Ansible Do Task If Apt Package Is Missing - conditional-statements

Question

I'm looking to do a series of tasks if a specific apt package is missing.
for example:
if graphite-carbon is NOT installed do:
- apt: name=debconf-utils state=present
- shell: echo 'graphite-carbon/postrm_remove_databases boolean false' | debconf-set-selections
- apt: name=debconf-utils state=absent
another example:
if statsd is NOT installed do:
- file: path=/tmp/build state=directory
- shell: cd /tmp/build ; git clone https://github.com/etsy/statsd.git ; cd statsd ; dpkg-buildpackage
- shell: dpkg -i /tmp/build/statsd*.deb
How would I begin to crack this?
I'm thinking maybe I can do a -shell: dpkg -l|grep <package name> and capture the return code somehow.

Answer 1

You can use the package_facts module (requires Ansible 2.5):
- name: Gather package facts
package_facts:
manager: apt
- name: Install debconf-utils if graphite-carbon is absent
apt:
name: debconf-utils
state: present
when: '"graphite-carbon" not in ansible_facts.packages'
...

Answer 2

It looks like my solution is working.
This is an example of how I have it working:
- shell: dpkg-query -W 'statsd'
ignore_errors: True
register: is_statd
- name: create build dir
file: path=/tmp/build state=directory
when: is_statd|failed
- name: install dev packages for statd build
apt: name={{ item }}
with_items:
- git
- devscripts
- debhelper
when: is_statd|failed
- shell: cd /tmp/build ; git clone https://github.com/etsy/statsd.git ; cd statsd ; dpkg-buildpackage
when: is_statd|failed
....
Here is another example:
- name: test if create_superuser.sh exists
stat: path=/tmp/create_superuser.sh
ignore_errors: True
register: f
- name: create graphite superuser
command: /tmp/create_superuser.sh
when: f.stat.exists == True
...and one more
- stat: path=/tmp/build
ignore_errors: True
register: build_dir
- name: destroy build dir
shell: rm -fvR /tmp/build
when: build_dir.stat.isdir is defined and build_dir.stat.isdir

Answer 3

I think you're on the right track with the dpkg | grep, only that the return code will be 0 in any case. But you can simply check the output.
- shell: dpkg-query -l '<package name>'
register: dpkg_result
- do_something:
when: dpkg_result.stdout != ""

Answer 4

I'm a bit late to this party but here's another example that uses exit codes - ensure you explicitly match the desired status text in the dpkg-query results:
- name: Check if SystemD is installed
command: dpkg-query -s systemd | grep 'install ok installed'
register: dpkg_check
tags: ntp
- name: Update repositories cache & install SystemD if it is not installed
apt:
name: systemd
update_cache: yes
when: dpkg_check.rc == 1
tags: ntp