ubuntu dapper,gnome 进不去，连man等程序都无法运行

uniqueman

xsession-error 显示如下：

Your session only lasted less than 10 seconds.  If you have not logged out
yourself this could mean that there is some installation problemor that you may
be out of diskspace.  Try logging in with one of the failsafe sessions to see   
if you can fix this problem.

then there's a checkbox with "View details (~/.Xsession-errors file)" written
next to it. I click on the box and get this:

/etc/gdm/PreSession/Default: Registering your session with wtmp and utmp
/etc/gdm/PreSession/Default: running: /usr/bin/X11/Sessreg -a -w /var/log/wtmp
-u /var/run/wtmp -x "/var/lib/gdm/:0.Xservers" -h "" -l ":0" "robert"           
/etc/gdm/Xsession: /etc/gdm/Xsession:can not execute binary file

并且运行man ls，会显示/usr/bin/troff :cannot execute binary file
这个是什么问题？
好多命令都不能运行，不过还有一些可以运行
很奇怪