今天使用sudo apt-get 安装包的时候，出现gpg错误，如下：

gpg: symbol lookup error: /usr/local/lib/libreadline.so.6: undefined symbol: UP

google了一下，看到：

////////////////////////////////////////////////////////////////////////////

I fought this problem from synaptic on down to gpg, only to find it was readline all along. My solution was somewhat simpler than building or patching.

in terminal:

ls /usr/local/lib

there was a bunch of readline libs in there (libreadline.so.BLAH-BLAH) so i:

su
mkdir temp
mv /usr/local/lib/libreadline* temp
ldconfig 
apt-get update

and voila. went without a hitch. then i deleted my temporary directory (rm -rf /usr/local/lib/temp), exited su.

~2 hours fighting to no avail, and for it work with a few seconds of commands... 
I'm not complaining, but it does seem somewhat of a cruel irony.

///////////////////////////////////////////////////////////////////////////////////

根据上面的网友回答，问题解决了，apt-get 能够正常工作，看来是因为存在多个版本的libreadline.so导致，但是不知道这个libreadline.so在什么时候出现的，头疼。