linux – 对现有会话不尊重的组成员身份的更改

>我有两个用户：alice和bob

alice@KAL:~$id alice
uid=3000(alice) gid=3000(alice) groups=3000(alice)
alice@KAL:~$id bob
uid=3001(bob) gid=3001(bob) groups=3001(bob)

>在alice的主目录中,有一个子目录,我想给bob写入权限.

(as alice)
alice@KAL:~$mkdir shared
alice@KAL:~$chmod g+w shared
alice@KAL:~$ls -l
total 4
drwxrwxr-x 2 alice alice 4096 2012-05-15 23:56 shared

>我将group alice(gid = 3000)添加为bob的二级组之一

(as root)
root@KAL:~# id bob
uid=3001(bob) gid=3001(bob) groups=3001(bob)
root@KAL:~# usermod -G 3000 bob
root@KAL:~# id bob
uid=3001(bob) gid=3001(bob) groups=3001(bob),3000(alice)

>我打开一个新的终端,su作为bob,并在alice的主目录中测试我的权限.

(initially as kp,su'ing as bob)
kp@KAL:~$sudo su bob
bob@KAL:/home/kp$cd /home/alice
bob@KAL:/home/alice$ls -l
total 4
drwxrwxr-x 2 alice alice 4096 2012-05-15 23:56 shared
bob@KAL:/home/alice$touch test
touch: cannot touch `test': Permission denied    <-- fails as expected
bob@KAL:/home/alice$cd shared
bob@KAL:/home/alice/shared$touch test     <-- succeeds as expected
bob@KAL:/home/alice/shared$ls -l
total 0
-rw-r--r-- 1 bob bob 0 2012-05-16 00:02 test

>在一个单独的终端中,作为root,我撤销了bob在group alice中的成员资格.

(root)
root@KAL:~# usermod -G 3001 bob
root@KAL:~# id bob
uid=3001(bob) gid=3001(bob) groups=3001(bob)

>现在,回到我被称为bob的终端,很明显会员资格的撤销被认可但没有得到尊重.

(as bob)
bob@KAL:/home/alice/shared$id bob
uid=3001(bob) gid=3001(bob) groups=3001(bob)   <-- group 3000 no longer secondary group
bob@KAL:/home/alice/shared$touch test2        <-- should fail
bob@KAL:/home/alice/shared$ls -l
total 0
-rw-r--r-- 1 bob bob 0 2012-05-16 00:02 test
-rw-r--r-- 1 bob bob 0 2012-05-16 00:20 test2
bob@KAL:/home/alice/shared$rm test            <-- this should also fail
bob@KAL:/home/alice/shared$ls -l
total 0
-rw-r--r-- 1 bob bob 0 2012-05-16 00:20 test2

>如果我现在退出,并且su再次作为bob,则更改是组成员资格现在得到尊重.

(as bob)
bob@KAL:/home/alice/shared$exit
exit
kp@KAL:~$sudo su bob
bob@KAL:/home/kp$cd /home/alice/shared
bob@KAL:/home/alice/shared$ls -l
total 0
-rw-r--r-- 1 bob bob 0 2012-05-16 00:20 test2
bob@KAL:/home/alice/shared$touch test3
touch: cannot touch `test3': Permission denied   <-- now fails as expected
bob@KAL:/home/alice/shared$id bob
uid=3001(bob) gid=3001(bob) groups=3001(bob)
bob@KAL:/home/alice/shared$

这是使用su的一些神器吗？组成员资格是否仅在shell开始时确定？

(这是在运行Ubuntu Maverick 10.10 x86_64 2.6.35-32-generic并运行bash shell的机器上.)