Safely limiting Ansible playbooks to a single machine

Question

I m using Ansible for some simple user management tasks with a small group of computers  Currently  I have my playbooks set to hosts  all and my hosts file is just a single group with all machines listed     file  hosts  office  imac-1 local imac-2 local imac-3 local   I ve found myself frequently having to target a single machine  The ansible-playbook command can limit plays like this   ansible-playbook --limit imac-2 local user yml   But that seems kind of fragile  especially for a potentially destructive playbook  Leaving out the limit flag means the playbook would be run everywhere  Since these tools only get used occasionally  it seems worth taking steps to foolproof playback so we don t accidentally nuke something months from now   Is there a best practice for limiting playbook runs to a single machine  Ideally the playbooks should be harmless if some important detail was left out

User · Answer

This approach will exit if more than a single host is provided by checking the play_hosts variable. The fail module is used to exit if the single host condition is not met. The examples below use a hosts file with two hosts alice and bob.

user.yml (playbook)

---
- hosts: all
  tasks:
    - name: Check for single host
      fail: msg="Single host check failed."
      when: "{{ play_hosts|length }} != 1"
    - debug: msg='I got executed!'

Run playbook with no host filters

$ ansible-playbook user.yml
PLAY [all] ****************************************************************
TASK: [Check for single host] *********************************************
failed: [alice] => {"failed": true}
msg: Single host check failed.
failed: [bob] => {"failed": true}
msg: Single host check failed.
FATAL: all hosts have already failed -- aborting

Run playbook on single host

$ ansible-playbook user.yml --limit=alice

PLAY [all] ****************************************************************

TASK: [Check for single host] *********************************************
skipping: [alice]

TASK: [debug msg='I got executed!'] ***************************************
ok: [alice] => {
    "msg": "I got executed!"
}

User · Answer

There s also a cute little trick that lets you specify a single host on the command line  or multiple hosts  I guess   without an intermediary inventory   ansible-playbook -i  imac1-local   user yml   Note the comma     at the end  this signals that it s a list  not a file   Now  this won t protect you if you accidentally pass a real inventory file in  so it may not be a good solution to this specific problem  But it s a handy trick to know

User · Answer

Since version 1 7 ansible has the run once option  Section also contains some discussion of various other techniques

User · Answer

There s IMHO a more convenient way  You can indeed interactively prompt the user for the machine s  he wants to apply the playbook to thanks to vars prompt   ---  - hosts      setupHosts       vars prompt      - name   setupHosts        prompt   Which hosts would you like to setup         private  no   tasks

User · Answer

AWS users using the EC2 External Inventory Script can simply filter by instance id   ansible-playbook sample-playbook yml --limit i-c98d5a71 --list-hosts   This works because the inventory script creates default groups

User · Answer

To expand on joemailer s answer  if you want to have the pattern-matching ability to match any subset of remote machines  just as the ansible command does   but still want to make it very difficult to accidentally run the playbook on all machines  this is what I ve come up with   Same playbook as the in other answer     file  user yml   playbook  --- - hosts      target       user        Let s have the following hosts   imac-10 local imac-11 local imac-22 local   Now  to run the command on all devices  you have to explicty set the target variable to  all   ansible-playbook user yml --extra-vars  target all    And to limit it down to a specific pattern  you can set target pattern here  or  alternatively  you can leave target all and append the --limit argument  eg   --limit imac-1    ie      ansible-playbook user yml --extra-vars  target all  --limit imac-1  --list-hosts  which results in   playbook  user yml    play  1  office   host count 2     imac-10 local     imac-11 local

User · Answer

Turns out it is possible to enter a host name directly into the playbook  so running the playbook with hosts  imac-2 local will work fine  But it s kind of clunky   A better solution might be defining the playbook s hosts using a variable  then passing in a specific host address via --extra-vars     file  user yml   playbook  --- - hosts      target       user        Running the playbook   ansible-playbook user yml --extra-vars  target imac-2 local    If    target    isn t defined  the playbook does nothing  A group from the hosts file can also be passed through if need be  Overall  this seems like a much safer way to construct a potentially destructive playbook   Playbook targeting a single host     ansible-playbook user yml --extra-vars  target imac-2 local  --list-hosts  playbook  user yml    play  1  imac-2 local   host count 1     imac-2 local   Playbook with a group of hosts     ansible-playbook user yml --extra-vars  target office  --list-hosts  playbook  user yml    play  1  office   host count 3     imac-1 local     imac-2 local     imac-3 local   Forgetting to define hosts is safe     ansible-playbook user yml --list-hosts  playbook  user yml    play  1    target     host count 0

