# New in 1.1 -- Diff mode for templates! Works with and without --check, but prefers --limit **URL:** \ **Category:** Ansible Project\ **Created:** [February 8, 2013, 3:59am UTC](https://forum.ansible.com/t/new-in-1-1-diff-mode-for-templates-works-with-and-without-check-but-prefers-limit/12312 "2013-02-08T03:59:35Z")\ **Posts on this page:** 10\ **Page:** 1
**Author:** ![Michael\_DeHaan](https://avatars.discourse-cdn.com/v4/letter/m/c4cdca/32.png) [@Michael\_DeHaan](https://forum.ansible.com/u/Michael_DeHaan)\ **Post date:** [February 8, 2013, 3:59am UTC](https://forum.ansible.com/t/new-in-1-1-diff-mode-for-templates-works-with-and-without-check-but-prefers-limit/12312/1 "2013-02-08T03:59:35Z")
So --check is a recently added "pseudo dry run" feature which I just recently talked about. So far, modules like file, template, and copy respond to --check and will give you whether things would change or not, and the other modules just get skipped over with --check. Whether or not you are using --check, I just added a --diff flag tonight, so far only to /usr/bin/ansible-playbook. When this is run, for any template files, it will present a basic unified diff of the changes made to those templates. To avoid running against binary files, it is not implemented for file types. I also suspect the output of diff mode is NOT optimal if running against multiple machines at the same time (it might interlace), so it is probably useful to use it with --limit. Example, to put it all in perspective ansible-playbook everything.yml --check # just see what might change on all boxes ansible-playbook everything.yml --check --limit foo.example.com # see what might change on just one box ansible-playbook everything.yml --check --diff # just see what might change and show diffs on all boxes ansible-playbook everything.yml --check --diff --limit foo.example.com # be sane, just show diffs for that one box There are TODO items here which I am hoping folks may want to help with, namely, teaching other modules to respond to the "check" attribute, just like file does now. Minimally I think this should be done for service, yum, and apt, and this would be pretty well decently complete. Given lots of folks may have command tasks and when statements they also want to simulate, perhaps we should also teach the command module that when run in 'check' mode that it should return changed with a 0 (success) error code. We can worry about the other modules having support for --check later, I'm not super concerned about those. I think most people want to know when files get modified out of band. ---
**Author:** ![Michael\_DeHaan](https://avatars.discourse-cdn.com/v4/letter/m/c4cdca/32.png) [@Michael\_DeHaan](https://forum.ansible.com/u/Michael_DeHaan)\ **Post date:** [February 8, 2013, 4:00am UTC](https://forum.ansible.com/t/new-in-1-1-diff-mode-for-templates-works-with-and-without-check-but-prefers-limit/12312/2 "2013-02-08T04:00:52Z")
Those developers interested should see this commit [https://github.com/ansible/ansible/commit/a9162a86f2e2b678eaef04501339c037bfc18319](https://github.com/ansible/ansible/commit/a9162a86f2e2b678eaef04501339c037bfc18319) Ideas for improvements in output are welcome. I didn't add this to /usr/bin/ansible because that doesn't really do templates. ---
**Author:** ![Chris\_Hoffman](https://avatars.discourse-cdn.com/v4/letter/c/85f322/32.png) [@Chris\_Hoffman](https://forum.ansible.com/u/Chris_Hoffman)\ **Post date:** [February 8, 2013, 6:52pm UTC](https://forum.ansible.com/t/new-in-1-1-diff-mode-for-templates-works-with-and-without-check-but-prefers-limit/12312/3 "2013-02-08T18:52:20Z")
One weird thing I have already ran into, and makes this hard, is dealing with register variables. Not sure the best way to handle there. ---
