How to make Ansible ignore changes in certain lines in file - ansible

Question

I have a regular ubuntu VM. My ansible playbook is the following
---
- hosts: linux
tasks:
- name: "Creating a file file.txt"
become: yes
copy: src=./file.txt dest=~/file.txt
notify: restart some_app
handlers:
- name: restart some_app
become: yes
service:
name=some_app
state=restarted
And the contents of my file are
First line
Second line
After creating my VM I apply my ansible playbook. Then I am doing some work on ubuntu VM which could change the contents of file.txt (in Ubuntu VM, not in host). And when my work is done I apply anible playbook again. But I want to restart some_app only when changes are made to the First line in my file and ignore any changes made to the Second line (during some work in ubuntu VM). For example:
If after my work I have (on Ubuntu VM)
First line
Changed Second line
I want ansible to do nothing to this file (I don't want it to be replaced by initial file.txt on my host) and I don't want some_app to be started.
But if I have
Changed First line
Changed Second line
I want ansible to replace only First line and restart some_app, so after applying ansible playbook I would have
First line
Changed Second line
Is it possible to create Second line only once and then make ansible ignore it completly? How can I make this possible using ansible playbook? I can't split my file.txt into two different files or smth like this.

Answer 1

You can create backup and diff the first lines of the files.
For example, if the destination does not exist the module copy reports changed in the registered dictionary. The attribute backup_file is undefined. In this case, changed_when evaluates to True and the handler is notified
- copy:
src: file.txt
dest: ~/file.txt
backup: true
register: copy_result
changed_when:
- copy_result is changed
- copy_result.backup_file is undefined
notify: restart some_app
If the destination exists you want to notify the handler if the first line changes. Create the block below to test the first line. The tasks in the block will be executed only if a backup file is created
- block:
- script:
cmd: '{{ playbook_dir}}/diff_first_line {{ copy_result.backup_file }}'
executable: /bin/bash
register: diff_result
ignore_errors: true
changed_when: diff_result.stdout_lines.0|d('') == '1c1'
notify: restart some_app
- debug:
var: diff_result
when: debug|d(false)|bool
when:
- copy_result.backup_file is defined
Create the script below. The script returns the diff between the first lines of the destination ~/file.txt and the backup file
shell> cat diff_first_line
diff <(head -n 1 ~/file.txt) <(head -n 1 $1)
exit 0
Create the handler for testing
handlers:
- name: restart some_app
debug:
msg: First line of ~/file.txt changed
Example of a complete playbook for testing
shell> cat pb.yml
- hosts: test_24
tasks:
- copy:
src: file.txt
dest: ~/file.txt
backup: true
register: copy_result
changed_when:
- copy_result is changed
- copy_result.backup_file is undefined
notify: restart some_app
- debug:
var: copy_result
when: debug|d(false)|bool
- block:
- script:
cmd: '{{ playbook_dir}}/diff_first_line {{ copy_result.backup_file }}'
executable: /bin/bash
register: diff_result
ignore_errors: true
changed_when: diff_result.stdout_lines.0|d('') == '1c1'
notify: restart some_app
- debug:
var: diff_result
when: debug|d(false)|bool
when:
- copy_result.backup_file is defined
handlers:
- name: restart some_app
debug:
msg: First line of ~/file.txt changed
If the destination does not exist the file will be created and the handler will be notified
shell> ansible-playbook pb.yml
PLAY [test_24] *******************************************************************************
TASK [copy] **********************************************************************************
changed: [test_24]
TASK [debug] *********************************************************************************
skipping: [test_24]
TASK [script] ********************************************************************************
skipping: [test_24]
TASK [debug] *********************************************************************************
skipping: [test_24]
RUNNING HANDLER [restart some_app] ***********************************************************
ok: [test_24] =>
msg: First line of ~/file.txt changed
PLAY RECAP ***********************************************************************************
test_24: ok=2 changed=1 unreachable=0 failed=0 skipped=3 rescued=0 ignored=0
shell> ssh admin#test_24 sudo cat /root/file.txt
First line
Second line
If the destination exists and the first line changed
shell> ssh admin#test_24 sudo cat /root/file.txt
First line changed
Second line
The script in the block will compare the first lines of the files and will notify the handler. Running the playbook with the variable debug=true gives
shell> ansible-playbook pb.yml -e debug=true
PLAY [test_24] *******************************************************************************
TASK [copy] **********************************************************************************
ok: [test_24]
TASK [debug] *********************************************************************************
ok: [test_24] =>
copy_result:
backup_file: /root/file.txt.1520674.2023-01-05#11:00:41~
changed: false
checksum: e23b0534d0708167023aba6f65ef83afe38d52df
dest: /root/file.txt
diff: []
failed: false
gid: 0
group: root
md5sum: cbfa34ef619be185bfbe2b8c8f104d97
mode: '0644'
owner: root
secontext: system_u:object_r:admin_home_t:s0
size: 23
src: /home/admin/.ansible/tmp/ansible-tmp-1672934439.424632-1552928-229786621684419/source
state: file
uid: 0
TASK [script] ********************************************************************************
changed: [test_24]
TASK [debug] *********************************************************************************
ok: [test_24] =>
diff_result:
changed: true
failed: false
rc: 0
stderr: |-
Shared connection to test_24 closed.
stderr_lines:
- Shared connection to test_24 closed.
stdout: |-
1c1
< First line
---
> First line changed
stdout_lines:
- 1c1
- < First line
- '---'
- '> First line changed'
RUNNING HANDLER [restart some_app] ***********************************************************
ok: [test_24] =>
msg: First line of ~/file.txt changed
PLAY RECAP ***********************************************************************************
test_24: ok=5 changed=1 unreachable=0 failed=0 skipped=0 rescued=0 ignored=0
The handler won't be notified if the first line doesn't change
shell> ssh admin#test_24 sudo cat /root/file.txt
First line
Second line changed
shell> ansible-playbook pb.yml -e debug=true
PLAY [test_24] *******************************************************************************
TASK [copy] **********************************************************************************
ok: [test_24]
TASK [debug] *********************************************************************************
ok: [test_24] =>
copy_result:
backup_file: /root/file.txt.1522339.2023-01-05#11:02:50~
changed: false
checksum: e23b0534d0708167023aba6f65ef83afe38d52df
dest: /root/file.txt
diff: []
failed: false
gid: 0
group: root
md5sum: cbfa34ef619be185bfbe2b8c8f104d97
mode: '0644'
owner: root
secontext: system_u:object_r:admin_home_t:s0
size: 23
src: /home/admin/.ansible/tmp/ansible-tmp-1672934567.8696983-1553096-127236704493791/source
state: file
uid: 0
TASK [script] ********************************************************************************
ok: [test_24]
TASK [debug] *********************************************************************************
ok: [test_24] =>
diff_result:
changed: false
failed: false
rc: 0
stderr: |-
Shared connection to test_24 closed.
stderr_lines:
- Shared connection to test_24 closed.
stdout: ''
stdout_lines: []
PLAY RECAP ***********************************************************************************
test_24: ok=4 changed=0 unreachable=0 failed=0 skipped=0 rescued=0 ignored=0