The Inventory File, Patterns, and Groups¶

Ansible works against multiple systems in your infrastructure at the same time. It does this by selecting portions of systems listed in Ansible’s inventory file, which defaults to /etc/ansible/hosts.

Hosts and Groups¶

The format for /etc/ansible/hosts is an INI format and looks like this:

mail.example.com

[webservers]
foo.example.com
bar.example.com

[dbservers]
one.example.com
two.example.com
three.example.com

The things in brackets are group names. You don’t have to have them, but they are useful.

If you have hosts that run on non-standard SSH ports you can put the port number after the hostname with a colon.

four.example.com:5309

Selecting Targets¶

We’ll go over how to use the command line in Command Line Examples section, however, basically it looks like this:

ansible <pattern_goes_here> -m <module_name> -a <arguments>

Such as:

ansible webservers -m service -a "name=httpd state=restarted"

Within Playbooks, these patterns can be used for even greater purposes.

Anyway, to use Ansible, you’ll first need to know how to tell Ansible which hosts in your inventory file to talk to. This is done by designating particular host names or groups of hosts.

The following patterns target all hosts in the inventory file:

all
*

Basically ‘all’ is an alias for ‘*’. It is also possible to address a specific host or hosts:

one.example.com
one.example.com:two.example.com
192.168.1.50
192.168.1.*

The following patterns address one or more groups, which are denoted with the aforementioned bracket headers in the inventory file:

webservers
webservers:dbservers

Individual host names (or IPs), but not groups, can also be referenced using wildcards:

*.example.com
*.com

It’s also ok to mix wildcard patterns and groups at the same time:

one*.com:dbservers

Note

It is not possible to target a host not in the inventory file, unless using playbooks with –override-hosts. More on that later. This is a safety feature.

Easy enough. See Command Line Examples and then Playbooks for how to do things to selected hosts.

Host Variables¶

Using the 0.4 branch of Ansible, it is easy to assign variables to hosts that will be used later in playbooks:

[atlanta]
host1 http_port=80 maxRequestsPerChild=808
host2 http_port=303 maxRequestsPerChild=909

Group Variables¶

Using the 0.4 branch of Ansible, variables can also be applied to an entire group at once:

[atlanta]
host1
host2

[atlanta:vars]
ntp_server=ntp.atlanta.example.com
proxy=proxy.atlanta.example.com

Groups of Groups¶

Using the 0.4 branch of Ansible, it is possible to make groups of groups:

[atlanta]
host1
host2

[raleigh]
host2
host3

[southeast:children]
alpha
beta

[southeast:vars]
some_server=foo.southeast.example.com

[usa:children]
southeast
northeast
southwest
southeast

YAML Inventory Format¶

For people using 0.3, or those that prefer to use it, the inventory file can also be expressed in YAML:

---

# some ungrouped hosts, either use the short string form or the "host: " prefix
- host: jupiter
- mars

# variables can be assigned like this...
- host: saturn
  vars:
  - moon: titan

# groups can also set variables to all hosts in the group
# here are a bunch of hosts using a non-standard SSH port
# and also defining a variable 'ntpserver'
- group: greek
  hosts:
  - zeus
  - hera
  - poseidon
  vars:
  - ansible_ssh_port: 3000
  - ntp_server: olympus.example.com

# individual hosts can still set variables inside of groups too
# so you aren't limited to just group variables and host variables.
- group: norse
  hosts:
  - host: thor
    vars:
    - hammer: True
  - odin
  - loki
  vars:
    - asdf: 1234

Tip: Be sure to start your YAML file with the YAML record designator ---.