Python subprocess get output and return code

#PYTHON SUBPROCESS GET OUTPUT AND RETURN CODE HOW TO#
#PYTHON SUBPROCESS GET OUTPUT AND RETURN CODE CODE#

Here we are instantiating a Popen and setting stdout and stderr to subprocess.PIPE to be able to read the output of our command, we are also setting universal_newline to True so we can work with strings. Proc = Popen(command(directory), stdout=PIPE, stderr=PIPE, universal_newlines=True) LetвЂ™s write a little example which letвЂ™s us see the size of a file/directory. So, what if we want to gain access to the output? Well, Popen will receive the stdout and stderr arguments, which now are safe to use, as we are going to read from them.

#PYTHON SUBPROCESS GET OUTPUT AND RETURN CODE CODE#

Now, PopenвЂs constructor will not raise any errors if the command exit code is not zero, its wait function will return the exit code and, again, itвЂ™s your responsibility to check it and perform any fallback. Now, if we execute this example, weвЂ™ll see it behaves the same as the one written with the subprocess.call. Sanitized_directory = shlex.quote(directory)Д¬ommand = 'du -sh '.format(sanitized_directory) LetвЂ™s see and example of a security issue this can cause: To use this tool in a safer way you can use shlex.quote() to properly escape whitespace and shell metacharacters in strings that are going to be used to construct shell commands. You can avoid this notation by setting the shell flag to a True value, which will spawn an intermediate shell process and tell it to run the command, like this:ДЄlthough the shell=True option might be useful to better exploit the most powerful tools the shell provides us, it can be a security risk, and you should keep this in mind. You will probably see the output of the command in the console as itвЂ™s sent to standard output, but it wonвЂ™t be available within your code using this command this way.ДЄnother thing to notice is that the command to run is being passed as an array, as the Popen interface will then concatenate and process the elements saving us the trouble of escaping quote marks and other special characters. LetвЂ™s see a trivial example:ДЄs you see, itвЂ™s returning 0, as itвЂ™s the return code of the ls -l command. This function runs the command described by args, waits for the command to complete and returns the exit code attribute. The TimeoutExpired exception will be re-raised after the child process has terminated. The only exception is the timeout which is passed to Popen.wait(), if the timeout expires, the child process will be killed and then waited for again. Although these are the most commonly used arguments, this function takes roughly as many arguments as Popen does, and it passes almost them all directly through to that interface. The most commonly used arguments it takes are (args, *, stdin=None, stdout=None, stderr=None, shell=False, timeout=None). The first function weвЂ™ll overview is subprocess.call.

#PYTHON SUBPROCESS GET OUTPUT AND RETURN CODE HOW TO#

Then weвЂ™ll see how to use Popen directly to ease our thirst for knowledge.ДЎ.

WeвЂ™ll first take a look to all the wrapper functions and see what they do and how to call them. But for more advanced use cases, the Popen interface can be used directly. The subprocess module defines a class called Popen and some wrapper functions which take almost the same arguments as its constructor does. The examples provided here were tested on a Unix based system. One thing to notice is that the API is roughly the same, but the underlying implementation is slightly different between Unix and Windows.

The API is consistent for all uses, and operations such as closing files and pipes are built in instead of being handled by the application code separately. It provides all of the functionality of the other modules and functions it replaces. This module is intended to be a replacement to the old os.system and such modules. In this article, using Python 3.4, weвЂ™ll learn about PythonвЂ™s subprocess module, which provides a high-level interface to interact with external commands.

Python subprocess get output and return code