Winning at Ansible: How to manipulate items in a list!

The Problem

Ansible is a great configuration management platform with a very, very extensible language for expressing yoru infrastructure as code. It works really well for common workflows (deploying files, adding authorized_keys, creating new EC2 instances, etc), but its limitations become readily apparent as you begin embarking in more custom and complex plays.

Here’s a quick example. Let’s say you have a playbook that uses a variable (or var in Ansible-speak) that contains a list of tables, like this:

important_files:
- file_name: ssh_config
file_path: /usr/shared/ssh_keys
file_purpose: Shared SSH config for all mapped users.
- file_name: bash_profile
file_path: /usr/shared/bash_profile
file_purpose: Shared .bash_profile for all mapped users.

(You probably wouldn’t manage files in Ansible this way, as it already comes with a fleshed-out module for doing things with files; I just wanted to pick something that was easy to work with for this post.)

If you wanted to get a list of file_names from this var, you can do so pretty easily with set_fact and map:

- name: "Get file_names."
set_fact:
file_names: "{{ important_files | map(attribute='file_name') }}"

This should return:

[ u'/usr/shared/ssh_keys', u'/usr/shared/bash_profile' ]

However, what if you wanted to modify every file path to add some sort of identifier, like this:

[ u'/usr/shared/ssh_keys_12345', u'/usr/shared/bash_profile_12345' ]

The answer isn’t as clear. One of the top answers for this approach suggested extending upon the map Jinja2 filter to make this happen, but (a) I’m too lazy for that, and (b) I don’t want to depend on code that might not be on an actual production Ansible management host.

The solution

It turns out that the solution for this is more straightforward than it seems:

- name: "Set file suffix"
set_fact:
file_suffix: "12345"

- name: "Get and modify file_names."
set_fact:
file_names: "{{ important_files | map(attribute='file_name') | list | map('regex_replace','(.*)','\\1_{{ file_suffix }}') | list }}"

Let’s break this down and explain why (I think) this works:

  • map(attribute='file_name') selects items in the list whose key matches the attribute given.
  • list casts the generated data structure back into a list (I’ll explain this below)
  • map('regex_replace','$1','$2') replaces every string in the list with the pattern given. This is what actually does what you want.
  • list casts the results back down to a list again.

The thing that’s important to note about this (and the thing that had me hung up on this for a while) is that every call to map (or most other Jinja2 filters) returns the raw Python objects, NOT the objects that they point to!

What this means is that if you did this:

- name: "Set file suffix"
set_fact:
file_suffix: "12345"

- name: "Get and modify file_names."
set_fact:
file_names: "{{ important_files | map(attribute='file_name') | map('regex_replace','(.*)','\\1_{{ file_suffix }}') }}"

You might not get what you were expecting:

ok: [localhost] => {
    "msg": "Test - <generator object do_map at 0x7f9c15982e10>."
}

This is sort-of, kind-of explained in this bug post, but it’s not very well documented.

Conclusion

This is the first of a few blog posts on my experiences of using and failing at Ansible in real life. I hope that these save someone a few hours!

About Me

Carlos Nunez is a site reliability engineer for Namely, a modern take on human capital management, benefits and payroll. He loves bikes, brews and all things Windows DevOps and occasionally helps companies plan and execute their technology strategies.