why is call to ssh with Python subprocess breaking outer bash loop

Question

I have a bash script myscript.sh:

#!/bin/bash
while read line; do
    myprog.py
done

calling a python program myprog.py

#!/usr/bin/env python
import subprocess
output = subprocess.check_output(['ssh', 'user@host', 'cmd'])

The ssh command that is called by subprocess executes without error, the output is correct. But when called like this the loop in myscript.sh only runs through the first line of input and then exits with status 0. If I replace the subprocess.check_output(...) call with a subprocess.Popen(...) and don't subsequently call Popen.wait() then the outer loop works as expected and the output from the ssh command is dumped to standard out some time after any output from the bash script. With the Popen.wait() behavior is the same as with check_output: bash loop only goes through one iteration before exiting without error.

If instead of ssh another command, e.g. ls, is called with check_output then the bash loop works as expected.

Can anyone help me understand why the code as shown isn't working as expected?

Note: this is a simplified version of what I am trying to do, though I do experience the same behavior with this code. In reality I am doing something with "$line" in the bash script and the subprocess call is wrapped in a try/except block.

Answer 1

As @larsmans guessed the ssh call was consuming stdin, breaking the outer bash loop. Adding the -n option to the ssh command resolved the issue:

output = subprocess.check_output(['ssh', '-n', 'user@host', 'cmd'])

Answer 2

The problem is that ssh reads from standard input, therefore it "eats" all the remaining lines in the loop. You can just connect its standard input to nowhere using the -n flag:

output = subprocess.check_output(['ssh', '-n', 'user@host', 'cmd'])

Look for the details on the man pages of ssh here https://linux.die.net/man/1/ssh and https://man.openbsd.org/ssh