How to merge two files using AWK? [duplicate]

by 
$ awk 'FNR==NR{a[$1]=$2 FS $3;next}{ print $0, a[$1]}' file2 file1
4050 S00001 31228 3286 0 12.1 23.6
4050 S00012 31227 4251 0 12.1 23.6
4049 S00001 28342 3021 1 14.4 47.8
4048 S00001 46578 4210 0 23.2 43.9
4048 S00113 31221 4250 0 23.2 43.9
4047 S00122 31225 4249 0 45.5 21.6
4046 S00344 31322 4000 1

Explanation: (Partly based on another question. A bit late though.)

FNR refers to the record number (typically the line number) in the current file and NR refers to the total record number. The operator == is a comparison operator, which returns true when the two surrounding operands are equal. So FNR==NR{commands} means that the commands inside the brackets only executed while processing the first file (file2 now).

FS refers to the field separator and $1, $2 etc. are the 1st, 2nd etc. fields in a line. a[$1]=$2 FS $3 means that a dictionary(/array) (named a) is filled with $1 key and $2 FS $3 value.

; separates the commands

next means that any other commands are ignored for the current line. (The processing continues on the next line.)

$0 is the whole line

{print $0, a[$1]} simply prints out the whole line and the value of a[$1] (if $1 is in the dictionary, otherwise only $0 is printed). Now it is only executed for the 2nd file (file1 now), because of FNR==NR{...;next}.

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