Bash script for resolving Docker ports

Docker containers can expose ports to outer world when needed. This is done by giving "-P" flag to docker run -command. This will publish all exposed ports to "a random high port from the range 49000 to 49900" (from Docker userguide). Even though the user guide doesn't explicitly say that the docker daemon will track what ports are published, I would guess that it does.

Publishing to random ports is useful as then you can have multiple containers running at the same. We need this for our CI -setup. But there's also requirement for accessing container from outside, so we need to resolve the port published by -P in build scripts.

"docker inspect" is a command which can be used for this. It takes "--format=template" parameter, which can be used to output information about container.

So following bash script resolves public port for given container name and exposed port.

#!/bin/bash
set -o nounset
set -o errexit

function resolvePort() {
  local container=$1
  local exposedPort=$2
  local port=$(docker inspect --format='{{range $p, $conf := .NetworkSettings.Ports}}{{if eq $p "'$exposedPort'/tcp"}}{{(index $conf 0).HostPort}}{{end}} {{end}}' $container)
  echo $port
}

 The magic happens in

{{range $p, $conf := .NetworkSettings.Ports}}{{if eq $p "'$exposedPort'/tcp"}}{{(index $conf 0).HostPort}}{{end}} {{end}}

In easier to read format:

{{range $p, $conf := .NetworkSettings.Ports}}
    {{if eq $p "'$exposedPort'/tcp"}}
        {{(index $conf 0).HostPort}}
    {{end}}
{{end}}

The "{{range $p, $conf := .NetworkSettings.Ports}}" iterates over ports configuration. It is like map, and one key-value -pair looks something like this

"80/tcp": [
  {
      "HostIp": "0.0.0.0",
      "HostPort": "49101"
  }
] 

$p is the key and $conf is the value. $p is the exposed port and its value is something like "80/tcp".

Then there's {{if eq $p "'$exposedPort'/tcp"}}, which is simple comparison.

The value of $conf is a array, and in this use case, we just need the first value (also there is just one). So in

{{(index $conf 0).HostPort}}

(index $conf 0) gives just that, and then (index $conf 0).HostPort returns 49101.

This can be then used

readonly publicPort=$(resolvePort containerName 80)
curl localhost:$publicPort

We might have been able to avoid this by using another container for tests and doing container linking, but this seems to work okay. And I wanted to learn about docker inspect --format :)