Using AWK to Process Input from Multiple Files

by 
awk 'FNR==NR{a[$1]=$2 FS $3;next}

here we handle the 1st input (file2). say, FS is space, we build an array(a) up, index is column1, value is column2 " " column3 the FNR==NR and next means, this part of codes work only for file2. you could man gawk check what are NR and FNR

{ print $0, a[$1]}' file2 file1

When NR != FNR it’s time to process 2nd input, file1. here we print the line of file1, and take column1 as index, find out the value in array(a) print. in another word, file1 and file2 are joined by column1 in both files.

for NR and FNR, shortly,

1st input has 5 lines
2nd input has 10 lines,

NR would be 1,2,3...15
FNR would be 1...5 then 1...10

you see the trick of FNR==NR check.

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