect to the running server on my container from a program on my host.
• docker container run --name my-postgres -p 5433:5432 -it --rm -e POSTGRES_PASSWORD=password -d postgres
  • Here I am saying expose port 5432 to my host via port 5433.
  • I now should be able to use any postgres gui client installed on my laptop to connect to the running postgres server in my container.