近日遇到一怪事，执行usermod更改用户家目录时，被提示usermod: user xxx is currently logged in，可是用户根本没有登陆到系统里啊。查了下，网上普遍的解决方法是备份以后清空/var/run/utmp文件，再重启以后就好了。可说的容易，服务器哪是想重启就重启的？
$ usermod -d /some/opther/path userfoo
usermod: user userfoo is currently logged in
$ who
myuser pts/0 2013-06-17 11:29 (1.2.3.4)
$ ps aux | grep userfoo
root 16724 0.0 0.0 103236 884 pts/0 S+ 11:36 0:00 grep userfoo
根本原因：chmod是根据lsof判断用户是否已经logged in的。通过lsof找出“正在登陆”的用户，kill掉相关进程就可以了。
$ lsof | grep userfoo
dbus-daem 29904 userfoo cwd unknown /proc/29904/cwd (readlink: Permission denied)
dbus-daem 29904 userfoo rtd unknown /proc/29904/root (readlink: Permission denied)
dbus-daem 29904 userfoo txt unknown /proc/29904/exe (readlink: Permission denied)
dbus-daem 29904 userfoo NOFD /proc/29904/fd (opendir: Permission denied)
dbus-daem 31889 userfoo cwd unknown /proc/31889/cwd (readlink: Permission denied)
dbus-daem 31889 userfoo rtd unknown /proc/31889/root (readlink: Permission denied)
dbus-daem 31889 userfoo txt unknown /proc/31889/exe (readlink: Permission denied)
dbus-daem 31889 userfoo NOFD /proc/31