Shell Programming and Scripting

cat big 24
cat small   13
cat red 63
dog big 34
chicken plays   39
fish    red 294

cat big 24
dog big 34
cat red 63
fish    red 294

awk

awk 'NR==FNR{A[$2]++;next}A[$2]>1' file file

$ awk 'FNR==NR{A[$2]++;next}A[$2]>1' file file
cat big 24
cat red 63
dog big 34
fish    red 294

awk 'NR==FNR{a[$2]++;next} ($2 in a){if(a[$2] > 1) print $0}' file_name file_name

cat big 24
cat red 63
dog big 34
fish    red 294

$ cat file
cat big 24
cat small   13
cat red 63
dog big 34
chicken plays   39
fish    red 294

$ awk '{if(A[$2]) {printf "%s\n%s\n",A[$2],$0;A[$2]="";next};A[$2]=$0}' file
cat big 24
dog big 34
cat red 63
fish    red 294

awk '!($2 in A){A[$2]=$0; next} A[$2]{print A[$2]; A[$2]=x}1' file