Describe JoinFiles here.

Join two files on a common field and re-arrange the output:

$ cat a.txt
cust1, ac1, 100
cust2, ac2, 200
$ cat b.txt
100000, cust1, 100
2000000, cust2, 200
$ join -a 1 -a 2 -1 3 -2 3 -t"," -o 1.1 1.2 1.3 2.1 a.txt b.txt
cust1, ac1, 100,100000
cust2, ac2, 200,2000000