Extract Ansible Fields with selectattr

When working with Ansible, processing task results to extract meaningful information is often necessary. This article demonstrates how to filter attributes and extract a field from any Ansible variables / outputs.

Intro

Let's imagine we have a registered Ansible variable called output, the structure might look like this:

ok: [localhost] => {
    "output": {
        "changed": true,
        "msg": "All items completed",
        "results": [
            {
                "ansible_loop_var": "item",
                "changed": true,
                << truncated >>
                "item": {
                    "key": "nova-api",
                    "value": null
                },
                "rc": 1,
            },
            {
                << truncated >>
                "item": {
                    "key": "nova-scheduler",
                    "value": null
                },
                "rc": 0,
            },
            {
                << truncated >>
                "item": {
                    "key": "nova-conductor",
                    "value": null
                },
                "rc": 0,
            }
        ]
    }
}

Our goal is to create a list of service names where rc is 1. Simply enough we can:

- name: Set fact for services rc == 1
  debug:
    msg: "{{ output.results | selectattr('rc', 'equalto', 1) | map(attribute='item.key') | list }}"

# output.results                  : will get the list value of results
# selectattr('rc', 'equalto', 1)  : for each element, selects one which has rc == 1
#   beside `equalto` we have:
#     match: use for regex
#     contains: check if keyword is inside a list or a string
# map(attribute='item.key')       : get the value in item.key of that element
# list:                           :form a new list of it

If we run the output will be like this:

ok: [localhost] => {
    "msg": [
        "nova-api"
    ]
}

In this example, only the nova-api service had a return code of 1, indicating an issue with this service.

Multi-condition - list active users in group docker

Suppose we have a variable with this data structure:

users:
  - name: Alice
    active: true
    groups: alice, docker
  - name: Bob
    active: false
    groups: bob, sudo
  - name: Charlie
    active: true
    groups: charlie

Now we want to filter not only active status is true but also the groups name of the users contain docker. All we need is add one more selectattr:

- name: Get list of active users in docker group
  debug:
    msg: "{{ users | selectattr('active', 'equalto', true) | selectattr('groups', 'match', '.*docker.*') | map(attribute='name') | list }}"

The output will look like this:

ok: [localhost] => {
    "msg": [
        "Alice"
    ]
}