Awk syntax, strange variable?

Question

I have this awk command:

awk -F, 'NR==FNR{_1[$1]++;next}_1[$1]' file1 file2

I actually almost know what the program is doing, but I don't know what _1 means. The intention is to compare two files, one has 1 field and the other has 4 fields.

Here's what I'm trying to do. I have these files, I need to know which numbers on file1 is in file 2 and then print the record of file 2.

file1

89611862
89613696
84126597
88690669
60147342

file2

84126597,12345300943529348333,2805058C30730000,28
88767631,12345712012010684248,2805058C30730002,28
83218719,12345712012012622594,2805058C30730006,28
89611862,12345712012010174449,2805058C30730014,28
85933710,12345712012007778032,2805058C3073001A,26
88690669,12345712012012060159,2805058C3073001E,28
60147342,12345712012011235922,2805058C30730030,28
89613696,12345712012008043980,2805058C3073003C,28
84783612,12345712012031117926,2805058C3073004C,28

awk -F, 'NR==FNR{arr[$1]++;next}arr[$1]' file1 file2

But it only prints the last found record:

60147342,12345712012011235922,2805058C30730030,28 

Thanks!

Answer 1

awk does not allow digit-only variable names (and variable names can't begin with a digit as well), so _1 is a trick to prepend _ in front of 1; so, _1 is a variable name in your awk program, which happens to be an array in this case.

Here is an example:

% awk 'BEGIN {1 = 3}'
awk: cmd. line:1: BEGIN {1 = 3}
awk: cmd. line:1:          ^ syntax error

% awk 'BEGIN {_1 = 3}'  # runs fine

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As an aside, you should use readable variable names in your program to have better maintainability unless you're golfing intentionally.