combine two files into one

pet@cow:~/xx$ ls
aa  bb
pet@cow:~/xx$ cat aa bb
aaa
bbb
ccc
ddd
pet@cow:~/xx$ cat aa
aaa
bbb
pet@cow:~/xx$ cat bb
ccc
ddd
pet@cow:~/xx$ cat aa bb >cc
pet@cow:~/xx$ cat cc
aaa
bbb
ccc
ddd