Interacting programs stdin and stdout (create a robot program to play a game)

Question

The setup: I have an executable file, that I "cannot edit", since I don't have the source code. When I execute the program it reveals a game, where it hands me some numbers via stdout, for me to calculate, when I have done my calculation I can give my answer on stdin and it will tell rather I got the calculation right or not. Then I get a new calculation and it all repeats.

The problem: I could just manually play this game, but I would like to automate this with a program. The available tools on the system is Bash and a C-compiler. What is the easiest way to interact with this program?

What I have done so far: I have been looking at pipes in C, and have a little program running where I fork the process and use pipes to communicate between the two processes. (something like this program). But I cannot get this to work when i call an external binary from the child process.

Answer 1

You can do this using pipes and FIFO.

For example, here is my test script blackbox.sh:

#!/bin/bash

# Here i output the value to be added.
echo "10 30"

# Here i read the result and depending on it, write good or not good in a file
read result
if [ "${result}" -eq 40 ]; then
    echo good > result.txt
else
    echo not good > result.txt
fi

Ran manually:

$ ./blackbox.sh 
10 30 <~~~~ Script output
40 <~~~~~~~ My input
$ cat result.txt 
good
$ 

Now, i create a FIFO with mkfifo coin and run again with the following line:

$ rm result.txt && cat coin |./blackbox.sh | ( read a b ; echo $((a+b)) > coin )
$ cat result.txt 
good

Note that:

• Any write to the FIFO will block as long as it's not opened for reading.
• Piping the output of blackbox.sh to something will, obviously, make it invisible to you.
• I voluntary choose an simple example, where output of blackbox.sh is easy to parse... I let you adapt this to your need.
• $(( )) allow arithmetic in Bash.