Shell

1. Replace text across multiple lines

E.g. replace text between <<< and >>> to text "COMMENT" in a test.txt file with below info

  file content
  aabbcc<<<comment part 1
  comment part 2>>>
  ddeeff

Create a replace.sed file including code below

  :begin
  /<<</,/>>>/ {
    />>>/! {
        $! {
            N;
             b begin
        }
    }
    s/<<<.*>>>/COMMENT/;
  }

Execute command: sed -f replace.sed test.txt

2. Delete a pattern-matched line and N lines after it in a text file.
  sed -i '/<replac with PATTERN>/I, +<replace with N> d' <replace with input filename>

3. Delete a pattern-matched line and N lines before it in a text file.
  tac <replace with input filename> | sed '/<replac with PATTERN>/I, +<replace with N> d' |tac > <replace with output filename>

4. Delete a pattern-matched line and M lines before it and N lines after it in a text file.
  grep -v "$(grep -B <replace with M> -A <replace with N> '<replac with PATTERN>' <replace with input filename>)" <replace with input filename> > <replace with output filename>

5. Read ini config file.
  File format:
    [section_1]
    key_1=value_1
    key_2=value_2
    ...

    [section_2]
    key_1=value_1
    key_2=value_2
    ...
  
  grep -Pzo "(?s)(\s*)\N*\[<replace with section name>].*?\[|\[<replace with section name>\].*" <replace with ini filename>