Shell Programming and Scripting

test_a component_b 
test_b component_k
test_c component_d
test_g component_b
test_k component_a
....

test_a note1 
test_b note5 
test_c note1 
test_p note a

test_a component_b note 1
test_b component_k note 5
test_c component_d note 1
test_g component_b default
test_k component_a default

$
$ cat fileA
test_a component_b
test_b component_k
test_c component_d
test_g component_b
test_k component_a
$
$
$ cat fileB
test_a note1
test_b note5
test_c note1
test_p note a
$
$
$ join fileA fileB ; join fileA fileB -v 1 | sed 's/$/ default/'
test_a component_b  note1
test_b component_k note5
test_c component_d note1
test_g component_b default
test_k component_a default
$

awk 'NR==FNR{a[$1]=$2;next}{print $0, (a[$1])?a[$1]:"default"}' FileB FileA

BEGIN { 
     FS = ",";
     OFS = ",";
     print ("first","second", "third", "fourth");
     }
     FNR==NR { 
         a[$1]=$1; 
         b[$1]=$2;
         c[$1]=$3;
         next };
{
    if ($1 in a) {print (a[$1], $2, b[$1], c[$1])}
       
 else {print ($1, $2, "default", "0")}
       }

print ("first","second", "third", "fourth");

===> print "first second third fourth";

a[$1]=$1

if ($1 in a) {print $1, $2, b[$1], c[$1]}