Ansible Quickstart

Posted by Ryan Himmelwright on Thu, Apr 30, 2020
Tags dev, linux, devops, ansible
Ansible Office Balcony, Durham NC

A long time ago, I briefly explained how to configure Ansible, in a post about building a raspberry pi cluster. All in all… it was by no means a great introduction to the basics of ansible.

A month ago, I drafted a progression of examples with notes, to teach a co-worker the basics of writing and using ansible roles and playbooks. After reading through them, I realized it wouldn’t take much to turn them into an actual Ansible quickstart post. So here we are.

I am not an Ansible genius, and reading this will not make you one either. However, the goal of this post is to provide enough understating to get started with writing some ansible playbooks.

Installing

Lets start by installing ansible. It should be in most distro’s main repos these days:

Fedora Linux:

sudo dnf install ansible

MacOS:

… I have no idea. I usually always ssh to Linux boxes from my macbook.

I think it can be installed with pip though, so possibly:

pip3 install ansible

Remote Node Requirements

In order for ansible to connect to a remote node, that node usually needs 3 things:

  • 1) Python installed
  • 2) password-less sudo permissions
  • 3) ssh keys configured (if running against remote hosts. Not needed if just running playbooks against localhost)

python

Python should already be installed on most systems. If not, check your package manager, or try searching the documentation on python.org to learn the best install method for your system.

Passwordless sudo

This will allow a user to run sudo commands, without having to type in a password each time. I shouldn’t have to say this, but… please use with care!

Granting password-less sudo permissions are most easily accomplished with visudo:

sudo visudo

This will open up the sudo settings in your $EDITOR. Once opened, find the following line and uncomment it (it’s usually near the bottom of the file).

## Same thing without a password
%wheel        ALL=(ALL)       NOPASSWD: ALL

ssh

Lastly, exchange ssh-keys with the remote node. This will allow ansible to ssh into the node without having to deal with those pesky passwords. The easiest way to exchange keys is using the ssh-copy-id command, as such:

ssh-copy-id username@hostname

Ansible Basics

Hosts File

A host inventory file is a yaml file that defines hosts ansible can connect to. The default file is located at /etc/ansible/hosts. An alternative inventory file may be provided using the -i flag.

Example file:

[VMs]
## Server VMs
192.168.10.50
192.168.10.71
192.168.10.118

[Hosts]
## Hosts
192.168.10.12

[cluster]
192.168.112.205
192.168.112.206
192.168.112.207

Modules

Modules are premade functionality used in ansible that can be imported into a playbook. Simply, they do what you want done. Some examples are ping, dnf, apt, redhat_subscription.

Feel free to search the ansible documentation to learn more.

Ad-hoc Ansible Commands

Simple and straight ansible executions can be called with the ansible command. Ad-hoc commands are usually called with a module, using the -m flag. For example, ping:

➜  ansible -m ping localhost

localhost | SUCCESS => {
    "changed": false,
    "ping": "pong"
}

For a more complicated example, lets use the dnf module to install htop:

ansible -m dnf -a "name=htop state=latest" localhost --become

This module requires some parameters to be defined. We are able to supply them using the -a flag, followed by a string of the key/values pairs.

Also, because the dnf module requires root permissions to function, we supply the --become flag, to become root.

Note, if I want to run this against another machine (beyond localhost), it has to be defined in whatever inventory file we are using.

So, if I define an inventory file (./hosts.yaml) containing my desktop computer:

[charmelon]
192.168.1.5

I can install htop on my desktop, using:

ansible -i ./hosts -m dnf -a "name=htop state=latest" charmeleon --become

… and it works!

192.168.1.5 | CHANGED => {
    "ansible_facts": {
        "discovered_interpreter_python": "/usr/bin/python"
    },
    "changed": true,
    "msg": "",
    "rc": 0,
    "results": [
        "Installed: htop-2.2.0-8.fc32.x86_64"
    ]
}

Playbooks

As you can imagine, doing everything from the command line isn’t always helpful, or easily reproducible. That’s what playbooks are for. In a nutshell, playbooks are ansible scripts. They are a yaml file which ansible runs, instead of running a series of ad-hoc commands.

To demonstrate, lets convert the dnf command from above, into a simple playbook named install-htop.yaml.

---
- hosts: charmeleon
  become: true

  tasks:
    - name: Install Htop
      dnf:
        name: htop
        state: latest

Being a yaml file, the first line starts with ---. Next, we define some meta information for the entire playbook. For example, this is were we put the --become flag, by turning it into become: true. This is also where we define what hosts the playbook will run against. If I’m providing a hosts file, I can alternatively use hosts: all to run against all hosts defined in the inventory file.

local connections

If the playbook is to run only locally, the connection type can be set to local (by default, it is set to ssh.)

  hosts: 127.0.0.1
  connection: local

tasks

Below the header information, we can define a set of tasks to run. In the tasks section, a block is defined for each task, usually by calling a module with parameters. It is best practice to describe each task using name:. This will make it easier to trace the logs.

For example, lets add theping module to the playbook so we have more than one task….

