#!/bin/ksh
awk '
NR > 1 {for(i = length($2) - 5; i >= 1; i--)
                c[substr($2, i, 6)]++
}
END {   for(i in c)
                print c[i], i | "sort -rn"
}' file

13 byebye
8 yebyeb
8 ebyeby
5 ypehel
5 typehe
5 pehell
5 llobye
5 hellob
5 elloby
5 ehello
5 catcat
4 tcatdo
4 ogcatc
4 gcatca
4 dogcat
4 catdog
4 atcatd
3 yedogc
3 ogbyeb
3 obyedo
3 lobyed
3 gbyeby
3 edogca
3 dogbye
3 byedog
2 tdogby
2 obyeby
2 lobyeb
2 atdogb
1 ttypeh
1 tdogdo
1 tcatty
1 ogdogb
1 gdogby
1 dogdog
1 cattyp
1 attype
1 atdogd
1 atcatt

#!/bin/ksh
awk '
NR > 1 {for(i = length($2) - 5; i >= 1; i--)
                c[substr($2, i, 6)]++
}
END {   for(i in c)
                print c[i], i | "sort -rn"
}' file | awk '
NR == 1 {c = $1}
$1 == c {print $2}'

byebye