我正在通过redis上的tutorial，遇到一个没有意义的命令。从我的下面的代码中，我得到一个确实仍然存在的密钥的-2生存时间返回值。我的代码不应该返回-1永不过期？初学者Redis命令
的教程说：
Redis can be told that a key should only exist for a certain length of time. This is accomplished with the EXPIRE and TTL commands.
SET resource:lock "Redis Demo" EXPIRE resource:lock 120
This causes the key resource:lock to be deleted in 120 seconds. You can test how long a key will exist with the TTL command. It returns the number of seconds until it will be deleted.
TTL resource:lock => 113 // after 113s TTL resource:lock => -2
The -2 for the TTL of the key means that the key does not exist (anymore). A -1 for the TTL of the key means that it will never expire. Note that if you SET a key, its TTL will be reset.
SET resource:lock "Redis Demo 1"
EXPIRE resource:lock 120
TTL resource:lock => 119
SET resource:lock "Redis Demo 2"
TTL resource:lock => -1
这是我键入到交互终端的代码。我的假设是，第三行应该让我回到-1，永不过期。我从来没有设定过期时间，所以我不知道为什么我会回到-2。
> SET loggedIn "True"
OK
> TTL logggedIn
(integer) -2
> GET loggedIn
"True"