Shell Programming and Scripting

$ cat f1
servera,1,2
serverb,25,30
serverc,35,45

$ cat f2
servera,15
serverb,10
serverc,5

$ awk -F',' '{if(NR == FNR) {ar[$1]=$0;next} print ar[$1]","$2}' f1 f2
servera,1,2,15
serverb,25,30,10
serverc,35,45,5
$

join -t',' -j 1 f1 f2

$ awk -F',' 'NR==FNR {ar[$1]=$0;next} 
            {  if($1 in ar) {  print ar[$1] "," $2;delete ar[$1] } 
                else print $0
            } 
            END { for (i in ar) print ar[i] }' master_file file1 > master_file.tmp

$ mv master_file.tmp master_file