Input_File=mat.txt
Input_Lines=`wc -l $Input_File | awk '{print $1}'` 
awk -v LINES=$Input_Lines '
# Initalisations
BEGIN {
   # compute #lines of matrix
   delta = 1 + 8*LINES;
   if (delta < 0) {
      print "Invalid input lines count"
      exit;
   }
   matrix_lines = (sqrt(delta) -1)/2;
   col   = 1
   line  = 1
}
# Memorize value in matrix
{
   matrix[line] = matrix[line] " " $1;
   line += 1;
   if (line > matrix_lines)
      line = ++col;
}
# Print full matrix
END {
   for (line=1; line<=matrix_lines; line++)
       print matrix[line];
}
' $Input_File

awk '
$0==0{i=++j}
{rows[i++]=rows[i]sprintf("%5s",$0)}
END {
         for (i=1; i<=j; i++) {
           print rows[i]
         }
}' yourFile

awk '
$0==0{i=++j}
{rows[i++]=rows[i]sprintf("%5s",$0)}
END {
        for (i=1; i<=j; i++) {
          printf rows[i]
          for (k=i+1; k<=j; k++) {
            $0=rows[k]
            printf("%5s",$i)
          }
          printf"\n"
       }
}' yourFile

awk '
$0==0{i=++j}
{rows[i++]=rows[i]sprintf(formatString,$0)}
END {
         for (i=1; i<=j; i++) {
           print rows[i]
         }
}' yourFile formatString="%5s"