Ansible – some random useful things.

Some random useful things for Ansible users:

inventory_hostname

inventory_hostname‘ contains the name of the current node being worked on…. (as in, what it is defined in your hosts file as) so if you want to skip a task for a single node –

- name: Restart amavis
  service: name=amavis state=restarted
  when: inventory_hostname != "boris"

(Don’t restart Amavis for boris,  do for all others).

You could also use :

...
  when: inventory_hostname not in groups['group_name']
...

if your aim was to (perhaps skip) a task for some nodes in the specified group.

 

Need to check whether you need to reboot for a kernel update?

  1. If /vmlinuz doesn’t resolve to the same kernel as we’re running
  2. Reboot
  3. Wait 45 seconds before carrying on…
- name: Check for reboot hint.
  shell: if [ $(readlink -f /vmlinuz) != /boot/vmlinuz-$(uname -r) ]; then echo 'reboot'; else echo 'no'; fi
  ignore_errors: true
  register: reboot_hint

- name: Rebooting ...
  command: shutdown -r now "Ansible kernel update applied"
  async: 0
  poll: 0
  ignore_errors: true
  when: kernelup|changed or reboot_hint.stdout.find("reboot") != -1
  register: rebooting

- name: Wait for thing to reboot...
  pause: seconds=45
  when: rebooting|changed

Fixing ~/.ssh/known_hosts

Often an ansible script may create a remote node – and often it’ll have the same IP/name as a previous entity. This confuses SSH — so after creating :

- name: Fix .ssh/known_hosts. (1)
  local_action: command  ssh-keygen -f "~/.ssh/known_hosts" -R hostname

If you’re using ec2, for instance, you could do something like :

- name: Fix .ssh/known_hosts.
  local_action: command  ssh-keygen -f "~/.ssh/known_hosts" -R {{ item.public_ip }} 
  with_items: ec2_info.instances

Where ec2_info is your registered variable from calling the ‘ec2’ module.

Debug/Dump a variable?

- name: What's in reboot_hint?
  debug: var=reboot_hint

which might output something like :

"reboot_hint": {
        "changed": true, 
        "cmd": "if [ $(readlink -f /vmlinuz) != /boot/vmlinuz-$(uname -r) ]; then echo 'reboot'; else echo 'no'; fi", 
        "delta": "0:00:00.024759", 
        "end": "2014-07-29 09:05:06.564505", 
        "invocation": {
            "module_args": "if [ $(readlink -f /vmlinuz) != /boot/vmlinuz-$(uname -r) ]; then echo 'reboot'; else echo 'no'; fi", 
            "module_name": "shell"
        }, 
        "rc": 0, 
        "start": "2014-07-29 09:05:06.539746", 
        "stderr": "", 
        "stdout": "reboot", 
        "stdout_lines": [
            "reboot"
        ]
    }

Which leads on to —

Want to run a shell command do something with the output?

Registered variables have useful attributes like :

  • changed – set to boolean true if something happened (useful to tell when a task has done something on a remote machine).
  • stderr – contains stringy output from stderr
  • stdout – contains stringy output from stdout
  • stdout_lines – contains a list of lines (i.e. stdout split on \n).

(see above)

- name: Do something
  shell: /usr/bin/something | grep -c foo || true
  register: shell_output

So – we could :

- name: Catch some fish (there are at least 5)
  shell: /usr/bin/somethingelse 
  when: shell_output.stdout > "5"

Default values for a Variable, and host specific values.

Perhaps you’ll override a variable, or perhaps not … so you can do something like the following in a template :

...
max_allowed_packet = {{ mysql_max_allowed_packet|default('128M') }}
...

And for the annoying hosts that need a larger mysql_max_allowed_packet, just define it within the inventory hosts file like :

[linux_servers]
beech
busy-web-server mysql_max_allowed_packet=256M

 

4 thoughts on “Ansible – some random useful things.”

  1. Thank you for sharing.

    I was looking for a solution for rebooting only when there is a kernel change. Your solution helped me save a lot of time. I had to modify it to the following. Using CentOS here.
    —- start ————
    tasks:

    # Package installation.
    – name: yum upgrade all packages.
    yum: name=* state=latest

    – name: Check for reboot hint.
    shell: if [ $(rpm -q kernel | sort -Vr | head -n 1) != kernel-$(uname -r) ]; then echo ‘reboot’; else echo ‘no’; fi
    ignore_errors: true
    register: reboot_hint

    – name: Rebooting …
    command: shutdown -r now “Ansible kernel update applied”
    async: 0
    poll: 0
    ignore_errors: true
    when: reboot_hint == ‘reboot’
    register: rebooting

    – name: Wait for server to reboot…
    pause: seconds=300
    when: rebooting|changed

    —- end ————-
    Please comment and make it better.

    Thank you again for sharing.

  2. I find it useful to monitor when the target host actually goes down, instead of just pausing 300 seconds or whatever. Usually it is quicker and we waste time, sometimes it takes longer, and the server is seen as rebooted before it goes down!

    I use:

    – name: Reboot | Wait for {{ inventory_hostname }} to go down
    local_action: wait_for host={{ inventory_hostname }} port=22 state=stopped
    register: timedown

    – debug: var=timedown

    – name: Reboot | Wait for {{ inventory_hostname }} to reboot
    local_action: wait_for host={{ inventory_hostname }} port=22 state=started delay=30
    register: timeup

    – debug: msg=”Up after {{ timeup.elapsed }} seconds”

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