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awk with 2 files
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NR is the number of records in current file. It is reset to 0 when
a new file is opened. FNR is the number of records since awk was
launched.

$ cat a
A
B
C
1

$ cat b
1
2
3

$ awk 'NR==FNR{a$[0];next}; $0 in a{print $0}' a b
1

The condition NR==FNR is true only for the first file. So an
associative array a is formed with index given by the first line
of the first file. The command "next" reads the next line and
restarts the program. So all lines of file a are stored in associative
array a. When file b is read the second half of the program is
executed. If the current line is found in array then a then the
current line is printed. The above program can be simplified to

$ awk 'NR==FNR{a$[0];next}; $0 in a' a b

since the default action in awk is to print the entire line