While using AWK, I'd like to verify that the NF is >= 10. If not, write the line to a reject file

I have an AWK script that is embedded in BASH code.

cat MyFile.csv | awk -v reject_file=MyRejectFile.csv "${AWK_SCRIPT}" | <generic loader>

And here is my AWK_SCRIPT...

   AWK_SCRIPT='BEGIN {
   FS=","
   OFS=","
   }
   {
      if (NF < 10) {
         print $0 >> reject_file
      }
   print $0
   }
   '

MyFile.csv looks something like this...

This,is,a,sample,line,that,has,more,than,10,fields
This,line,has,less,than,10,fields
This,is,a,second,line,that,has,more,than,10,fields
This,line,also,has,less,than,10,fields

I am trying to get the first and third lines to print to stdout and the second and fourth lines to print to the reject file.

There's a few ways to skin this cat:

You could use an else:

   AWK_SCRIPT='BEGIN {
   FS=","
   OFS=","
   }
   {
      if (NF < 10) {
         print $0 >> reject_file
      }
      else {
         print $0
      }
   }
   '

You could also just drop next inside the if block to stop processing the current line and skip to the next line:

   AWK_SCRIPT='BEGIN {
   FS=","
   OFS=","
   }
   {
      if (NF < 10) {
         print $0 >> reject_file
         next
      }
   print $0
   }
   '

You could also just spell out each condition and resulting action:

   AWK_SCRIPT='BEGIN {
   FS=","
   OFS=","
   }
   NF < 10 {
      print $0 >> reject_file
   }
   NF >= 10 {
      print $0
   }
   '