# Using Ansible to calculate a total that aggregates a value across hosts in inventory

**URL:** <https://forum.ansible.com/t/using-ansible-to-calculate-a-total-that-aggregates-a-value-across-hosts-in-inventory/18748>\
**Category:** Ansible Developer\
**Created:** [July 13, 2015, 9:13pm UTC](https://forum.ansible.com/t/using-ansible-to-calculate-a-total-that-aggregates-a-value-across-hosts-in-inventory/18748 "2015-07-13T21:13:31Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![Tim\_Chambers](https://avatars.discourse-cdn.com/v4/letter/t/a587f6/32.png) [@Tim\_Chambers](https://forum.ansible.com/u/Tim_Chambers)\
**Post date:** [July 13, 2015, 9:13pm UTC](https://forum.ansible.com/t/using-ansible-to-calculate-a-total-that-aggregates-a-value-across-hosts-in-inventory/18748/1 "2015-07-13T21:13:31Z")

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I’m relatively new to Ansible. I’m looking for a pattern for gathering some custom, dynamic facts from hosts and then processing them locally. For instance, can anyone point me to an example of a simple play that runs df(1), calculates the total disk used, and prints the df output and the total for all the hosts? That example would get me started in the direction I want to go.

The script I’m running on my hosts returns multiple lines of output, I want to gather that output and display it. The last line of the output is a number. I want to add up all those values and have the play print the total at the end of the play.

Googling [ansible calculate total disk usage](https://www.google.com/search?q=ansible%20calculate%20total%20disk%20usage) did not return helpful results.

Or is my problem that Ansible isn’t designed for such things. I know I can run the play from a local wrapper script and then post-process the JSON to my heart’s content. I was hoping to do this all native inside Ansible, though.

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**Author:** ![Henry\_Finucane](https://avatars.discourse-cdn.com/v4/letter/h/94ad74/32.png) [@Henry\_Finucane](https://forum.ansible.com/u/Henry_Finucane)\
**Post date:** [July 14, 2015, 3:42am UTC](https://forum.ansible.com/t/using-ansible-to-calculate-a-total-that-aggregates-a-value-across-hosts-in-inventory/18748/2 "2015-07-14T03:42:42Z")

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If you're literally looking for total disk used, that comes in the  
default ansible facts- try running this against a host or two:

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**Author:** ![Tim\_Chambers](https://avatars.discourse-cdn.com/v4/letter/t/a587f6/32.png) [@Tim\_Chambers](https://forum.ansible.com/u/Tim_Chambers)\
**Post date:** [July 14, 2015, 6:00pm UTC](https://forum.ansible.com/t/using-ansible-to-calculate-a-total-that-aggregates-a-value-across-hosts-in-inventory/18748/3 "2015-07-14T18:00:42Z")

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I think I understand how facts work and how your example will give me disk space per host. My problem is how to then add up those host-specific numbers into a single number for the entire play.

I think the answer to my question, reinforced by advice from a work colleague, is that Ansible is the right tool for performing tasks on multiple hosts, but there’s no way in a playbook to manipulate host results to make calculations across hosts. Today I’m tackling this with the API in a Python script.

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**Author:** ![Henry\_Finucane](https://avatars.discourse-cdn.com/v4/letter/h/94ad74/32.png) [@Henry\_Finucane](https://forum.ansible.com/u/Henry_Finucane)\
**Post date:** [July 14, 2015, 6:08pm UTC](https://forum.ansible.com/t/using-ansible-to-calculate-a-total-that-aggregates-a-value-across-hosts-in-inventory/18748/4 "2015-07-14T18:08:56Z")

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This isn't true- using the `hostvars` dictionary (keyed by inventory  
names), you can look at arbitrary values for arbitrary hosts. There's  
also a globally available 'groups' dictionary (the values are lists of  
hosts) if you want to sort and categorize.

Seriously, try a callback plugin. You can then make sure you get  
updated information after all provisioning runs, instead of requiring  
a separate step.

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<div class="post-metadata">

**Author:** ![Tim\_Chambers](https://avatars.discourse-cdn.com/v4/letter/t/a587f6/32.png) [@Tim\_Chambers](https://forum.ansible.com/u/Tim_Chambers)\
**Post date:** [July 14, 2015, 6:28pm UTC](https://forum.ansible.com/t/using-ansible-to-calculate-a-total-that-aggregates-a-value-across-hosts-in-inventory/18748/5 "2015-07-14T18:28:33Z")

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Thanks for bearing with me. I see what you mean now and am studying callbacks.