Using awk how do I combine data in two files and substitute values from the second file to the first file?

Question

Any ideas how to the following using awk?

Two input files, data.txt and keys.txt:

data.txt contains some data:

A;1
B;2
A;3

Answer 1

awk solution:

awk -F';' 'NR==FNR{a[$1]=$2; next}{if($1 in a) $0=$0 FS a[$1]; print}' file2 file1

The output:

A1;1;2
A2;2;1
A3;3;0.5
A1;1;2
A2;2;1
A3;3;0.5

---

• NR==FNR - processing the first file i.e. file2

• a[$1]=$2 - accumulating additional values for each key

• if($1 in a) $0=$0 FS a[$1] - appending value if first column matches

Answer 2

awk to the rescue!

assumes the second file has unique keys unlike first file (if not you need to specify what happens then)

$ awk 'BEGIN   {FS=OFS=";"} 
       NR==FNR {a[$1]=$2; next} 
       $1 in a {print $0,a[$1]}' file2 file1

A;1;30
B;2;20
A;3;30

ps. note the order of files...