User · Answer

This shows how to run the playbooks on the target server itself   This is a bit trickier if you want to use a local connection   But this should be OK if you use a variable for the hosts setting and in the hosts file create a special entry for localhost   In  all  playbooks have the hosts  line set to   - hosts      target   default  no hosts        In the inventory hosts file add an entry for the localhost which sets the connection to be local    localhost  127 0 0 1  ansible connection local   Then on the command line run commands explicitly setting the target - for example     ansible-playbook --extra-vars  target localhost  test yml   This will also work when using ansible-pull     ansible-pull -U  lt git-repo-here gt  -d   ansible --extra-vars  target localhost  test yml   If you forget to set the variable on the command line the command will error safely  as long as you ve not created a hosts group called  no hosts    with a warning of   skipping  no hosts matched   And as mentioned above you can target a single machine  as long as it is in your hosts file  with     ansible-playbook --extra-vars  target server domain  test yml   or a group with something like     ansible-playbook --extra-vars  target web-servers  test yml

User · Answer

A slightly different solution is to use the special variable ansible limit which is the contents of the --limit CLI option for the current execution of Ansible   - hosts      ansible limit   default omit        No need to define an extra variable here  just run the playbook with the --limit flag   ansible-playbook --limit imac-2 local user yml

User · Answer

We have some generic playbooks that are usable by a large number of teams  We also have environment specific inventory files  that contain multiple group declarations   To force someone calling a playbook to specify a group to run against  we seed a dummy entry at the top of the playbook    ansible-dummy-group  dummy-server   We then include the following check as a first step in the shared playbook   - hosts  all   gather facts  False   run once  true   tasks    - fail        msg   Please specify a group to run this playbook against      when    dummy-server  in ansible play batch    If the dummy-server shows up in the list of hosts this playbook is scheduled to run against  ansible play batch   then the caller didn t specify a group and the playbook execution will fail

User · Answer

I really don t understand how all the answers are so complicated  the way to do it is simply   ansible-playbook user yml -i hosts hosts --limit imac-2 local --check   The check mode allows you to run in dry-run mode  without making any change

User · Answer

I have a wrapper script called provision forces you to choose the target  so I don t have to handle it elsewhere   For those that are curious  I use ENV vars for options that my vagrantfile uses  adding the corresponding ansible arg for cloud systems  and let the rest of the ansible args pass through  Where I am creating and provisioning more than 10 servers at a time I include an auto retry on failed servers  as long as progress is being made - I found when creating 100 or so servers at a time often a few would fail the first time around     echo  Usage   VAR value  bin provision  options  dev all TARGET vagrant  echo    bootstrap - Bootstrap servers ssh port and initial security provisioning  echo    dev - Provision localhost for development and control  echo    TARGET - specify specific host or group of hosts  echo    all - provision all servers  echo    vagrant - Provision local vagrant machine  environment vars only   echo echo  Environment VARS  echo    BOOTSTRAP - use cloud providers default user settings if set  echo    TAGS - if TAGS env variable is set  then only tasks with these tags are run  echo    SKIP TAGS - only run plays and tasks whose tags do not match these values  echo    START AT TASK - start the playbook at the task matching this name  echo ansible-playbook --help   sed -e  1d     s   etc ansible hosts  set by bin provision argument       -k s     use for fresh systems        --tags s     use TAGS var instead        --skip-tags s     use SKIP TAGS var instead        --start-at-task s     use START AT TASK var instead

User · Answer

I would suggest using --limit  lt hostname or ip gt

[ansible] Safely limiting Ansible playbooks to a single machine?

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