subprocess ssh command fails for some commands but not others (command works in terminal)

Question

As part of a python script, I am hoping to capture the output of a shell command executed via ssh, namely

ssh User@999 screen -list

If I execute the above command directly in terminal, I get the results I need. However, when executing through subprocess.check_output as below, I get a non-zero exit status 1 error.

I am able to execute other commands via ssh and capture the output without problem.
Is there something specific about screen -list that does not like being called in this fashion?

import subprocess 

srvr = '[email protected]'

print("CMD 1: ===============")
cmd1 = "ssh " + srvr + " ls -l"
print ("COMMAND IS .....  " + cmd1 + "\n")
out1 = subprocess.check_output(cmd1, shell=True)
print(out1 + "\n")

print("CMD 2: ===============")
cmd2 = "ssh " + srvr + " screen -list"
print ("COMMAND IS .....  " + cmd2 + "\n")
out2 = subprocess.check_output(cmd2, shell=True)
print(out2 + "\n")

Error:

subprocess.CalledProcessError: Command '['ssh [email protected] screen', '-list']' returned non-zero exit status 1

Have you tried capturing some more output say from stderr of the child process(es)? This may give you a clue as to what's going on... — James Mills
– James Mills, Commented Dec 10, 2013 at 15:41

falsetru · Accepted Answer · 2013-12-10 15:42:40Z

1

subprocess.check_output check the exit code of the subprocess; and it raises exception if the exit code is not zero.

If you don't care about exit code, use subprocess.Popen.communicate:

out1, err1 = subprocess.Popen(cmd1,
                              stdout=subprocess.PIPE,
                              stderr=subprocess.PIPE).communicate()

answered Dec 10, 2013 at 15:42

falsetru

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2 Comments

Ricardo Saporta Over a year ago

I was about to update my question that appending ; exit 0 to my command gets me the desired output. Any drawback to that method?

falsetru Over a year ago

@RicardoSaporta, If you don't care about the exit code, I think it's okay. BTW, it will not work in Windows cmd.

William Denman · Accepted Answer · 2013-12-10 15:43:42Z

0

That's how subprocess.check_output() is supposed to work. See: http://docs.python.org/2/library/subprocess.html

The command on your server is returning a non zero return code and thus is raising the appropriate Exception CalledProcessError.

answered Dec 10, 2013 at 15:43

William Denman

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2 Comments

Ricardo Saporta Over a year ago

while what you say is accurate, it really provides no useful information

William Denman Over a year ago

Sorry about that. Was writing while on the go.

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