Shell Programming and Scripting

ColumnA  ColumnB ColumnC 
Client 1     XXXXX  Server A
Client 2     XXXXX  Server B 
Client 3     XXXXX   Server C

ColumnA  CloumnB   
Server A    value 1
Server B     value 2 
Server C     value 1

ColumnA  ColumnB ColumnC   CoulmnD
Client 1     XXXXX  Server A    Value 1
Client 2     XXXXX  Server B     Value 2 
Client 3     XXXXX   Server C    value1

awk 'FNR==NR{a[$1]=$0 FS $0;next}{ print $0, a[$1]}' file1 file2

awk 'NR==FNR{A[$1]=$2} $3 in A{print $0, A[$3]}' FS='\t' OFS='\t'  file2 file1

$ cat file1
"ColumnA","ColumnB","ColumnC"
"Client 1","XXXXX","Server A"
"Client 2","XXXXX","Server B"
"Client 3","XXXXX","Server C"

$ cat file2
"ColumnA","ColumnB"
"Server A",1
"Server B",2
"Server C",1