CODEWITHSUNDEEP
pythonsubprocesspipestdin
obskyr
obskyr

Reputation: 1459

How to pass subprocess control to regular stdin after using a pipe?

What I'd like to do is to, in Python, programmatically send a few initial commands via stdin to a process, and then pass input to the user to let them control the program afterward. The Python program should simply wait until the subprocess exits due to user input. In essence, what I want to do is something along the lines of:

import subprocess

p = subprocess.Popen(['cat'], stdin=subprocess.PIPE)

# Send initial commands.
p.stdin.write(b"three\ninitial\ncommands\n")
p.stdin.flush()

# Give over control to the user.
# …Although stdin can't simply be reassigned
# in post like this, it seems.
p.stdin = sys.stdin

# Wait for the subprocess to finish.
p.wait()

How can I pass stdin back to the user (not using raw_input, since I need the user's input to come into effect every keypress and not just after pressing enter)?

Upvotes: 1

Views: 622

Answers (2)

Kevin Joyce
Kevin Joyce

Reputation: 111

So searching for this same thing, at least in my case, the pexpect library takes care of this:

https://pexpect.readthedocs.io/en/stable/

p = pexpect.spawn("ssh myhost")
p.sendline("some_line")
p.interact()

As by its name you can automate a lot of interaction before handing it over to the user.

Note, in your case you may want an output filter: Using expect() and interact() simultaneously in pexpect

Upvotes: 0

torek
torek

Reputation: 487755

Unfortunately, there is no standard way to splice your own stdin to some other process's stdin for the duration of that process, other than to read from your own stdin and write to that process, once you have chosen to write to that process in the first place.

That is, you can do this:

proc = subprocess.Popen(...)  # no stdin=

and the process will inherit your stdin; or you can do this:

proc = subprocess.Popen(..., stdin=subprocess.PIPE, ...)

and then you supply the stdin to that process. But once you have chosen to supply any of its stdin, you supply all of its stdin, even if that means you have to read your own stdin.

Linux offers a splice system call (documentation at man7.org, documentation at linux.die.net, Wikipedia, linux pipe data from file descriptor into a fifo) but your best bet is probably a background thread to copy the data.

Upvotes: 3

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