When you iterate over the same file twice you can use line numbers as convenient indices; it can make for cleaner logic.

awk 'NR == FNR {if ($2 in z) { y[z[$2]]; y[FNR] } z[$2]=FNR; next} (FNR in y)' file file

I used a similar trick in my answer to this question:

• Print only unique lines that appear last in logfile based on date/time

The basis of this trick is that Awk will create a variable simply by referencing it, and the index in arrayname construct returns true or false depending on whether an array element has been created with the specified index.

When you iterate over the same file twice you can use line numbers as convenient indices; it can make for cleaner logic.

awk 'NR == FNR {if ($2 in z) { y[z[$2]]; y[FNR] } z[$2]=FNR; next} (FNR in y)' file file

I used a similar trick in my answer to this question:

• Print only unique lines that appear last in logfile based on date/time

The basis of this trick is that Awk will create a variable simply by referencing it, and the index in arrayname construct returns true or false depending on whether an array element has been created with the specified index.