executing 2 scripts with connection

I've been given some challenge: there is an application that provides me some postfix expression and expects to get a solution within 5 seconds. If there is no solution, it quits, otherwise, it provides additional expression. The interface is like follow: etc. If there was only single expression I've thought to use tail -1 command to get the last line to get the expression and use the dc command to evaluate it. However, as there are unknown number of expressions (so far), I can't use it and have to do some loop till the string which is read is not starting with a number (i.e expression). Therefore I've though to use 2 scripts: 1. start the application always send text output to file1 and execute it in the background (script 1) 2. read the last row of a script the file1 (script 2), evaluate a solution and sent it to file2 (script 2) 3. read solution from file2, delete file2 (for next solutions), bring the application to foreground, send a solution and then send the application to background (script 1) However, it seems to be little bit complicate for me. Any suggestions how to simplify the procedure to solve it?