---
- hosts: 127.0.0.1
  connection: local
  become: true

  tasks:
	- name: Ping host first...
	  ping:

    - name: Install Htop
      dnf:
        name: htop
        state: latest

Now the playbook will run both tasks, using name as the header for the output of each one:

➜  /tmp ansible-playbook install-htop.yaml

PLAY [all] ****************************************

TASK [Gathering Facts] ****************************************
ok: [192.168.1.5]

TASK [Ping host first...] ****************************************
ok: [192.168.1.5]

TASK [Install Htop] ****************************************
ok: [192.168.1.5]

PLAY RECAP ****************************************
192.168.1.5                : ok=3    changed=0    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0
Variables

We can define sections other than tasks. A useful section to add is vars:, which defines variables for use in the playbook. To illustrate, lets replace the hard-coded htop in the dnf task, to a variable named package. We can even use the package variable in the name string, to dynamically change the output in the log:

---
- hosts: 127.0.0.1
  connection: local
  become: true

  vars:
    package: htop

  tasks:
    - name: Ping host first...
      ping:

    - name: Install {{ package }}
      dnf:
        name: "{{ package }}"
        state: latest

And the output:

➜  ansible-playbook install-htop.yaml

PLAY [127.0.0.1] ****************************************

TASK [Gathering Facts] ****************************************
ok: [127.0.0.1]

TASK [Ping host first...] ****************************************
ok: [127.0.0.1]

TASK [Install htop] ****************************************
changed: [127.0.0.1]

PLAY RECAP ****************************************
127.0.0.1                  : ok=3    changed=1    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0

One great feature of variables is that they can be swapped out when calling the playbook. The -e flag allows you to provide an alternative value for a variable. For example, lets say we want to install nano instead of htop:

➜  ansible-playbook install-htop.yaml -e package=nano

PLAY [127.0.0.1] ****************************************

TASK [Gathering Facts] ****************************************
ok: [127.0.0.1]

TASK [Ping host first...] ****************************************
ok: [127.0.0.1]

TASK [Install nano] ****************************************
changed: [127.0.0.1]

PLAY RECAP ****************************************
127.0.0.1                  : ok=3    changed=1    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0

Note how the task name has changed accordingly in the output. So Fancy!

Creating some structure

Roles

As nice as scripts are, they don’t scale well. To combat impending chaos, we break functionality down into roles. Roles are collections of tasks, variables, and other resources that can be mixed and matched in playbooks.

A role is defined by a directory of it’s name, and usually contains a tasks sub-directory, where all of it’s tasks are defined. Each sub-directory requires a main.yaml to be the root file for that directory. So, at the vary least, a tasks directory will have a file named tasks/main.yaml which contains the role’s tasks.

If there are a BUNCH of tasks defined, they can be broken out into seperate files, and included in the main.yaml task file.

In addition to tasks, a role might include a defaults or vars sub directory. These are again structured with a main.yaml file that may, or may not, import other files, depending on the size and organization of the role.

## Example structure of a 'subscriptions' role
roles
└── subscriptions
    ├── defaults
    │   └── main.yaml
    ├── README.md
    └── tasks
        └── main.yaml

It is important to note that these yaml files contain just their item. For example, the task files contain just tasks. This is because when a role is imported into a playbook, its items are simply inserted accordingly.

ansible.cfg

Before we start writing some roles, it is important to know that if you are using roles, you need to tell ansible where to find them. The easiest way to do this is to define an ansible.cfg file in the directory you will run ansible-playbook from. For example:

[defaults]
roles_path = roles/

Our role

As it stands, our example playbook is a massive 13 lines long! I can hardly open the file without crashing my text editor. So, lets try to break up the functionality into roles.

First, lets make the directories:

mkdir -p roles/install-htop/{tasks,defaults}

Next, we can add our variables to a default file, roles/install-htop/defaults/main.yaml:

---
package: htop

With the package variable set, lets create the tasks. To demonstrate including other files in the main.yaml, I’m going to be overly-complicated and extract our ping task into its own file, and then include it in the main.yaml.

So first, roles/install-htop/tasks/ping.yaml

---
- name: Ping host first...
  ping:

And then, roles/install-htop/tasks/main.yaml, which will also include our dnf install task…

---
- include_tasks: ping.yaml

- name: Install {{ package }}
  dnf:
    name: "{{ package }}"
    state: latest

Congrats, we have an install-htop role defined!

Including roles in playbooks

Just as we included vars and tasks in the playbook, if we already have tasks and vars defined in a role, we can instead include that role:

---
- hosts: 127.0.0.1
  connection: local
  become: true

  roles:
    - install-htop

Easy. Let’s run it:

➜  ansible-playbook install-htop.yaml
[WARNING]: Ansible is being run in a world writable directory (/tmp), ignoring it as an ansible.cfg source. For more information see
https://docs.ansible.com/ansible/devel/reference_appendices/config.html#cfg-in-world-writable-dir

PLAY [127.0.0.1] ****************************************

TASK [Gathering Facts] ****************************************
ok: [127.0.0.1]

TASK [install-htop : include_tasks] ****************************************
included: /tmp/roles/install-htop/tasks/ping.yaml for 127.0.0.1

TASK [install-htop : Ping host first...] ****************************************
ok: [127.0.0.1]

TASK [install-htop : Install htop] ****************************************
ok: [127.0.0.1]

PLAY RECAP ****************************************
127.0.0.1                  : ok=4    changed=0    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0

and it still works :)

Conclusion

While only the tip of the iceburg, I think we have covered enough basics to make something useful. Using this small amount of Ansible knowledge, I have been able to create playbooks that configure applications, update all my computers, and setup each of my machines when I reformat them. However, don’t let that stop you from learning even more! Ansible is a powerful tool and worth any amount of time invested into it. Enjoy!

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