sed insert line with spaces to a specific line

Question

I have a line with spaces in the start for example \" Hello world\". I want to insert this line to a specific line in a file. for example insert \" hello world\" to the next f

Answer 1

$ a="  some string  "
$ echo -e "hello\nworld"
hello
world
$ echo -e "hello\nworld" | sed "/world/ s/.*/${a}.\n&/" 
hello
  some string  .
world

The . was added in the substitution above to demonstrate that the trailing whitepsaces are preserved. Use sed "/world/ s/.*/${a}\n&/" instead.