$ join -j 100 a.txt 1.txt
 a 1
 a 2
 a 3
 a 4
 b 1
 b 2
 b 3
 b 4
 c 1
 c 2
 c 3
 c 4
 d 1
 d 2
 d 3
 d 4

cat a.txt | while read LINE_A
do
  cat 1.txt | while read LINE_1
  do
    prinft “${LINE_A}  と ${LINE_1} を比較したら ”
    if [ “${LINE_A}” = “LINE_1” ] ; then
      printf “同じもの”
    else
       printf “違うもの”
    fi
    echo “でした”
  done
done