How to run a task when variable is undefined in ansible? -

i looking way perform task when ansible variable not registers /undefined e.g

-- name: task    command:  sed -n '5p' "{{app.dirs.includes}}/build.info" | awk '{print  $2}'    when: (! deployed_revision) , ( !deployed_revision.stdout )    register: deployed_revision 

from ansible docs: if required variable has not been set, can skip or fail using jinja2’s defined test. example:

tasks:  - shell: echo "i've got '{{ foo }}' , not afraid use it!"   when: foo defined  - fail: msg="bailing out. play requires 'bar'"   when: bar not defined 

so in case, when: deployed_revision not defined should work