Grep

grep -oE '<data>[^<]*</data>' yourxmlfile

Bash

tag='data'
tL="<$tag>" tR="</$tag>"
xml=$(< yourxmlfile)
while case $xml in *"$tL"* ) :;; * ) break;; esac; do
  t1=${xml#*"$tL"} t2=${t1%%"$tR"*} xml=${t1#*"$tR"}
  echo "${tL}${t2}${tR}"
done

Perl

perl -lne "print for/<$tag>.*?<\/$tag>/g" yourxmlfile

Sed

sed -e "
  s|<$tag>|\n&|
  s/.*\n//
  s|</$tag>|&\n|
  /\n/P;D
" yourxmlfile

Вывод

 <data>asdf</data>
 <data>asdf</data>
 <data>asdf</data>
 <data>asdf</data>