DevOps Hunter: Ansible 2: Ad-hoc commands and Getting start Writing a Playbook

Saturday, July 24, 2021

Ansible 2: Ad-hoc commands and Getting start Writing a Playbook

Overview of Ansible PLAYBOOK

An Ansible playbook is a single YAML file that contains multiple plays.

Each Play will be defined with a set of activities that are treated as tasks, and these tasks can be executed at the remote host that is the Ansible client.

The task can be a single action that can be one of:

Execute a command
Run a script
install patch or package
Reboot VM/box
Restart services

Simple ansible play can be

check the timestamp
reboot server
wait for connect back
check the uptime
check timestamp

Complex ansible play

Take a backup of files on 20 DB VMs
Deploy application on 100 App boxes
100 servers patch apply
100 VM reboot after patch
Mail and slack notifications on patch process

Ansible ad-hoc command

When you plan to write a playbook first you need to test the ad-hoc commands as trial and error will gives more confidence to run in a play

 
  # Ansible ad-hoc command syntax
ansible [-i inventry_file] server[group1:group2] -m module [-a argument]

Here is some, "Hello World" program from Ansible ad-hoc command and playbook

 
cd qa; ls 
# Using qa servers as targets 
ansible qa -m debug -a "Welcome To VybhavaTechnologies from remote"
ansible -i localhost -m debug -a "Welcome To VybhavaTechnologies from localhost"

We can also the ping to the specific host alias that is defined in the "/etc/ansible/hosts" file as shown below

[servers]
host1 ansible_ssh_host=192.168.33.200
host2 ansible_ssh_host=192.168.33.210
host3 ansible_ssh_host=192.168.33.220

Why do we run ad-hoc commands in Ansible Controller?

To use setup tasks to quickly bring a managed node to a desired state
To perform a quick test to verify that a playbook has executed successfuly
To run a discovery task to verify that a node meets certain criteria

Sample ad-hoc commands with Ansible Shell Module

Let's experiment with 'shell' module send a terminal command to the remote host and retrieve the results. For instance, to find out the disk-space and uptime usage on our host2 machine, we could use:

 ansible -m shell -a 'df -h .;uptime' server

Ansible shell module execution example.

You can run the ansible commands on the selective hosts here host2 and host3.

ansible -m shell -a 'uptime' host2:host3

Converting the above ad-hoc command execution into a playbook as hello.yml file. You can use your favourite editor, vi hello.yml # File name: hello.yml

 
--- 
 - name: Hello from ansible playbook
   hosts: localhost
   
   tasks:
   - name: Prints message
     debug:
       msg: "Welcome To VybhavaTechnologies from remote"

Execute the playbook using ansible-playbook command with playbook yaml file as :

 
ansible-playbook hello.yml

More Examples to practice different automation needs:

 
     ansible -i prod_inv prod -m shell -a "uptime"
     ansible -i prod_inv prod:web:db -m shell -a "uptime" # multiple group
     ansible web -m shell -a "free -m" # get the RAM size of web group

Execution of the above commands are

Ansible shell module ad-hoc command execution

How to converting the ad-hoc commands to playbook?

Here I've tried to get converted simple uptime, free commands to run in playbook

 
# File: fun-play.yaml

---
 - name: Monitor CPU and Mem
   hosts: all
   tasks:
   - name: Find CPU load
     shell: uptime
     register: up_time
   - debug:
       var: up_time.stdout_lines
       
   - name: Find RAM Size
     shell: free -m
     register: free_ram
   - debug:
       var: free_ram.stdout_lines

Execution of the above play book as follows:

ansible-playbook fun-play.yaml

You can add df command to this one more play into the playbook and give try.

How a PLAYBOOK structure can be?

A playbook can have multiple play sections.

Multiple play sections in a Ansible Playbook

Creating your first Playbook

Here is a simple play book with very simple tasks

---
 -  name: First playbook
    hosts: db
    tasks:
      - name: test command
        command: hostname
        register: output

      - debug: var=output

      - name: test a script
        script: remoterun.sh
        register: scriptout

      - debug:
          msg: "{{ scriptout.stdout }}"

 - name: another play
   hosts: db
   tasks:
     - name: Install webserver
       become: yes
       become_user: root
       yum:
         name: httpd
         state: present

     - name: Start webserver
       become: yes
       become_user: root
       service:
          name: httpd
          state: started

Create a script File remoterun.sh

 
  echo "Welcome from shell script"

It's execution output is as shown below:

First playbook execution

continue...

Apache web server started

How does Ansible built-in Debug works?

A debug module block can be added to any of the tasks, which will help us to understand the Ansible execution flow.

Sample example where debug uses ansible facts to retrieve the remote host IP addresses.

---
- name: Testing
  hosts: all
  gather_facts: yes

  tasks:
    - debug: var=hostvars[inventory_hostname]['ansible_env'].SSH_CONNECTION.split(' ')[2]

The execution output is image

_/\_

Hope you enjoyed this post, Keep learning, Keep smiling Keep sharing ... :)

References

official debug builtin

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