Using Ansible's command and shell modules properly

21 Sep 2016

I realise I have quite strong opinions on the command and shell modules in Ansible. There are now four independent checks in ansible-lint for ways to use the modules badly. Let me count the ways…

Using command/shell instead of a better module

There are a number of modules that can be used instead of commands. Obvious candidates include most package installation modules (yum, rpm, pip, etc.), most version control modules (git, hg, etc.), OS control modules such as service.

There are also commands that can be run when an argument to the file module would be better - e.g file state=absent rather than rm, file state=link rather than ln.

I also managed to introduce a command_warnings check into Ansible that will warn you at runtime.

With the following playbook:

- hosts: target
  gather_facts: no

  tasks:
  - name: get coreutils version
    command: rpm -q coreutils

ansible-playbook outputs a warning when command_warnings is enabled:

$ ansible-playbook playbook.yml

PLAY [target] ******************************************************************

TASK [get coreutils version] ***************************************************
changed: [target]
 [WARNING]: Consider using yum, dnf or zypper module rather than running rpm

PLAY RECAP *********************************************************************
target                     : ok=1    changed=1    unreachable=0    failed=0

Because of the great potential for false positives (e.g. you need to run a command with an equivalent module in a way that the module does not support), it’s easy to switch off warnings in a way that works with both ansible and ansible-lint - just add warn: no to the command arguments

- name: get coreutils version
  command: rpm -q coreutils
  args:
    warn: no
  register: coreutils_version

(Using non-YAML notation, the middle three lines would just be command: warn=no rpm -q coreutils, but I’ve moved over to full YAML form in my playbooks now - and I have an ansible-review check for that! For simplicity I use the key-value form in inline examples here.)

Using shell instead of command

The shell module is potentially more dangerous than the command module (ok, nothing is really stopping you doing command: rm -rf --no-preserve-root) and should only be used when you actually need shell functionality. So if you’re not stringing two commands together (using pipes or even just && or ;), you don’t really need the shell module. Similarly, expanding shell variables or file globs require the shell module. If you’re not using these features, don’t use the shell module. If you are using these features, think twice if you can rewrite the shell command to make it more Ansibley.

Convergence and command/shell

When you run command or shell, they always set changed to True. This is because Ansible has no mechanism for understanding whether your command changed anything or not. Some commands are genuinely read only (e.g. git status) and others have side effects.

Generally, one expects with Ansible that when a playbook is run twice, no changes should happen on the second run. There are (at least) four ways to achieve this (ansible-lint only checks these four, so if there’s another mechanism, let me know).

1. changed_when

If a command is read only, set changed_when to False. If you can tell whether a command changed something based on its return code or its stdout or stderr, you can use this with changed_when:

- name: clear yum cache
  command: yum clear metadata
  register: yum_clear
  changed_when: '"\n0 metadata files removed" not in yum_clear.stdout'

2. and 3. creates and removes

If a command creates a file after it is first run, or removes a file after it is first run, you can use the creates or removes argument with command or shell. Then it won’t run a second time.

- name: trivially create a file
  shell: echo "hello" > /tmp/hello
  args:
    creates: /tmp/hello

(don’t use the above example, use copy: content=hello dest=/tmp/hello)

4. when

Often a command behaves from the outside world no differently if it puts something into a state or it’s already in that state. In such cases, it might always return a 0 exit status, and print no output.

In these cases, you might need a read-only pre-check command that determines whether the system is already in the desired state, and then not do the changing task if it is.

- name: check tuned profile
  command: tuned-adm active
  register: tuned_adm
  changed_when: False

- name: set tuned profile
  command: tuned-adm profile virtual-guest
  when: "'Current active profile: virtual-guest' not in tuned-adm.stdout"