Ansible: Set variable to file content

You can use the slurp module to fetch a file from the remote host: (Thanks to @mlissner for suggesting it)

vars:
  amazon_linux_ami: "ami-fb8e9292"
  user_data_file: "base-ami-userdata.sh"
tasks:
- name: Load data
  slurp:
    src: "{{ user_data_file }}"
  register: slurped_user_data
- name: Decode data and store as fact # You can skip this if you want to use the right hand side directly...
  set_fact:
    user_data: "{{ slurped_user_data.content | b64decode }}"

You can use fetch module to copy files from remote hosts to local, and lookup module to read the content of fetched files.

You can use lookups in Ansible in order to get the contents of a file, e.g.

user_data: "{{ lookup('file', user_data_file) }}"

Caveat: This lookup will work with local files, not remote files.

Here's a complete example from the docs:

- hosts: all
  vars:
     contents: "{{ lookup('file', '/etc/foo.txt') }}"
  tasks:
     - debug: msg="the value of foo.txt is {{ contents }}"