[root@localhost ~]# !ll
ll /proc/5825/fd
total 0
lrwx------ 1 root root 64 Oct 28 10:46 0 -> /dev/pts/0
lrwx------ 1 root root 64 Oct 28 10:46 1 -> /dev/pts/0
lrwx------ 1 root root 64 Oct 28 10:46 2 -> /dev/pts/0
lrwx------ 1 root root 64 Oct 28 10:46 255 -> /dev/pts/0
[root@localhost ~]# ps
PID TTY TIME CMD
5825 pts/0 00:00:00 bash
6031 pts/0 00:00:00 ps

what is value of 255 for file descriptors?

Sorry, we've got absolutely no idea what you're talking about, or what you're asking.

Write a clear question, and provide complete information, such as version/distro of Linux, and what you're trying to do/accomplish.

my system version is RHEL5.2,kernel version is 2.6.18-92.el5. I want to know bash by default for this file descriptor for the meaning of 255

Usually 255 is a bad or deleted file descriptor, If thats what you are talking about.

Why bash each terminal are open?

What?

if bad or deleted file descriptor,why each terminal are open?
[root@localhost ~]# tty
/dev/pts/1
[root@localhost ~]# ps
PID TTY TIME CMD
6860 pts/1 00:00:00 bash
7200 pts/1 00:00:00 ps
[root@localhost ~]# ll /proc/6860/fd
total 0
lrwx------ 1 root root 64 Oct 28 17:29 0 -> /dev/pts/1
lrwx------ 1 root root 64 Oct 28 17:51 1 -> /dev/pts/1
lrwx------ 1 root root 64 Oct 28 17:50 2 -> /dev/pts/1
lrwx------ 1 root root 64 Oct 28 17:51 255 -> /dev/pts/1
[root@localhost ~]#

[root@localhost ~]# tty
/dev/pts/2
[root@localhost ~]# ps
PID TTY TIME CMD
7171 pts/2 00:00:00 bash
7203 pts/2 00:00:00 ps
[root@localhost ~]# ll /proc/7171/fd
total 0
lrwx------ 1 root root 64 Oct 28 18:19 0 -> /dev/pts/2
lrwx------ 1 root root 64 Oct 28 18:20 1 -> /dev/pts/2
lrwx------ 1 root root 64 Oct 28 18:20 2 -> /dev/pts/2
lrwx------ 1 root root 64 Oct 28 18:20 255 -> /dev/pts/2