The content of directory /dev/fd have difference file in one time

Question

I have such command.

#echo -e "ls\npwd" >test
#bash
#exec <test

After this code.You will find

#ls /dev/fd

0 1 2 3

#cd /dev/fd;ls

0 1 2 255

Fun,the directory /dev/fd have difference content..What's happen?

Answer 1

/dev/fd is not a fixed directory, but a symlink that points to a directory on a per-process basis. In your two invocations of ls, you are listing the contents of two different "directories".

Your first output gives the file descriptors associated with ls. The second output gives the file descriptors associated with bash. The echo and exec preceding the directory listings do not change this.

Consider:

foo:~$ ls -ln /dev/fd/
total 0
lrwx------ 1 1000 1000 64 Jul 10 12:13 0 -> /dev/pts/19
lrwx------ 1 1000 1000 64 Jul 10 12:13 1 -> /dev/pts/19
lrwx------ 1 1000 1000 64 Jul 10 12:13 2 -> /dev/pts/19
lr-x------ 1 1000 1000 64 Jul 10 12:13 3 -> /proc/20115/fd
foo:~$ cat /dev/fd/../cmdline && echo
cat/dev/fd/../cmdline

foo:~$ cd /dev/fd ; ls -ln
total 0
lrwx------ 1 1000 1000 64 Jul  8 05:01 0 -> /dev/pts/19
lrwx------ 1 1000 1000 64 Jul  8 05:01 1 -> /dev/pts/19
lrwx------ 1 1000 1000 64 Jul  8 05:01 2 -> /dev/pts/19
lrwx------ 1 1000 1000 64 Jul  8 05:01 255 -> /dev/pts/19
foo:/dev/fd$ cat ../cmdline && echo
bash
foo:/dev/fd$ pwd -P
/proc/2689/fd