Python - subprocess.Popen不返回输出

Question

I am using a Windows 7 machine right now have have STS installed with PyDev utility for python development.

My code wants to login to a machine and return the output. My current code is:

        ssh = subprocess.Popen(["uname -a"], shell=True, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
        print (ssh)
        result = ssh.stdout.readlines()
        print (result)

This yileds an output of :

<subprocess.Popen object at 0x02B35810>
[]

But the uname -a command should yield Linux 3.10.0-514.10.2.el7.x86_64 #1 SMP Mon Feb 20 02:37:52 EST 2017 x86_64 x86_64 x86_64 GNU/Linux

Any idea why I cant capture the output ?

Answer 1

You can call uname with command/args properly split in a list like this:

subprocess.Popen(["uname","-a"] ...

or quick&dirty in a string like this (you have to handle quoting, but here there's none):

subprocess.Popen("uname -a", ...

but not like this:

subprocess.Popen(["uname -a"] ...

else the actual issued command is "uname -a" (literally). Of course, it doesn't exist, which should throw an exception, but since you're using shell=True it doesn't since it runs the nonexisting command in a shell (which exists, so subprocess doesn't complain)

that's one of the numerous reasons why you shouldn't throw shell=True in without being absolutely forced to (for instance to run a shell internal command)

Checking the error stream (using result = ssh.stderr.readlines() ) would reveal something like "uname -a": command not found

Note that using PIPE for both output and error streams may lead to deadlock unless you're using ssh.communicate() to handle streams in separate threads internally.

My suggestion for your problem would simply to use check_output as you don't need to run the command in background, you just need its output:

result = subprocess.check_output(["uname","-a"])

(which is slightly different because it doesn't create lines but a sole buffer, that you can easily apply strip / splitline onto)

Note that if you're looking for a portable way to get system type or platform, I recommend you the os module ( os.name yields nt for windows for instance) and the platform module, where you can get system architecture, linux distribution, etc...