Ansible Patterns for Ad-Hoc Commands and Playbooks: Complete Guide for Beginners

Mastering Ansible Patterns: Targeting Hosts and Groups

Ansible patterns are highly flexible, enabling you to execute commands or run playbooks against a precise subset of your infrastructure. Whether you're dealing with a single host, a group of hosts, or a more complex combination of groups with exclusions or intersections, understanding patterns is crucial for optimizing your workflow.

What Are Ansible Patterns?

Ansible patterns are used to specify which hosts or groups you want to target during the execution of your commands or playbooks. Patterns allow you to:

• Target a single host
• Target multiple hosts
• Target groups of hosts
• Exclude hosts or groups
• Perform regular expression matching
• Combine patterns for more granular control

Let's dive into the details and see how these patterns work in both ad-hoc commands and playbooks.

Common Ansible Patterns

Ansible patterns can be simple or highly specific, depending on your needs. Here's a breakdown of the most common patterns you can use:

Description	Pattern(s)	Targets
All hosts	all (or *)	All hosts in your inventory
One host	host1	A single host named host1
Multiple hosts	host1:host2 (or host1,host2)	Multiple hosts host1 and host2
One group	webservers	All hosts in the webservers group
Multiple groups	webservers:dbservers	All hosts in webservers and dbservers
Excluding groups	webservers:!atlanta	All hosts in webservers, except those in atlanta
Intersection of groups	webservers:&staging	Hosts in both webservers and staging

Pattern Examples in Ad-Hoc Commands

To use these patterns with Ansible ad-hoc commands, you can apply them directly using the ansible command. Here's how to use some of the common patterns:

1. Target All Hosts

   • Command:

    ansible all -m ping
   

2. Target One Host

   • Command:

    ansible host1 -m ping
   

3. Target Multiple Hosts

   • Command:

    ansible host1:host2 -m ping
   

4. Target One Group

   • Command:

    ansible webservers -m ping
   

5. Target Multiple Groups

   • Command:

    ansible webservers:dbservers -m ping
   

6. Exclude Hosts from a Group

   • Command:

    ansible webservers:!atlanta -m ping
   

7. Intersection of Groups

   • Command:

    ansible webservers:&staging -m ping
   

8. Combine Multiple Patterns (Complex Example)

   • Command:

    ansible webservers:dbservers:&staging:!phoenix -m ping
   

This command targets hosts in both webservers and dbservers groups, ensures they are also in the staging group, and excludes any host in the phoenix group.

Pattern Examples in Playbooks

You can use the same patterns in your playbooks as well. The syntax for targeting hosts or groups in playbooks is very similar to the ad-hoc command format.

Basic Playbook Example

---
- name: Ping all hosts
  hosts: all
  tasks:
    - name: Ping each host
      ping:

Playbook Targeting One Host

---
- name: Configure host1
  hosts: host1
  tasks:
    - name: Install Nginx
      apt:
        name: nginx
        state: present

Playbook Targeting Multiple Hosts

---
- name: Configure multiple hosts
  hosts: host1:host2
  tasks:
    - name: Install Apache
      apt:
        name: apache2
        state: present

Playbook Targeting a Group

---
- name: Configure Web Servers
  hosts: webservers
  tasks:
    - name: Install PHP
      apt:
        name: php
        state: present

Excluding a Group in a Playbook

---
- name: Configure Web Servers excluding Atlanta
  hosts: webservers:!atlanta
  tasks:
    - name: Install MySQL
      apt:
        name: mysql-server
        state: present

Combining Patterns in a Playbook

---
- name: Configure servers
  hosts: webservers:dbservers:&staging:!phoenix
  tasks:
    - name: Install Redis
      apt:
        name: redis-server
        state: present

This playbook installs Redis on all hosts in webservers and dbservers that are also part of the staging group, while excluding any host in the phoenix group.

Advanced Pattern Options

Using Variables in Patterns

Ansible allows the use of variables to dynamically target groups. For example, you can pass variables into your playbook using the -e argument:

ansible-playbook -e "excluded=atlanta required=staging" my_playbook.yml

In your playbook, you can reference these variables in the hosts section:

---
- name: Configure servers
  hosts: webservers:!{{ excluded }}:&{{ required }}
  tasks:
    - name: Install Nginx
      apt:
        name: nginx
        state: present

Using Group Position in Patterns

Ansible also supports indexing into groups to target specific hosts based on their position in the group.

Example Group

[webservers]
cobweb
webbing
weber

You can then target hosts by their index:

1. First Host:

   hosts: webservers[0]

1. Last Host:

   hosts: webservers[-1]

1. Range of Hosts:

   hosts: webservers[0:2]

Using Regular Expressions in Patterns

Ansible also supports regular expressions to match hosts. You can use the ~ symbol to indicate a regular expression pattern:

hosts: ~web.*\.example\.com

This pattern will match any host whose name begins with web and ends with .example.com.

Conclusion

Ansible patterns provide a powerful way to target specific hosts, groups, or combinations of both within your inventory. Whether you're running ad-hoc commands or creating complex playbooks, patterns offer the flexibility to manage infrastructure efficiently. By mastering these patterns, you can streamline your automation processes, saving time and effort in your day-to-day tasks.