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Ansible tutorial: Grouping hosts

Hosts in inventory can be grouped arbitrarily. For instance, you could have a debian group, a web-servers group, a production group, etc...

[debian]
host0
host1
host2

This can even be expressed shorter:

[debian]
host[0:2]

If you wish to use child groups, just define a [groupname:children] and add child groups in it. For instance, let's say we have various flavors of linux running, we could organize our inventory like this:

[ubuntu]
host0

[debian]
host[1:2]

[linux:children]
ubuntu
debian

Grouping, of course, leverages configuration mutualization.

Setting variables

You can assign variables to hosts in several places: inventory file, host vars files, group vars files, etc...

I usually set most of my variables in group/host vars files (more on that later). However, I often use some variables directly in the inventory file, such as ansible_host which sets the IP address for the host. Ansible by default resolves hosts' name when it attempts to connect via SSH. But when you're bootstrapping a host, it might not have its definitive IP address yet. ansible_host comes in handy here.

When using ansible or ansible-playbook command, variables can also be set with --extra-vars (or -e) command line switch as a whitespace-separated list of key=val pairs (it's possible to use the switch multiple times for each variable initialization). We'll talk about ansible-playbook in the next step.

ansible_port, as you can guess, has the same function regarding the SSH port ansible will try to connect at.

[ubuntu]
host0 ansible_host=192.168.0.12 ansible_port=2222

Ansible will look for additional variables definitions in group and host variable files. These files will be searched in directories group_vars and host_vars, below the directory where the main inventory file is located.

The files will be searched by name. For instance, using the previously mentioned inventory file, host0 variables will be searched in those files:

  • group_vars/linux
  • group_vars/ubuntu
  • host_vars/host0

It doesn't matter if those files do not exist, but if they do, ansible will use them.

Now that we know the basics of modules, inventories and variables, let's explore the real power of Ansible with playbooks.

Head to step-04.