How to connect pieces of sentences in one line?

Question

bash

There is a text file in which sentences are word-wrapped to the next line.
We need a recipe that replaces the newline with a space if the end of the line does not have an end-of-sentence sign (.?!) and the line is not empty.

Answer 1

sed ':x {N ; s/\([^.!?]\)\n/\1 / ; /\n/!bx ; P ; D}' -i file.txt