Ansible - Advanced shell command execution format

Question

I have 3 variables named IPOctet, ServerIPRange and epcrange. If I perform the following operation in my terminal, it works perfectly

IPOctet=$(echo "$ServerIPRange/$epcrange+$IPOctet" | bc)

How to I do something similar in a ansible inside a task, for e.g

---
- hosts: localhost
  gather_facts: False

  vars_prompt:
    - name: epcrange
      prompt: Enter the number of EPCs that you want to configure
      private: False
      default: "1"
    - name: serverrange
      prompt: Enter the number of Clients that you want to configure
      private: False
      default: "1"
    - name: ServerIPRange
      prompt: Enter the ServerIP range
      private: False
      default: '128'
    - name: LastIPOctet
      prompt: Enter The last Octet of the IP you just entered
      private: False
      default: '10'

  pre_tasks:


    - name: Set some facts
      set_fact:
        ServerIP1: "{{ServerIP}}"
        ServerIPRange1: "{{ServerIPRange}}"
        IPOctet: "{{LastIPOctet}}"

    - name: local action math
      local_action: shell {{IPOctet}}=$(echo "${{ServerIPRange}}/${{epcrange}}+${{IPOctet}}" | bc)  # Proper Syntax?
      with_sequence: start=1 end=4
      register: result
      ignore_errors: yes

What is the proper syntax for this command? Maybe using shell echo "......." . I just need to save the contents of this command into the IPOctet variable and IPOctet will change with each loop iteration and the results should be stored in my result register

P.S: how can I access the individual items in the array separately?

Edit: Is anything like this possible, currently it just does the calculation once and stores it 4 times in the register...

- name: bashless math
  set_fact:
    IPOctet: "{{ (ServerIPRange|int/epcrange|int)+IPOctet|int }}"
  register: IPOctet
  with_sequence: "start=1 end={{stop}} "
  register: my_ip_octet

Solution 1

1. Your terminal expression reassigns the IPOctet shell variable, so it gives a different result each time it is executed. This is fine, but difficult to reproduce in Ansible:

   $ IPOctet=10 ServerIPRange=128 epcrange=1
   $ IPOctet=$(echo "$ServerIPRange/$epcrange+$IPOctet" | bc); echo $IPOctet
   138
   
   $ IPOctet=$(echo "$ServerIPRange/$epcrange+$IPOctet" | bc); echo $IPOctet
   266
   

2. The syntax: "shell {{IPOctet}}=$(echo ..." does NOT assign to the Ansible variable. The shell attempts to execute a command like "10=138", which is not found.

3. When register is used within a loop, the target variable is not set until the loop completes - so your expression always sees the original value for {{IPOctet}}.

4. A solution is to run the whole loop as a single shell command:

   - name: local action math2
     local_action: shell IPOctet={{IPOctet}}; for i in 1 2 3 4; do IPOctet=$(expr {{ServerIPRange}} / {{epcrange}} + $IPOctet); echo $IPOctet; done
     register: result
   

   NOTE: I've used the expr command rather than bc, but the results are the same.

5. You can iterate over these results using result.stdout_lines:

   - name: iterate results
     local_action: debug msg={{item}}
     with_items: result.stdout_lines
   

Solution 2

Firstly your Jinja template is incorrect, every single variable needs to be surrounded with a pair of brackets. You can not use multiple variables within single pair of brackets. For example,

{{ ServerIPRange }}

Secondly, set_fact is used only to set a fact value. You can not run shell commands using set_fact. You should use shell module instead.

- name: local action math
  local_action: shell {{ IPOctet }}=$(echo {{ ServerIPRange|int }}/{{ epcrange|int }}+{{ IPOctet|int }}" | bc)
  with_sequence: start=1 end=4
  register: result
  ignore_errors: yes

Ansible will do the calculation 4 times and store it in a list as 4 different elements. You can check what all is stored inside this list and can even access it by looping over it.

- debug: msg={{ result }}

Hope this helps :)