i've checked the link, and it makes it better. but it doesn't include all the information. i'll continue searching the internet. However i have seen an example of creating a fd:


as in the page, this was intended to redirect the stdout to the fd 5 and create it, and close it. i have the following questions:
- what is exactly the meaning of second command? is it to redirect the command stdout "test" to the fd 5? and how i can see the contents of the fd 5?
- in the first command, why the < is used instead if > and what is the difference between the below two commands as in the info bash *Redirection section

It will be helpful if anyone could include a graph for file descriptor before and after different command execution.

I've played around, and this is what I've come up with:

cmd >word redirects stdout to file word
cmd >&num redirects stdout to file descriptor num
cmd num>word changes file descriptor num to point to file word
cmd num1>&num2 changes file descriptor num1 to point to where file descriptor num2 points
if cmd is "exec", the redirection applies to the shell, otherwise it applies only to that command.

in general, i need to know what is the difference between

for example, if we apply the below direction to the shell by executing them with exec, what is the effect:
4<&1
4>&1
1>&4
1<&4
0>&4
0<&4
4>&0
4<&0

Use num>something when you intend to write to your file descriptor, use num<something when you intend to read from it. The direction of the arrow determines how bash will open the file (see man 2 open).

i'm asking about the special case of redirection (using redirection between file descriptors). for the FD vs File usage it is clear.

As far as I can tell, bash duplicates the file descriptor (see man 2 dup), and I think that the arrow doesn't matter. I would still follow the same guidelines as for files though, so for duplicating stdin (0) use <, for duplicating stdout (1) or stderr (2) use >.