Interacting with a running interactive console program from a different process

Question

I have a console program with an interactive shell, similar to say, the Python interactive shell. Is there an easy way to start this interactive program A and then use another program B to run A? I want to do something like this:

$ /usr/bin/A&
$ #find PID of A somehow
$ somecommand "PID of A" "input string to send to A"
output string from A
$

What kind of "somecommand" could do this? Is this what "expect" is supposed to facilitate? I read the expect man page but still have no idea what it does.

Solution

expect is for a different purpose. It runs commands on a captive program. You, by contrast, are asking for a way to send commands to a process already running in the background.

As a bare-bones minimal example of what you want, let's create a FIFO:

$ mkfifo in

A FIFO is a special file that one process can write to while a different process reads from it. Let's create a process to read from our FIFO file in:

$ python <in &
[1] 3264

Now, let's send python a command to run from the current shell:

$ echo "print 1+2" >in
$ 3

The output from python is 3 and appears here on stdout. If we had redirected python's stdout, it could be sent elsewhere.

What expect does

expect allows you to automate interaction with a captive command. As an example of what expect can do, create a file:

#!/usr/bin/expect --
spawn python
expect ">>>"
send "print 1+2\r"
expect ">>>"

Then, run this file with expect:

$ expect myfile
spawn python
Python 2.7.3 (default, Mar 13 2014, 11:03:55) 
[GCC 4.7.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> print 1+2
3
>>> 

Comments

[user3268289]

Thank you sir, that was exactly what I was wondering. It's too bad man pages can't be as well written as your comment!

[phil294]

this was very helpful, however some interactive prompts would need a tail -f in | python &-like syntax

[John1024]

@Blauhirn Interesting. Can you give me an example?

[phil294]

@John1024 yes: try your python example yourself. the print command can be executed only once, the python shell stops after that. but concerning my previous comment: you'd need that for wish: while wish <in & does work, it only accepts the first interaction (echo, cat instance). I needed to write tail -f in |wish & for it to handle multiple echos. -thanks

[John1024]

Yes, I see that. On the other hand, tail -f in | python is not working for me at all: the python process hangs without ever responding.