Adding strings to an array in Ansible

Jun 16, 2017

From time to time, I need to dynamically build a list of strings (or a list of other things) using Ansible's set_fact module.

Since set_fact is a module like any other, you can use a with_items loop to loop over an existing list, and pull out a value from that list to add to another list.

For example, today I needed to retrieve a list of all the AWS EC2 security groups in a region, then loop through them, building a list of all the security group names. Here's the playbook I used:

- hosts: localhost
  connection: local
  gather_facts: no

  vars:
    ec2_security_group_names: []

  tasks:
    - name: Get security groups from EC2.
      ec2_group_facts:
        region: us-east-1
        filters:
          "tag:myapp": "example"
      register: ec2_security_groups

    - name: Build a list of all the security group names.
      set_fact:
        ec2_security_group_names: "{{ ec2_security_group_names }} + [ '{{ item.group_name }}' ]"
      with_items: "{{ ec2_security_groups.security_groups }}"

    - name: Print the security group names to the console.
      debug: var=ec2_security_group_names

Before building the ec2_security_group_names list, I made sure to set it as an empty list in vars (ec2_security_group_names: []). That way the first time the loop adds a group_name, it has something to add to (an empty list)!

When run, you get something like:

TASK [debug] ***********************************************************************************************************
ok: [localhost] => {
    "changed": false, 
    "ec2_security_group_names": [
        "security-group-1", 
        "security-group-2", 
        "security-group-3", 
    ]
}

Further reading: