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

This might work for you (GNU sed):

sed ':a;N;s/\n_/ /;ta;P;D' file

This avoids slurping the file into memory.

or:

sed -e ':a' -e 'N' -e 's/\n_/ /' -e 'ta' -e 'P' -e 'D' file

Answer 2

Try following solution:

In sed the loop is done creating a label (:a), and while not match last line ($!) append next one (N) and return to label a:

:a
$! {
  N
  b a
}

After this we have the whole file into memory, so do a global substitution for each _ preceded by a newline:

s/\n_/ _/g
p

All together is:

sed -ne ':a ; $! { N ; ba }; s/\n_/ _/g ; p' infile

That yields:

title1 _1 texthere
title2 _2 texthere

Answer 3

If your whole file is like your sample (pairs of lines), then the simplest answer is

paste - - < file

Otherwise

awk '
    NR > 1 &&  /^_/ {printf "%s", OFS} 
    NR > 1 && !/^_/ {print ""} 
    {printf "%s", $0} 
    END {print ""}
' file 

Answer 4

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.