**Author:** ![Michael\_DeHaan](https://avatars.discourse-cdn.com/v4/letter/m/c4cdca/32.png) [@Michael\_DeHaan](https://forum.ansible.com/u/Michael_DeHaan)\ **Post date:** [February 8, 2013, 6:56pm UTC](https://forum.ansible.com/t/new-in-1-1-diff-mode-for-templates-works-with-and-without-check-but-prefers-limit/12312/4 "2013-02-08T18:56:45Z")
--check mode and register variables you mean? Definitely, I think it's just going to be a limitation (a limitation as a result of capability does not GREATLY concern me, it's just going to be a feature with caveats...other tools can't even do this so it's not an issue to me). I would welcome creative approaches to making this more useful in those situations, but it's also one of the reasons I put it off originally... the whole 'it's not going to be very accurate because it's a simulation' kind of thing. ---
**Author:** ![Chris\_Hoffman](https://avatars.discourse-cdn.com/v4/letter/c/85f322/32.png) [@Chris\_Hoffman](https://forum.ansible.com/u/Chris_Hoffman)\ **Post date:** [February 8, 2013, 8:25pm UTC](https://forum.ansible.com/t/new-in-1-1-diff-mode-for-templates-works-with-and-without-check-but-prefers-limit/12312/5 "2013-02-08T20:25:56Z")
I checked changes to the RabbitMQ modules to support --check mode and it works as expected. It was pretty trivial to add the support for it since modules have to be structured for idempotentance. ---
**Author:** ![Tin\_Tvrtkovic](https://avatars.discourse-cdn.com/v4/letter/t/bc8723/32.png) [@Tin\_Tvrtkovic](https://forum.ansible.com/u/Tin_Tvrtkovic)\ **Post date:** [February 8, 2013, 9:00pm UTC](https://forum.ansible.com/t/new-in-1-1-diff-mode-for-templates-works-with-and-without-check-but-prefers-limit/12312/6 "2013-02-08T21:00:31Z")
Here’s an idea related to this. Instead of just showing a diff, how about letting Ansible (in dry run mode, or something similar) parse all templates and generate local copies? A number of my configuration files are Jinja 2 templates right now. If someone asked me to show them an example config file for something I’m deploying, I’d have to either fetch one from a server where it’s been deployed, or make a copy of the template and “manually” render it (i.e. edit out all the J2 tags and put in dummy values). I think it might be useful to have Ansible do the template file generation offline, locally. I realize templates that use dynamically fetched data wouldn’t work, but I find all my templates use data from the hosts file, and my vars. ---
**Author:** ![Brian\_Coca1](https://avatars.discourse-cdn.com/v4/letter/b/278dde/32.png) [@Brian\_Coca1](https://forum.ansible.com/u/Brian_Coca1)\ **Post date:** [February 8, 2013, 9:05pm UTC](https://forum.ansible.com/t/new-in-1-1-diff-mode-for-templates-works-with-and-without-check-but-prefers-limit/12312/7 "2013-02-08T21:05:24Z")
this is its very easy with a local action. I also have a hack that does this but changes how ansible works with stdin/stdout ---
**Author:** ![Brian\_Coca1](https://avatars.discourse-cdn.com/v4/letter/b/278dde/32.png) [@Brian\_Coca1](https://forum.ansible.com/u/Brian_Coca1)\ **Post date:** [February 8, 2013, 9:16pm UTC](https://forum.ansible.com/t/new-in-1-1-diff-mode-for-templates-works-with-and-without-check-but-prefers-limit/12312/8 "2013-02-08T21:16:50Z")
I actually setup a playbook that does this: ansible-playbook gen\_template.yml -e "target=webserver1" --- gen\_template.yml - hosts: $target   vars\_prompt:     - name: template       prompt: "Enter template file name"       private: false     - name: templateloc       prompt: "Enter template file location"       private: false   tasks:       - name: gen template         local\_action: template src=$templateloc/$template dest=/tmp/$template ---
**Author:** ![Michael\_DeHaan](https://avatars.discourse-cdn.com/v4/letter/m/c4cdca/32.png) [@Michael\_DeHaan](https://forum.ansible.com/u/Michael_DeHaan)\ **Post date:** [February 8, 2013, 9:18pm UTC](https://forum.ansible.com/t/new-in-1-1-diff-mode-for-templates-works-with-and-without-check-but-prefers-limit/12312/9 "2013-02-08T21:18:06Z")
Sounds like a great thing to use a callback plugin for. It would show all the other diffs on screen still, but you could also save them. ---
**Author:** ![Tin\_Tvrtkovic](https://avatars.discourse-cdn.com/v4/letter/t/bc8723/32.png) [@Tin\_Tvrtkovic](https://forum.ansible.com/u/Tin_Tvrtkovic)\ **Post date:** [February 9, 2013, 2:46pm UTC](https://forum.ansible.com/t/new-in-1-1-diff-mode-for-templates-works-with-and-without-check-but-prefers-limit/12312/10 "2013-02-09T14:46:11Z")
This is quite clever, thanks.