Add a few notes about variables to the FAQ.

pull/5144/head
Michael DeHaan 11 years ago
parent 81fdd5d02a
commit 12ecff71ba

@ -138,6 +138,42 @@ Then you can use the facts inside your template, like this::
{{ hostvars[host]['ansible_eth0']['ipv4']['address'] }} {{ hostvars[host]['ansible_eth0']['ipv4']['address'] }}
{% endfor %} {% endfor %}
.. _programatic_access_to_a_variable::
How do I access a variable name programatically?
++++++++++++++++++++++++++++++++++++++++++++++++
An example may come up where we need to get the ipv4 address of an arbitrary interface, where the interface to be used may be supplied
via a role parameter or other input. Variable names can be built by adding strings together, like so::
{{ hostvars[inventory_hostname]['ansible_' + which_interface]['ipv4']['address'] }}
The trick about going through hostvars is neccessary because it's a dictionary of the entire namespace of variables. 'inventory_hostname'
is a magic variable that indiciates the current host you are looping over in the host loop.
.. _first_host_in_a_group::
How do I access a variable of the first host in a group?
++++++++++++++++++++++++++++++++++++++++++++++++++++++++
What happens if we want the ip address of the first webserver in the webservers group? Well, we can do that too. Note that if we
are using dynamic inventory, which host is the 'first' may not be consistent, so you wouldn't want to do this unless your inventory
was static and predictable. (If you are using AWX, it will use database order, so this isn't a problem even if you are using cloud
based inventory scripts).
Anyway, here's the trick::
{{ hostvars[groups['webservers'][0]]['ansible_eth0']['ipv4']['address'] }}
Notice how we're pulling out the hostname of the first machine of the webservers group. If you are doing this in a template, you
could use the Jinja2 '#set' directive to simplify this, or in a playbook, you could also use set_fact:
- set_fact: headnode={{ groups[['webservers'][0]] }}
- debug: msg={{ hostvars[headnode].ansible_eth0.ipv4.address }}
Notice how we interchanged the bracket syntax for dots -- that can be done anywhere.
.. _file_recursion: .. _file_recursion:
How do I copy files recursively onto a target host? How do I copy files recursively onto a target host?

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