diff --git a/docsite/rst/faq.rst b/docsite/rst/faq.rst index 167100f2405..4a252a76ec0 100644 --- a/docsite/rst/faq.rst +++ b/docsite/rst/faq.rst @@ -138,6 +138,42 @@ Then you can use the facts inside your template, like this:: {{ hostvars[host]['ansible_eth0']['ipv4']['address'] }} {% 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: How do I copy files recursively onto a target host?