How to detect that foreground process is waiting f

Question

I have to create a script (ksh or perl) that starts certain number of parallel jobs (another scripts), each of them runs as a foreground process in a separate session. Plus I start monitoring job that has to determine if any of those scripts is expecting input from operator, and switch to the corresponding session if necessary. My problem is that I have not found a good way to determine that process is expecting input. For the background process it's pretty easy: process state is "stopped" and this can be easily checked with 'ps' command. In case of foreground process this does not work. So far I tried to attach to the process with dbx or truss to see if it's hanging on 'read', but this approach seems too heavyweight. Could you suggest some better solution? Perl, shell, C, Java, etc. … is ok as long as it’s standard and does not require extra 3rd party or OS-specific stuff to install.

Thank you.

Answer 1

What you're asking isn't possible, at least not reliably. The process may be using select or other polling method rather than blocking on a read call. You can't know whether it's waiting for operator input or busy doing other stuff, and in general it could be both (doing stuff in the background while being responsive to operator input).

The normal way for a program to signal that it's waiting for operator input is to print a prompt. Thus you should consider a session to be active if it's displayed a prompt since the last time you fed it input.

If your programs don't behave this way, you'll need to find some other program-specific way to know that these processes are waiting for input.