> cat bld_csv 
#! /bin/bash

echo "host,parent" >host.csv
while read zf
  do
  hck=$(echo "$zf" | cut -d" " -f1)
  if [ "$hck" = "host_name" ]
    then
    hnm=$(echo "$zf" | cut -d" " -f2)           
  fi
  if [ "$hck" = "parents" ]
    then
    pnm=$(echo "$zf" | cut -d" " -f2)
    echo $hnm","$pnm >>host.csv
  fi

done < sample

awk ' 
BEGIN{print "host,parent"}
/host_name/{p=$2;getline;print p","$2}
' sample

$ cat temp.txt | perl -ne 'chomp; print "host,parent\n" if $. == 1; print $_ if $_ =~ s/^host_name\s+(.*)$/$1/;  print ",$_\n" if $_ =~ s/^parents\s+(.*)$/$1/;'

host,parent
hostA,hostNull
hostB,hostA
hostC,hostB

nawk 'BEGIN{RS="--";n=1;host[n]=sprintf("host,parent")}
{
n=n+1
host[n]=sprintf("%s,%s",$4,$6)
}
END{
for(i=1;i<=n;i++)
if(host[i]!=",")
print host[i]
}
' filename