C:\>awk "FNR == NR { if (FNR > 1) word[$1] = $2; next } $1 in word { print $0, word[$1] }" DataSet2.txt DataSet1.txt
A 12 45 hello
A 10 12 hello
A 15 74 hello
C 11 41 blue
C 18 12 blue
B 23 15 goodbye
D 19 10 happy
D 11 12 happy
E 14 5 black
E 34 31 black

C:\>