How to implement if nesting in nginx

# 是否为移动端
set $mobile 0;
if ($http_user_agent ~* (mobile|nokia|iphone|ipad|android|samsung|htc|blackberry)) {
  set $mobile 1;
}

# 获取子域名
set $prefix 1;
if ($host ~* "sub1.test.com") {
  set $prefix 2;
}
if ($host ~* "sub2.test.com") {
  set $prefix 3;
}
if ($host ~* "sub3.test.com") {
  set $prefix 4;
}
set $sign "${mobile}${prefix}";
if ($sign = 11) {
  rewrite ^(.*) http://m.test.com$1 permanent;
}
if ($sign = 12) {
  rewrite ^(.*) http://m.test.com/sub1$1 permanent;
}
if ($sign = 13) {
  rewrite ^(.*) http://m.test.com/sub2$1 permanent;
}
if ($sign = 14) {
  rewrite ^(.*) http://m.test.com/sub3$1 permanent;
}