start process in background, get PID, and write into stdin in Shell

Question

I need to do a buffer overflow for my system security course. Therefore I do have a program(called canary) I need start which asks for a input string (read()). I need to calculate a canary(random canary built with PID and time) for a successful buffer overflow. I already wrote a program(getcanary) which gets me the right canary. The problem: I try to start canary in a extra terminal, then get the PID of it, then calculate the program followed by a write to canary's STDIN. The last thing is where I have a Problem.

#!/bin/bash
echo "start canary"
x-terminal-emulator -e ./canary &
sleep 1
PID=$(pgrep canary)
CANARY=$(./getcanary $PID)

How can I write the command to the extra terminal? I already tried several solutions,

echo "cmd" > /proc/$PID/fd/0

is one of it I also tried

mkfifo fifo
cat > fifo &
./canary < fifo
echo "cmd" > fifo

some other solutions are not allowed by my environment, as the script must run on a clean install of xubuntu, so I can't use screen or tmux

I hope you can help me, Thank you! :)

PS.: I'm sorry if I misunderstood any of these solutions I tried, I'm not very familiar with shell scripting.

• Charles Duffy: Put the write into the code that's performed inside the terminal emulator, rather than trying to do it from outside. For example: x-terminal-emulator -e 'echo "cmd" | ./canary'
• C4ptainC4mp: Thank you, the problem is, I need the canary first to build the right canary (otherwise the buffer overflow won't be successful) therefore I need the PID of the canary process and therefore I need to start the process at first
• Charles Duffy: Let's back up here a bit. Why are you running a terminal emulator at all?
• Charles Duffy: The easy way to do this is to run the program as a bash coproc, assuming you don't really need the terminal.
• Charles Duffy: coproc { canary; } puts the PID of the canary in $COPROC_PID, lets you write to it with echo "something" >&${COPROC[1]}, and lets you read from it with read varname <&${COPROC[0]}.

Answer 1

Write to the terminal, not to the running process!

#!/bin/bash
echo "start canary"
x-terminal-emulator -e ./canary &
termpid=$!
sleep 1
xvkbd -window $(xdotool search --sync --pid $termpid) -text "echo Hello world!\n"