Shell Programming and Scripting

apple
banana
pear
walnut

#!/usr/bin/awk -f
BEGIN { FS="," }
# put search terms (file1) into array
FNR == NR {a[$1];next}
# do this for all file2 entries
1 {
        for (i = 1; i <= NF; i++)
                if ($i in a)    # we had this term in file1
                        a[$i] = (length(a[$i]) ? (a[$i] SUBSEP) : "") $1
}

END {
        for (i in a)
        {
                print i;
                split(a[i], matches, SUBSEP)
                for (j in matches)
                        print matches[j]
                print ""
        }
}

awk '
NR==FNR{a[$1]=$1;out[$1]=$1;next}
{ for(i=1;i<=NF;i++)
      if($i in a){ 
        out[a[$i]]=out[a[$i]]"\n" $1
      }
}
END{
  for(i in out){
    print out[i]"\n"}
}' file1 FS="," file2

awk '
NR==FNR{a[$1]=++p;out[p]=$1;next}
{ for(i=1;i<=NF;i++)
      if($i in a){ 
        out[a[$i]]=out[a[$i]]"\n" $1
      }
}
END{
  for(i=1;i<=p;i++) {
    print out[i]"\n"}
}' file1 FS="," file2