awk ' FILENAME=="fileb" {arr[$1]+=$2}
        FILENAME=="filea" {arr{$1]+=$2}
        END{for (i in arr {print i, arr[i] } ' filea fileb > filec

awk '{a[$1]+=$2}END{for(i in a) print i, a[i]}' file1 file2 file3 .... fileX > output_file

nawk '{
if(NR==FNR)
	arr[$1]=$2
else
	arr[$1]+=$2
}
END{
for(i in arr)
	print i" "arr[i]
}' a b

open(FH,"<a");
while(<FH>){
	$_=~tr/\n//d;
	@arr=split(" ",$_);
	$hash{$arr[0]}=$arr[1];
}
close(FH);
open(FH1,"<b");
while(<FH1>){
	$_=~tr/\n//d;
	@arr=split(" ",$_);
	$hash{$arr[0]}+=$arr[1];
}
close(FH1);
for $key (sort keys %hash){
	print $key," ",$hash{$key},"\n";
}

File A
SGD 8080000001 400
USD 8080000002 300
HK 8080000003 200


File B
SGD 8080000001 600
SGD 8080000002 200
HK 8080000004 200


I want one file where

File OUT
SGD 8080000001 1000
USD 8080000002 300
SGD 8080000002 200
HK 8080000003 200
HK 8080000004 200

awk '{a[$1 FS $2]+=$3}END{for(i in a) print i, a[i]}'  file1 file2 file3 .... fileX> output_file