Ansible: how to construct a variable from another

Question

Here is my problem I need to use one variable 'target_host' and then append '_host' to it's value to get another variable name whose value I need. If you look at my playbook. Task nbr 1,2,3 fetch the value of variable however nbr 4 is not able to do what I expect. Is there any other way to achieve the same in ansible?

   ---
    - name: "Play to for dynamic groups"
      hosts: local 
      vars:
        - target_host: smtp
        - smtp_host: smtp.max.com
      tasks:
        - name: testing
          debug: msg={{ target_host }}
        - name: testing
          debug: msg={{ smtp_host }}
        - name: testing
          debug: msg={{ target_host }}_host
        - name: testing
          debug: msg={{ {{ target_host }}_host }}


Output:

TASK: [testing] *************************************************************** 
ok: [127.0.0.1] => {
    "msg": "smtp"
}

TASK: [testing] *************************************************************** 
ok: [127.0.0.1] => {
    "msg": "smtp.max.com"
}

TASK: [testing] *************************************************************** 
ok: [127.0.0.1] => {
    "msg": "smtp_host"
}

TASK: [testing] *************************************************************** 
ok: [127.0.0.1] => {
    "msg": "{{{{target_host}}_host}}"
}

Answer 1

Seems to me you can just use the var option instead of msg:

  debug: var="{{ target_host }}_host"

Gives:

TASK [testing] ********************************************************************************************************************************
ok: [localhost] => {
    "smtp_host": "smtp.max.com"
}

Answer 2

You need to put quotes around it:

- hosts: local
  vars: [ target_host: smtp ]
  tasks:
    debug: msg="{{ target_host }}_host"

-- edit --

Kashyap I need to go one more level than this. Imagine there is another variable 'smtp_host' and I want to construct that variable at runtime using another variable(target_host) and attaching a string '_host' to it. = {{ {{ target_host }}_host }} – Max

My bad. Didn't read carefully enough.

This (AFAIK) isn't possible. The primary limitation that stops us doing this (no matter how you spin it), is 'variable expansion' in ansible is a single pass process and what you want requires multiple-passes.

Only [seriously hacky] ways I can think of are:

• Create the playbook dynamically from your playbook using template and execute it.
• I heard that Jinja2 engine does multi-pass evaluation. May be if you put these strings in a template and then use the lookup('template', ...) filter. Unfortunately I have no experience with Jinja2 templates so not quite sure if this is even an option.

Answer 3

I'm currently using the array-like syntax of Jinja 2. I don't think this is a great solution, but I've yet to find something better.

Let me give an example with one of my abstracted tasks. See my variable configuration and example task below:

# Variables file, available in the task context
containers:
  app:
    image: mynamespace/myappcontainer:snapshot
  web:
    image: nginx:latest
  db:
    image: mariadb:latest



# Example task
- name: Start containers
  docker_container:
    name: "{{ item }}"
    image: "{{ containers[item].image }}"
  with_items:
    - app
    - web
    - db

In the above example I'm using the with_items Ansible loop, which runs the task for each item and makes the {{ item }} variable available accordingly.
This results in creating 3 Docker containers each with the proper container name based on the items list, and the proper image retrieved from the external variables I've configured.

Even though this example uses with_items, it is of course adaptable to your problem with use of your own variables.

Although this works perfectly fine in this situation, I'm afraid this requires the variables you'd like to access to be part of some parent variable (containers in this example). Therefore I'd recommend to split variables with a . to construct a hierarchy, and not with a _.

A variable like a.b.c where b is dynamic, would be accessible using a[b].c.
A variable like a.b where b is dynamic, would be accessible using a[b].

---

A solution you would use might look like (untested):

- name: "Play to for dynamic groups"
  hosts: local 
  vars:
    - target: smtp
    - hosts:
        smtp: smtp.max.com
        imap: imap.max.com
  tasks:
    - name: testing
      debug: msg={{ hosts[target] }}

Note that the variables are configured slightly differently, because it's structure is hierarchical.

Answer 4

You have two ways to choose:
1. General using.

vars:
    - target_host: smtp
    - smtp: smtp.max.com
tasks: 
    - name: testing
        debug: msg={{ target_host }}
    - name: testing
        debug: msg={{ smtp }}
    - name: testing
        debug: msg={{ vars[target_host] }}

2. Using fact

tasks: 
    - set_fact: target_host=smtp
    - set_fact: smtp=smtp.max.com
    - name: testing
        debug: msg={{ target_host }}
    - name: testing
        debug: msg={{ smtp }}
    - name: testing
        debug: msg={{hostvars[inventory_hostname][target_host]}}

Answer 5

You can nest your lookups like so:

---
- hosts: local
  connection: local
  gather_facts: no
  vars:
    target_host: smtp
    lookup_host: "{{ target_host }}_host"
    smtp_host: smtp.max.com
  tasks:
    - debug: var="{{ lookup_host }}"

Answer 6

If you have a variable like

vars: myvar: xxx xxx_var: anothervalue

the working Ansible syntax:

- debug: msg={{ vars[myvar + '_var'] }}

will give you the analogue of:

- debug: msg={{ xxx_var }}