Python- How To Get Output Of Sudo Command Using Popen

Question

sudo command validates the password of the user against the user name. I am writing a simple program which checks if the user's input is valid according to the sudo command. I am using the -S tag in order to pass an input externally, and i am using Popen() to run the script:

import getpass
import subprocess
import time

password = getpass.getpass()
proc = subprocess.Popen('sudo -k -S -l'.split(), stdin=subprocess.PIPE, stderr=subprocess.PIPE, stdout=subprocess.PIPE)
output = proc.communicate(password.encode())
print(f'This is the value of the output {output[0]}')

If the user inputs the wrong password in the getpass.getpass() I need to validate it against the actual user password. In this case, sudo should output Sorry Please Try Again. Could someone please let me know how I can read that error?

When I run this in the terminal:

➜  Desktop python3.8 test.py
Password: 
Sorry, try again.
This is the value of the output b''

Thanks, cheers and I really appreciate your help.

The command gets stuck and nothing happens

XtremeDevX
– XtremeDevX

2020-08-19 17:42:00 +00:00
Commented Aug 19, 2020 at 17:42 — XtremeDevX
– XtremeDevX, Commented Aug 19, 2020 at 17:42

that other guy · Accepted Answer · 2020-08-19 17:59:12Z

1

The Python Documentation says:

Note that if you want to send data to the process’s stdin, you need to create the Popen object with stdin=PIPE.

So use

proc = subprocess.Popen(['sudo', '-S', '-l'], stdin=subprocess.PIPE, stderr=subprocess.PIPE, stdout=subprocess.PIPE)

to allow writing a password with communicate, otherwise it just waits for you to enter it on the terminal (without a prompt since it's being captured).

Any error messages are available in the output variable, specifically output[1] which corresponds to stderr.

edited Aug 19, 2020 at 17:59

answered Aug 19, 2020 at 17:45

that other guy

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4 Comments

XtremeDevX Over a year ago

Yes, I have tried that, and now it is taking the input, however, how can i capture the "Sorry please try again" ?

that other guy Over a year ago

You have captured it. It's in your output variable, but in output[1] while you just looked at output[0]

XtremeDevX Over a year ago

thanks so much, what would be a valid method now to confirm if the password is wrong ? the output when i write the wrong password is this

[sudo] password for tejas: [sudo] password for tejas:  sudo: no password was provided sudo: 1 incorrect password attempt

that other guy Over a year ago

You can set the environment variable LC_ALL=C to make sure the language is always the same and then e.g. match the substring "incorrect password". Hopefully it's fairly stable over time. Not sure why you'd ever try to do this, but that's a different matter.

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