Patterns: targeting hosts and groups

When you execute Ansible through an ad hoc command or by running a playbook, you must choose which managed nodes or groups you want to execute against. Patterns let you run commands and playbooks against specific hosts and/or groups in your inventory. An Ansible pattern can refer to a single host, an IP address, an inventory group, a set of groups, or all hosts in your inventory. Patterns are highly flexible - you can exclude or require subsets of hosts, use wildcards or regular expressions, and more. Ansible executes on all inventory hosts included in the pattern.

• Using patterns
• Common patterns
• Limitations of patterns

• Advanced pattern options

  • Using variables in patterns
  • Using group position in patterns
  • Using regexes in patterns
• Patterns and ansible-playbook flags

Using patterns

You use a pattern almost any time you execute an ad hoc command or a playbook. The pattern is the only element of an ad hoc command that has no flag. It is usually the second element:

ansible <pattern> -m <module_name> -a "<module options>"

For example:

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

In a playbook the pattern is the content of the hosts: line for each play:

- name: <play_name>
  hosts: <pattern>

For example:

- name: restart webservers
  hosts: webservers

Since you often want to run a command or playbook against multiple hosts at once, patterns often refer to inventory groups. Both the ad hoc command and the playbook above will execute against all machines in the webservers group.

Common patterns

This table lists common patterns for targeting inventory hosts and groups.

Description	Pattern(s)	Targets
All hosts	all (or *)
One host	host1
Multiple hosts	host1:host2 (or host1,host2)
One group	webservers
Multiple groups	webservers:dbservers	all hosts in webservers plus all hosts in dbservers
Excluding groups	webservers:!atlanta	all hosts in webservers except those in atlanta
Intersection of groups	webservers:&staging	any hosts in webservers that are also in staging

Once you know the basic patterns, you can combine them. This example:

webservers:dbservers:&staging:!phoenix

targets all machines in the groups ‘webservers’ and ‘dbservers’ that are also in the group ‘staging’, except any machines in the group ‘phoenix’.

You can use wildcard patterns with FQDNs or IP addresses, as long as the hosts are named in your inventory by FQDN or IP address:

192.0.\*
\*.example.com
\*.com

You can mix wildcard patterns and groups at the same time:

one*.com:dbservers

Limitations of patterns

Patterns depend on inventory. If a host or group is not listed in your inventory, you cannot use a pattern to target it. If your pattern includes an IP address or hostname that does not appear in your inventory, you will see an error like this:

[WARNING]: No inventory was parsed, only implicit localhost is available
[WARNING]: Could not match supplied host pattern, ignoring: *.not_in_inventory.com

Your pattern must match your inventory syntax. If you define a host as an alias:

atlanta:
  host1:
    http_port: 80
    maxRequestsPerChild: 808
    host: 127.0.0.2

you must use the alias in your pattern. In the example above, you must use host1 in your pattern. If you use the IP address, you will once again get the error:

[WARNING]: Could not match supplied host pattern, ignoring: 127.0.0.2

Advanced pattern options

The common patterns described above will meet most of your needs, but Ansible offers several other ways to define the hosts and groups you want to target.

Using variables in patterns

You can use variables to enable passing group specifiers via the -e argument to ansible-playbook:

webservers:!{{ excluded }}:&{{ required }}

Using group position in patterns

You can define a host or subset of hosts by its position in a group. For example, given the following group:

[webservers]
cobweb
webbing
weber

you can use subscripts to select individual hosts or ranges within the webservers group:

webservers[0]       # == cobweb
webservers[-1]      # == weber
webservers[0:2]     # == webservers[0],webservers[1]
                    # == cobweb,webbing
webservers[1:]      # == webbing,weber
webservers[:3]      # == cobweb,webbing,weber

Using regexes in patterns

You can specify a pattern as a regular expression by starting the pattern with ~:

~(web|db).*\.example\.com

Patterns and ansible-playbook flags

You can change the behavior of the patterns defined in playbooks using command-line options. For example, you can run a playbook that defines hosts: all on a single host by specifying -i 127.0.0.2, (note the trailing comma). This works even if the host you target is not defined in your inventory. You can also limit the hosts you target on a particular run with the --limit flag:

ansible-playbook site.yml --limit datacenter2

Finally, you can use --limit to read the list of hosts from a file by prefixing the file name with @:

ansible-playbook site.yml --limit @retry_hosts.txt

If RETRY_FILES_ENABLED is set to True, a .retry file will be created after the ansible-playbook run containing a list of failed hosts from all plays. This file is overwritten each time ansible-playbook finishes running.

ansible-playbook site.yml –limit @site.retry

To apply your knowledge of patterns with Ansible commands and playbooks, read Introduction to ad hoc commands and Intro to playbooks.