Evaluating return code in ansible conditional

I'm working on automating a task which needs to append the latest version of software to a file. I don't want to it do this multiple times for the same version.

It looks at the following example file:

var software releases = new Array(
    "4.3.0",
    "4.4.0",
    "4.5.0",
    "4.7.0",
    "4.8.0",
    "4.11.0",
    "4.12.1",
    "4.14.0",
    "4.15.0",
    "4.16.0",
);

the defaults main.yml would pass in something like

VERSION: 4.16.2

code

- name: register version check
  shell: cat /root/versions.js | grep -q {{VERSION}}
  register: current_version

- debug: msg="The registered variable output is {{ current_version.rc }}"

- name: append to versions.js
  lineinfile:
    dest: /root/versions.js
    regexp: '^\);'
    insertbefore: '^#\);'
    line: "    \"{{VERSION}}\",\n);"
    owner: root
    state: present
    when: current_version.rc == 1

problem: the debug message is evaluating current_version.rc and showing me boolean values based on the grep commands output, but I cant re-use this in the when conditional to determine if the task should be run.

Edit: the output:

PLAY [localhost] **************************************************************

GATHERING FACTS ***************************************************************
ok: [localhost]

TASK: [test | register version check] *****************************************
failed: [localhost] => {"changed": true, "cmd": "cat /root/versions.js | grep -q 3.19.2", "delta": "0:00:00.003570", "end": "2015-12-17 00:24:49.729078", "rc": 1, "start": "2015-12-17 00:24:49.725508", "warnings": []}

FATAL: all hosts have already failed -- aborting

PLAY RECAP ********************************************************************
           to retry, use: --limit @/root/site.retry

localhost                  : ok=1    changed=0    unreachable=0    failed=1

As nikobelia pointed out in the comments, grep returns an exit code of 1 when it doesn't match any lines. Ansible then interprets this (actually any status code other than 0 from a shell/command task) as an error and so promptly fails.

You can tell Ansible to ignore the response code from the shell/command task by using ignore_errors. Although with grep this will ignore actual errors (given by a return code of 2) so instead you might want to use failed_when like this:

- name: register version check
  shell: cat /root/versions.js | grep -q {{VERSION}}
  register: current_version
  failed_when: current_version.rc == 2