If you are not familiar with modules, check out Ansible - Getting Started with Modules.

There are a few different modules that can be used to modify a file.

• The blockinfile module can be used to append, change, or remove a block of lines in a file. 
• The lineinfile module can be used to append, change, or remove a line in a file. 
• The replace module can be used to change or remove a line in a file.

AVOID TROUBLE

If the target file does not exist, invoking this playbook will return an error like the sample below.

For this reason, you will almost always want to use the create: true parameter, so that the file is created if it does not exist. Or use the stat module to determine if the file exists, and then used the when parameter to only execute the task when the file exists.

fatal: [server1.example.com]: FAILED! => {
  "ansible_facts": {
    "discovered_interpreter_python": "/usr/bin/python"
  },
  "changed": false,
  "msg": "Destination /tmp/foo.txt does not exist !",
  "rc": 257
}

At the bare minimum, the following is all that is needed to invoke this module. By default, text is appended as a new line at the end of the file (EOF).

- name: append 'Hello World' to /tmp/foo.txt
  lineinfile:
    path: /tmp/foo.txt
    line: "Hello World"
    create: true

If the target file exists, and the file was successfully updated, the folllowing should be displayed.

TASK [append 'Hello World' to /tmp/foo.txt]
ok: [server1.example.com]

If the file was not changed, the "changed" key should contain a value of false.

{
    "out": {
        "changed": false
}

Insert After

insertafter: EOF will append new lines to the bottom of the file.

- name: append 'World' to the end of /tmp/foo.txt
  lineinfile:
    path: /tmp/foo.txt
    line: World
    insertafter: EOF
    create: true

Or, you can use a regular expression. Let's say foo.txt contains the following:

Hello
World

The following will append "example" after the line beginning with Hello.

- name: append 'example' after line beginning with 'Hello'
  lineinfile:
    path: /tmp/foo.txt
    line: example
    insertafter: ^Hello
    create: true

Running this play should update foo.txt to have the following.

Hello
example
World

Beginning of file (BOF)

insertbefore: BOF is used to append to the beginning of the file (BOF).

- name: append 'World' to the beginning of /tmp/foo.txt
  lineinfile:
    path: /tmp/foo.txt
    line: World
    insertbefore: BOF
    create: true

In this example, state: present is used so that "new line" is appended if the file does not currently contain any lines matching "new line". If there is already a line matching "new line", nothing will be appended.

- name: append new line EOF if doesn't exist
  lineinfile:
    path: /tmp/foo.txt
    line: "new line"
    state: present
    create: true

In this example, line is used to replace the last line containing "old line" with "new line". If the file contains two or more lines containing "old line", only the last line containing "old line" will be updated to "new line". For this reason, the replace module is typically used to update a text in a file, as the replace module would replace every line containing "old line" with "new line".

- name: replace old line with new line
  lineinfile:
    path: /tmp/foo.txt
    regexp: "old line"
    line: "new line"
    create: true

Append to an existing line

Let's say foo.txt contains "Hello World". Here is how you would append "example" after Hello World.

- lineinfile:
    path: /tmp/foo.txt
    backrefs: true
    regexp: '^(old line)$'
    line: '\1 example'
    create: true

Remove a line

In this example, regexp and state: absent are used to remove the last line containing "Hello World". If the file contains two or more lines containing "Hello World", only the last line containing "Hello World" will be removed. 

- name: remove new line
  lineinfile:
    path: /tmp/foo.txt
    regexp: "Hello World"
    state: absent
    create: true