matching columns from two files

Question

Dear all,

I have two files, in which the first column of them might have same values. I would like to match the first column of both files, and print the lines in FILE1 for which there was a match.

FILE1:
xxx1 yyy yyy yyy
xxx2 yyy yyy yyy
xxx3 yyy yyy yyy

FILE2:
xxx3 zzzz
xxx4 zzzz

OUTPUT:
xxx3 yyy yyy yyy

Any suggestions are welcomed.

Best wishes

Answer 1

join

Answer 2

awk 'FNR==NR{ a[$1]=$0;next } ($1 in a)' file2 file1

Answer 3

Here is my recipe:

awk 'key[$1]; FNR==NR {key[$1]=1}' file2 file1

I assume that both lists are sorted by the key (first column) and each key only appears once in a file. The first pattern short for:

key[$1] != 0

In which case, the default action is to print the whole line. This pattern implicitly works for the second file (file1) only because for the first file, the key has not been marked. In the second pattern:

FNR==NR {key[$1]=1}

The FNR==NR means we are processing the first file (file2 in this case). In this case, we mark the key for later reference.