Shell Programming and Scripting

1.txt apple is
3.txt apple is
5.txt apple is
2.txt apple is a
7.txt apple is a
8.txt apple is a fruit
4.txt orange not a fruit
6.txt zero is

sort -k2 myfile.txt

1.txt_3.txt_5.txt apple is
2.txt_7.txt apple is a
8.txt apple is a fruit
4.txt orange not a fruit
6.txt zero is

awk -F' ' 'NF>2{a[$2] = a[$2]"_"$1}END{for(i in a){print a[i]" "i}}' myfile.txt

_4.txt orange
_1.txt_3.txt_5.txt_2.txt_7.txt_8.txt apple
_6.txt zero

awk 'FNR==NR{A=$1;$1="";array[$0]=array[$0]?array[$0] "_" A:A;next} {$1="";B=$0} (B in array){C=B;sub(/^[[:space:]]+/,X,B);print array[C] OFS B;delete array[C]}'   Input_file  Input_file

1.txt_3.txt_5.txt apple is
2.txt_7.txt apple is a
8.txt apple is a fruit
4.txt orange not a fruit
6.txt zero is

awk '{A=$1;$1="";array[$0]=array[$0]?array[$0] "_" A:A} END{for(i in array){j=i;sub(/^[[:space:]]+/,X,i);print array[j] OFS i}}'  Input_file

8.txt apple is a fruit
2.txt_7.txt apple is a
6.txt zero is
1.txt_3.txt_5.txt apple is
4.txt orange not a fruit

awk '
        {sub (" ", FS)
         $0=$0
         T[$2]=(T[$2]?T[$2]"_":"") $1
        }
END     {for (t in T) print T[t], t
        }
' FS="\001" file
8.txt apple is a fruit
1.txt_3.txt_5.txt apple is
2.txt_7.txt apple is a
4.txt orange not a fruit
6.txt zero is