Ansible modules - Don't reinvent the wheel : use a module!

Python 2.7.13

Is it already installed ?

(nothing)

Let's install it :

Even though we're dealing with assert in the context of Ansible, the overall concept is language-independent and is widely implemented. The bug/limitation/behavior we're discussing here _may be_ specific to Ansible.

1. I make a series of assertions
2. assert checks these
3. ...and yells at the wrong ones

• They call it LE Big Mac. ()
• [This ship] made the Kessel Run in less than twelve parsecs. ()
• The Answer to the Ultimate Question of Life, the Universe, and Everything is 12. ()

  
  - name: Check variable is specified
    assert:
      that:
      - "{{ requiredVariable }} is undefined"
  

      - "{{ requiredVariable }} is undefined"

1. for this playbook to work right, the requiredVariable variable is mandatory, but this code ensures it's undefined
2. the assertion requiredVariable is undefined should fail if the playbook is run while requiredVariable is actually defined (which is how it's always been run on our side)

  vars:
    myVariable: 'hello'



  - assert:
      that:
      - myVariable is defined
      - "{{ myVariable }} is undefined"
      - undefinedVariable is undefined
      - "{{ undefinedVariable }} is undefined"

1^st assertion : myVariable is defined

2^nd assertion : "{{ myVariable }} is undefined"

3^rd assertion : undefinedVariable is undefined

4^th assertion : "{{ undefinedVariable }} is undefined"

• This subtlety can be the cause of hard-to-find bugs.
• Using assert may lead to a false feeling of safety : the code I found at the beginning of our old playbook :
  • actually checks nothing (sh*t may happen someday...)
  • it doesn't even fail : this code WORKS. It just does not check what it was intended for

• Failing to fail doesn't mean you succeeded !

• shell is executed by /bin/sh on the remote node, therefore lacks some functionalities you may be used to :

  ---
  #	ANSIBLE_LOCALHOST_WARNING=false ansible-playbook test.yml
  - hosts: 127.0.0.1
    connection: local
    gather_facts: no
    tasks:
  
    - shell: echo $RANDOM
      register: shellResult
    - debug:
        var: shellResult.stdout		empty string since $RANDOM is a Bash variable
  
    - shell: bash -c 'echo $RANDOM'
      register: shellResult
    - debug:
        var: shellResult.stdout		has a value
  ...

• command is NOT executed in a shell on the remote node, so shell-specific variables (like $HOME) and commands (>, |, &, ...) are NOT supported.

Since both behave mostly the same way and accept the same hacks, I won't be repeating shell or command hereafter .

• skipped in check mode
• LANG-dependent output
• changed is always True
• the args keyword
• [WARNING]: Consider using yum module rather than running yum

skipped in check mode

LANG-dependent output

fr_FR : Quelle page de manuel voulez-vous ?
en_US : What manual page do you want?
es_MX : ¿Qué página de manual desea?
de_DE : Welche Handbuchseiten möchten Sie haben?
ja_JP : マニュアルページを指定してください
zh_CN : 您需要什么手册页？

- name: "A task that makes great things"
  shell: "some shell command"
  environment:
    LANG: C

changed is always True

- name: check whether ...
  shell: someCommand
  register: myVariable
  changed_when: false				never report this task as changed
  ignore_errors: true

- shell: someCommand
  register: myVariable
  changed_when: "myVariable.rc != 2"	change based on return code

shell warning : [WARNING]: Consider using yum module rather than running yum (source)

• use backrefs: yes

  - name: do something
    lineinfile:
      dest: /path/to/file
      regexp: '^what we are looking for$'
      line: 'the new line that will replace the whole line matched by the regexp above'
      backrefs: yes

• OR update the regexp so that it matches both the original AND changed lines