Is there any way to de-couple stdin into multiple input sources?

Question

I am looking for a way to have two stdins to a bash script, namely one that is interactive and another which could leverage redirection

Say I need to invoke a program which asks for credentials of the type username, password I want to feed that interactively and then I want to feed a while loop from a redirection. Something like:

$> ./myscript.sh < input.lst

or

$> cat input.lst | ./myscript.sh

In my script there would be something like (very simplified):

#!/bin/bash
./authentication_application
while read i
do 
  echo $i
done 

Where authentication_application is a binary I don't own (have no access to source code and haven't compiled it myself), I can't change and can't be replaced with any alternative authentication method.

And the invocation would result in

Username: some_user
Password: password 
... contents of input.lst ...

With some_user and password input interactively and contents of input.lst taken from the redirection

I tried redirecting stdin to a new fd and using read -u <fd> inside the script with fd being "7" for instance, and something like this:

#!/bin/bash
exec <fd><&0
exec 0<&-

... but here is where I have no idea of how to re-open (or reset) stdin to the interactive one

Any Ideas? you people even think it's possible?

BTW, I'm not looking for the following:

• prepending input.lst with username and password on cleartext (nor any kind of cypher hack)
• cat-ing two files into the script (the first one storing username and password on cleartext)
• A workaround where I have to use parameters to get_credentials.sh to pass username and password without prompting the user
• passing the file input.lst as a parameter to the script and using positional parameters to access the file in the while loop (it would defeat the purpose of redirecting the input and leveraging the pipelining nature of the script i.e. $> cat input.lst | ./filter_1.sh | ./filter_2.sh |... | ./filter_n.sh )

Answer 1

Something like this perhaps:

#! /bin/bash
echo -n "Password: "
read pwd < /dev/tty
echo
while read x ;do
echo $x
done
echo "Password was: " $pwd

You invoke it with ./foo.sh < infile; it then prints the prompt and reads a password from the terminal, but processes the loop from its stdin which is redirected to the file.

I'm sure there are gotchas but it is probably good enough for most cases.

Answer 2

NickD's answer gave me the insight to tackle the issue.

This is the working code for the exact specification

#!/bin/bash
./authentication_application < /dev/tty
while read i
do
  echo $i
done

Hoping someone finds it useful