Remove newline depending on the format of the next line

Question

I have a special file with this kind of format :

title1
_1 texthere
title2
_2 texthere

I would like all newlines starting with \"_\" to be

Answer 1

A Perl approach:

perl -00pe 's/\n_/ /g' file 

Here, the -00 causes perl to read the file in paragraph mode where a "line" is defined by two consecutive newlines. In your example, it will read the entire file into memory and therefore, a simple global substitution of \n_ with a space will work.

That is not very efficient for very large files though. If your data is too large to fit in memory, use this:

perl -ne 'chomp; 
          s/^_// ? print "$l " : print "$l\n" if $. > 1; 
          $l=$_; 
          END{print "$l\n"}' file 

Here, the file is read line by line (-n) and the trailing newline removed from all lines (chomp). At the end of each iteration, the current line is saved as $l ($l=$_). At each line, if the substitution is successful and a _ was removed from the beginning of the line (s/^_//), then the previous line is printed with a space in place of a newline print "$l ". If the substitution failed, the previous line is printed with a newline. The END{} block just prints the final line of the file.