Inventory Files / Hosts Files in Ansible¶

An inventory file is a list of hosts and groups of hosts. These files are used to run Ansible playbooks on multiple hosts at a time.

Table of Contents¶

Defining an Inventory using a Hosts File
Example Host Files
- Using ini
- Using yaml
Testing a Hosts File
Specifying Different Groups of Hosts (Grouping Hosts)
- Vizualize an Inventory
View the Inventory in yaml or json format

Defining an Inventory using a Hosts File¶

You can use a hosts file to define a group of hosts.
By default, Ansible looks for /etc/ansible/hosts for the inventory file.

Specify an inventory file by using the -i option when running an Ansible command.

ansible-playbook -i inventory.yml my_playbook.yml

The file can be in ini format (most common) or yaml format.

hosts.ini:
```
[servers]
controlplane
node01
```

hosts.yaml:

all:
  hosts:
    controlplane:
      ansible_host: 192.168.1.10
      ansible_user: ubuntu
    node01:
      ansible_host: 192.168.1.11
      ansible_user: ubuntu

Example Host Files¶

Using `ini`¶

Using ini format, hosts.ini:

# File: hosts.ini

# Define individual hosts
webserver ansible_host=192.168.1.10 ansible_user=ubuntu ansible_ssh_private_key_file=~/.ssh/id_rsa
dbserver ansible_host=192.168.1.20 ansible_user=root

# Define groups
[webservers]
web1 ansible_host=192.168.1.11
web2 ansible_host=192.168.1.12

[databases]
db1 ansible_host=192.168.1.21
db2 ansible_host=192.168.1.22

# Nested groups. The parent group will contain all hosts in all its child groups.
[all_servers:children]
webservers
databases

# Group variables (applied to all hosts in the group)
[webservers:vars]
ansible_user=deploy
ansible_port=2222

Host Entries:¶

Each line specifies a hostname or alias (e.g., webserver) and optional connection variables.

ansible_host: IP or domain name of the host.
ansible_user: User to connect as.
ansible_ssh_private_key_file: Path to the SSH private key.

Groups:¶

Group hosts together using descriptive names, like [webservers] or [databases].
Create nested groups with the :children suffix.
[all_servers:children] makes webservers and databases groups children of the all_servers group.

Group Variables:¶

Use the :vars suffix to define variables applied to all hosts in the group.
[webservers:vars] applies the variables to all hosts in the webservers group.

Using `yaml`¶

Using yaml format, hosts.yml:

# File: hosts.yml

all:
  children:
    webservers:
      hosts:
        web1:
          ansible_host: 192.168.1.11
          ansible_user: deploy
        web2:
          ansible_host: 192.168.1.12
    databases:
      hosts:
        db1:
          ansible_host: 192.168.1.21
          ansible_user: root

yaml uses a nested structure.

all: The top-level group containing all hosts and subgroups.
children: Subgroups under the all group.
hosts: List of hosts with their variables.

Testing a Hosts File¶

Test the inventory you've specified by using ansible servers.

ansible servers -i /root/hosts -m ping

You can see how Ansible looks at your hosts file:

ansible-inventory -i /path/to/hosts --list
ansible-inventory -i /path/to/hosts --graph

# To see it in yaml format:
ansible-inventory -i /path/to/hosts --list -y

Translating an Inventory to Yaml¶

Just like checking your hosts file, you can use ansible-inventory to translate an inventory to .yaml format.

ansible-inventory -i ./inventory.ini --list --yaml

By default, the --list option will output json.
You can also output in toml format with --toml.

Specifying Different Groups of Hosts (Grouping Hosts)¶

[servers]
controlplane
node01

[test_env]
controltest type=client
node01test  type=server

[prod_env]
controlprod type=client
node01prod  type=server

# Assign to a group
[non-prod:children]
servers
test_env

This makes both the servers and test_env groups part of the non-prod group.

Parent groups inherit all hosts from their children.

If I specify hosts as non-prod in a playbook, it will run on all hosts in the servers and test_env groups.

Vizualize an Inventory¶

To see a graph of the hosts file:

ansible-inventory -i /root/hosts --graph

We see that hosts file is interpreted by Ansible as:

@all:
  |--@non-prod:
  |  |--@servers:
  |  |  |--controlplane
  |  |  |--node01
  |  |--@test_env:
  |  |  |--controltest
  |  |  |--node01test
  |--@prod_env:
  |  |--controlprod
  |  |--node01prod
  |--@ungrouped:

View the Inventory in `yaml` or `json` format¶

You can specify the format of the output when using ansible-inventory to view the inventory in json or yaml data.
By default, ansible-inventory --list will output in json.
Add -y/--yaml to see it in yaml format.

# View inventory in json
ansible-inventory -i hosts.ini --list 
ansible-inventory -i hosts.ini --list 
# For yaml format:
ansible-inventory -i hosts.ini --list --yaml
ansible-inventory -i hosts.ini --list -y

Targeting a Single Host by Limiting the Hosts with `--limit`¶

Using --limit, you can target a specific host by name from a hosts file.

ansible all -i hosts.ini --limit host2 -a "/bin/echo Hello, world."

ansible all: Targets all hosts in the inventory.
-a: The argument to send to the host.
This will run echo Hello, world only on host2 (as it's defined in hosts.ini).

Ranges of IPs in Inventories¶

You can specify a range of IP addresses in inventory files.

[servers]

192.168.200.[1:254]