Shell Programming and Scripting

Field1,Field2,Field3,Field4,Field4
abc   ,123   ,xyz   ,000   ,pqr
mno   ,123   ,dfr   ,111   ,bbb

Field2,Field4
123   ,000
123   ,111

$ awk -F, 'NR==1 || $2==123{print $2,$4}' OFS="," file
Field2,Field4
123,000 
123,111

$ cat Header_list.txt
Field2
Field4

$ cat file
Field1,Field2,Field3,Field4,Field4
abc ,123 ,xyz ,000 ,pqr
mno ,123 ,dfr ,111 ,bbb

awk 'NR==FNR{
                Cols=Cols (Cols?"|":"")$1
                next
            }

     FNR==1{
                for (i=1;i<=NF;i++) 
                if (match($i,Cols)) 
                Ar[++n]=i
           }

           {
               for (i=1;i<=n;i++) 
               printf (i<n)? $(Ar[i])  FS : $(Ar[i])
               printf "\n"
            }' FS=","  Header_list.txt file

Field2 ,Field4, Field4 
123,  000,  pqr 
123,  111,  bbb

awk '
NR==1 {
   for (i=1;i<=NF;i++) {
      if ($i==SEARCHHDR) {
         srchfld=i;
      }
      if ($i==OUTHDR) {
         outfld=i;
      }
   }

   print $srchfld OFS $outfld
}

NR>1 && $srchfld ~ SEARCHVAL {
   print $srchfld OFS $outfld
}
' SEARCHHDR="Field2" SEARCHVAL="123" OUTHDR="Field4" FS=, OFS=, file