Let's use the Realms API to download our world backup!

try_realms_api.sh

#!/bin/bash

# you need httpie (apt-get install httpie)
# you need to replace <these things> with your details
# run with 'source' to export vars to your shell for experimentation

# 1. authenticate to get an access token

auth_server=https://authserver.mojang.com
user=<your login email>
password=<your account password>
client_token=any-constant-string

# log in
response=$(http POST ${auth_server}/authenticate username=${user} password=${password} clientToken=${client_token} agent:='{"name": "Minecraft", "version": 1}')
# strip double quotes
access_token=$(echo ${response} | jq .accessToken | sed 's/"//g')

# 2. Utility: validate an access token is still valid
# http POST ${auth_server}/validate accessToken=${access_token} clientToken=${client_token}

# 3. Utility: refresh the token, getting a new accessToken without having to give password
# http POST ${auth_server}/refresh accessToken=${access_token} clientToken=${client_token}

# 4. access Realms!

# server. Note the real Minecraft client uses pc.realms.minecraft.net
realms_server=https://pc.realms.minecraft.net
# your UUID from https://mcuuid.net, I believe the trimmed version is correct
uuid=<your trimmed UUID>
# your username, e.g. notch
user_id=<your username>
# Minecraft version
version=1.11.2

# is this user allowed to access Realms? see http://wiki.vg/Realms_API
http --verbose GET ${realms_server}/mco/available "Cookie:sid=token:${access_token}:${uuid};user=${user_id};version=${version}"

For those who may want to know, bedrock's endpoint is: https://bedrock.realms.minecraft.net/

Are you sure? I'm getting an ENOTFOUND Error when I try to use that. pocket.realms.minecraft.net however does at least return my requests, so I'd expect this one to be the correct endpoint.