Ansible-Playbook: Specify hosts on the command line

While we can specify what hosts to run the command through ansible command-line program, it's does not apply to ansible-playbook program. We always need to specify the hosts: attribute in our YAML playbook. It mean if we were to run someone's else playbook, we always have to edit the playbook to customize the hardcoded hosts: attribute.

Googling for this, I found the workaround (which a great resource in itself to gain insight what ansible capable for). We can use variables. So instead of the following playbook:-

- hosts: myhostname
  tasks:
    - name: Add user
      user: name=kamal
            groups=admin
            shell=/bin/bash

We can write it as:-

- hosts: $hosts
  tasks:
    - name: Add user
      user: name=kamal
            groups=admin
            shell=/bin/bash

And then run our playbook as:-

ansible-playbook -u root --extra-vars="hosts=myhostname"