Ansible Templates

Author: R Zach Feeser

Lab Objective

There are two ways to set up inventories for your Ansible playbooks: the smart way and the not-so-smart way. We’ll learn how the inventory directory works and what you should AVOID during set up.

Process

Let’s establish good habits for how we organize our TMUX environment for developing playbooks. Split your terminal so you have four panes (the TMUX commands you’ll want to know are below)

SPLIT SCREEN HORIZONTALLY: ctrl b (hands off keyboard) shift "

SPLIT SCREEN VERTICALLY: ctrl b (hands off keyboard) shift 5

SWITCH BETWEEN PANES: ctrl b (hands off keyboard) arrow key

When you’ve split your terminal, it should look like this. The panes have been labeled to demonstrate what each pane will be used for.

+----------------------------------------------+
|                      |                       |
|  PANE 1              |  PANE 2               |
|  "Tree" output       |  Command line         |
|                      |                       |
|                      |                       |
+----------------------------------------------+
|                      |                       |
|  PANE 3              |  PANE 4               |
|  Host files          |  Playbook             |
|                      |                       |
|                      |                       |
+----------------------------------------------+

In PANE 1, create a new directory for this lab.

PANE 1 ~$ mkdir -p ~/smurfdir && cd ~/smurfdir
Here we are going to make our host file using the file content below.

PANE 1 ~/smurfdir$ vim hosts
```
all:
  hosts:
  children:
      webserver:
          hosts:
              student1
```
We will now make three subdirectories, groups_vars, host_vars and templates.

PANE 1 ~/smurfdir$ mkdir group_vars host_vars templates
Let’s create a file called webserver inside our group_vars directory.

PANE 1 ~/smurfdir$ vim group_vars/webserver
```
smurfs: little pink people
```
Stifle your outrage for now–of COURSE Smurfs are not pink–but it’s all part of the point we are trying to make. Create another file called student1 inside of host_vars.

PANE 1 ~/smurfdir$ vim host_vars/student1
```
ansible_host: 192.168.6.18
smurfs: lil blue guys
user: ubuntu
```
BLUE Smurfs, that’s better. Inside of your templates directory, create a file named smurfcolor. {{ smurfs }} is declaring that smurfs is a variable, and that when this template is executed the value of smurfs should take its place.

PANE 1 ~/smurfdir$ vim templates/smurfcolor
```
Smurf Report: {{ smurfs }}
```
Last, let’s install and run a helpful program named tree to help us visualize our files and subdirectories.

PANE 1 ~/smurfdir$ sudo apt install tree

PANE 1 ~/smurfdir$ tree

Your display should now look like this:

.
├── group_vars
│   └── webserver
├── hosts
├── host_vars
│   └── student1
└── templates
    └── smurfcolor

Switch to PANE 3 (bottom left) and let’s have our webserver and student1 files open and visible to us.

PANE 3 ~$ vim /home/student/smurfdir/host_vars/student1
Now that you are in vim, we can open multiple files in vim as tabs! Enter this command while in command mode:

:tabe /home/student/smurfdir/group_vars/webserver
Cool! You now have a very 1990s looking tab structure at the top of this pane. Press gt to tab back and forth between webserver and student1 in PANE 3.

We’re now ready to set up our playbook. Move to PANE 4 and create our smurfmaker.yaml playbook.

PANE 4 ~$ vim /home/student/smurfmaker.yaml

- name: smurf color identification
  hosts: webserver
  remote_user: ubuntu

  tasks:
  - name: create a file
    template:
      src: smurfcolor
      dest: /home/student/smurfreport
      owner: "{{ user }}"
      mode: '0644'

DO NOT leave vim in PANE 4! As soon as you write-quit, you will lose all your undo abilities! Whenever you make a change to smurfmaker.yaml, use :w only.
Move to PANE 2. It’s time for us to call on this playbook, which will generate a file called smurfreport in /home/student. We are using smurfcolor as our template, which will replace the value of {{ smurfs }} when the new file is made. But which value will it take? Take an educated guess, then run the playbook.

PANE 2 ~$ ansible-playbook smurfmaker.yaml
Read out the file that was just created.

PANE 2 ~$ cat smurfreport
```
Smurf Report: lil blue guys
```
Ok! The value of {{ smurfs }} in our template was replaced by value in host_vars/student1. If you look at our yaml inventory we made in step 4, student1 is at the BOTTOM. When Ansible searches for the value of a key, it starts at the bottom and works its way up.
Let’s make a change to host_vars/student1. In PANE 3, select student1 file and comment out the line containing the smurfs key. Save with :w
```
ansible_host: 192.168.6.18
#smurfs: lil blue guys
user: ubuntu
```
In PANE 2, run your playbook again.

PANE 2 ~$ ansible-playbook smurfmaker.yaml
Read out the file that was just re-created.

PANE 2 ~$ cat smurfreport
```
Smurf Report: lil pink guys
```
PINK?? Blasphemy. But observe why this happened. Because smurfs is no longer defined by student1. It is now being defined by the group that student1 belongs to, webserver.

STOP. THINK. CONSIDER.

Imagine that we are not dealing with just one variable being defined in multiple places, but dozens. Imagine removing a variable definition in one place only to find out it’s being defined SOMEWHERE ELSE instead… and you don’t know where! EXTREME CAUTION should be exercised when declaring variables, and at Alta3 we definitely do not recommend seeding your directory structure with multiple definitions. Pain and frustration is sure to follow!

To demonstrate one additional level of hierarchy, let’s return to PANE 4 and make a change. Edit your playbook to look like this:

- name: smurf color identification
  hosts: webserver
  remote_user: ubuntu

  vars:
    smurfs: lil green guys

  tasks:
  - name: create a file
    template:
      src: smurfcolor
      dest: /home/student/smurfreport
      owner: "{{ user }}"
      mode: 0644

Save with a :w, then return to PANE 2 to run your playbook one more time.

PANE 2 ~$ ansible-playbook smurfmaker.yaml
Read out the file that was just re-created.

PANE 2 ~$ cat smurfreport
```
Smurf Report: lil green guys
```
You can see that variables defined in the Ansible playbook itself override all variables set elsewhere.

Ansible Templates

Lab Objective

Process

STOP. THINK. CONSIDER.