What's correct Jinja2 operator to do "less than" on a string?

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1

I’m setting an Ansible fact like this:

set_fact: server_num={{ ansible_hostname[-2:] }}

server_num can take these values:

‘00’
‘01’
‘02’
‘03’
‘04’
ad infinitum

If I want a conditional that will evaluate to True for all server numbers less than a particular server number ‘n’ I’d like to be able to do this:

when: server_num < ‘n’

However, this throws the error "error while evaluating conditional: server_num < ‘n’

I could do this but clearly it doesn’t scale:

when: server_num == ‘00’ or server_num == ‘01’ or … or server_num == ‘n-1’

Is there a more succinct way to do this? I can’t seem to find a Jinja2 “less than” operator.

Thanks.

2

Hmm, this trivial playbook works for me:

  - hosts: localhost
    tasks:
    - debug: msg="Yep!"
      when: server_num < '03'

With '--extra-vars server_num=02', I get

  TASK: [debug msg="Yep!"]

3

Yea, it’s weird. Sometimes “<” works for me and sometimes it doesn’t. I couldn’t find this operator documented in the Jinja2 or Ansible docs and I don’t recall where I first learned it so I figured maybe it’s an invalid operator. I put a debug statement right before the line where I’m referencing server_num and it said it was set as I expected it to be but the statement still failed.

4

The less than operator is documented at http://jinja.pocoo.org/docs/dev/templates/#comparisons

Although, trying to use mathematical operations on string values isn’t exactly always straight forward.

You might get better results from the version_compare filter such as:

when: server_num|version_compare(‘100’, ‘lt’)

See http://docs.ansible.com/ansible/playbooks_filters.html#version-comparison-filters

5

I see. I agree that doing math on strings is somewhat questionable. I’ll check out version_compare. Thanks Matt.

6

I see. I agree that doing math on strings is somewhat questionable.

Well, strings can be sorted, and compared in that way. I wouldn't bet on
what adding 1 to a string would do, but comparisons seem legit.

http://jinja.pocoo.org/docs/dev/templates/#comparisons is where I found
the comparisons too, FWIW.

                                      -Josh (jbs@care.com)

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