Python subprocess stdin pipe

Question

Hello i have such problem. I need to login via subprocess. So i want to create a input pipe and write login and password to them.So i try to do something like this:

sp = subprocess.Popen([cmd],stdout=subprocess.PIPE, stdin=subprocess.PIPE)
i, o = sp.communicate()

After first string i have such error tcgetattr: Inappropriate ioctl for device. And i have None value of o. How can i write smth to pipe in this case. Thank you for your help.

• Ashwini Chaudhary: You're not providing anything for stderr: stderr=subprocess.PIPE

Answer 1

From subprocess.communicate(input) docs:

The optional input argument should be data to be sent to the child process ...
Note that if you want to send data to the process’s stdin, you need to create the Popen object with stdin=PIPE. Similarly, to get anything other than None in the result tuple, you need to give stdout=PIPE and/or stderr=PIPE too.

i.e., to pass input to the subprocess, specify input parameter; to get a non-None o value, you must specify stderr=PIPE.

In many case password prompt and password itself are not written to/read from a pipe but directly to/from terminal i.e., you might need a pseudo-tty to pass the password unless the command allows to set an option to accept password via stdin. You could use pexpect to interact with a child process using pty. See the first reason in Why not just use a pipe (popen())?