$ cat file1.csv 
a,b,c
d,e,f
g,h,i
j,k,l

$ cat file2.csv 
0,w
1,x
2,y
3,z
[download]

$ cat merged.csv
a,b,c,w
d,e,f,x
g,h,i,y
j,k,l,z
[download]