How to append string to the end of a line in file on its first occurrence

Question

Having below file content

Label abc
First line
Second line
Append line at the end of this line
Append line at the end of this line
Append line at the end of this line
Last line

How to append a string added string to the first occurrence of Append line at the end of this line using Ansible playbook

Label abc
First line
Second line
Append line at the end of this line added string 
Append line at the end of this line
Append line at the end of this line
Last line

I tried the below but the entire file content was modified

    - name: Replace only the first occurrence of a line in a file and append values
      hosts: localhost
      tasks:
        - name: Read the file content
          slurp:
            path: /path/to/your/file
          register: file_content
    
        - name: Modify only the first occurrence of the line and append values
          set_fact:
            updated_lines: >-
              {%- set found = False -%}
              {%- for line in (file_content.content | b64decode).split('\n') -%}
                {%- if not found and line.startswith('label Linux') -%}
                  {%- set found = True -%}
                  {{ line + ' your_append_text' }}
                {%- else -%}
                  {{ line }}
                {%- endif -%}
              {%- endfor -%}
    
        - name: Write the modified content back to the file
          copy:
            content: "{{ updated_lines | join('\n') }}"
            dest: /path/to/your/file
            backup: yes

Answer 1

Given the file

shell> cat /tmp/ansible/file 
Label abc
First line
Second line
Append line at the end of this line
Append line at the end of this line
Append line at the end of this line
Last line

The below task does what you want

    - lineinfile:
        path: /tmp/ansible/file
        regexp: '^Append line at the end of this line.*$'
        line: 'Append line at the end of this line added string'
        firstmatch: true

gives

shell> cat /tmp/ansible/file 
Label abc
First line
Second line
Append line at the end of this line added string
Append line at the end of this line
Append line at the end of this line
Last line

The task is idempotent.

---

Q: "Match the line dynamically."

A: Substitute the variables. For example, the below task gives the same result

    - lineinfile:
        path: /tmp/ansible/file
        regexp: '^{{ target }}.*$'
        line: '{{ target }} {{ append }}'
        firstmatch: true
      vars:
        target: Append line at the end of this line
        append: added string