Shell Programming and Scripting

awk '{print $0 ""> "file" NR}' RS='' input-file

$ awk -v N=`cat FILE | wc -l` 'rand()<numberoflines/N' FILE

awk -f ow.awk myFile
or
awk -v p1=60 -v p2=40 -f ow.awk myFile

function genrand(n)
{
  return(int(n*rand())+1)
}

BEGIN {
  srand()
  if (!p1) p1=80
  if (!p2) p2=20
  perc[p1]="p1"
  perc[p2]="p2"
}
{ a[FNR]=$0;fnr=FNR }
END {
  for(i=1;i<=fnr;i++){
    g=int(genrand(fnr))
    if (!(g in a))
      i--
    else {
      out=((i/fnr)*100 <= p1)?perc[p1]:perc[p2]
      print a[g] >> out
      close(out)
      delete a[g]
    }
  }
}

 sort -R file | split -l $(($(wc -l <file) *8/10))

split -l $[ $(wc -l file |cut -d ' ' -f1) * 70 / 100  ] file output_prefix

awk 'BEGIN {srand()} {print > (rand() < .8 ? "f1" : "f2")}' file