Working with Python subprocess - Shells, Processes, Streams, Pipes, Redirects and More

Streams

In UNIX and Linux, there are three I/O channels known as streams which connect a computer program with its environment such as a text terminal (eg gnome-terminal' running Bash) or another computer program (eg a Python program using the ``subprocess` module). These I/O channels are called the standard input, standard output and standard error respectively and can also be refered to by their corresponding file descriptors which are the numbers 0, 1 and 2 respectively.

Here you can see that standarf input is often called stdin, standard output called stdout and standard error called stderr.

The streams work as follows: input from the terminal is sent via the standard input stream to the program, normal output is returned from the program via the standard output and error messages are returned to the environment standard error. The diagram from wikipedia below illustrates this:

Redirecting Standard input and Standard Output to Files

You can redirect the standard output from a program to a file using the shell operator > in Bash (other shells may have slightly different syntax). Here's an example:

$ program1 > file1

This results in program1 being executed and its standard output stream being written to file1, replacing any existing data in file1. If you'd wanted to append data to the end of the file instead you could use the shell operator >> in Bash:

$ program1 >> file1

The shell operator < can be used to read data from a file and send it to the standard input stream of a program:

$ program1 < file1

Again, program1 is executed but this time file1 is the source of the data for standard input instead of the keybaord.

You can actually combine shell operators to achieve more sophisticated results. In the example below program1 is executed and data from file1 is sent to its standard input. The standard output from running program1 with the input from file1 is then written to file2:

$ program1 < file1 > file2

There may be times when you want the output from one program to be read as the input to another program. You can achieve this using temporary files like this:

$ program1 > tempfile1
$ program2 < tempfile1
$ rm tempfile1

This is a bit cumbersome though so shells provide a facility called piping.

Piping

Piping allows the standard output from one program to be fed directly into the standard input of another without the need for a temporary file:

$ program1 | program2

The | character is known as the pipe character and so this process is known as piping.

Here's another picture from wikipedia illustrating this:

Introducing subprocess

Now that we've discussed the sort of functionality offered on the command line, let's experiment with the subprocess module. Here's a simple command you can run on the command line:

$ echo "Hello world!"
Hello world!

Let's try to run this from Python.

In the past, process management in Python was dealt with by a large range of different Python functions from all over the standard library. Since Python 2.4 all this functionality has been carefully and neatly packaged up into the subprocess module which provides one class called Popen which is all you need to use.

The Popen class takes the following options which are all described in detail at http://docs.python.org/library/subprocess.html#using-the-subprocess-module:

subprocess.Popen(args, bufsize=0, executable=None, stdin=None,
    stdout=None, stderr=None, preexec_fn=None, close_fds=False,
    shell=False, cwd=None, env=None, universal_newlines=False,
    startupinfo=None, creationflags=0
)

Luckily most of the time you will just need to use args and shell.

Using the Shell

Let's start with a simple example and run the Hello World! example in the same way as before, passing the command though the shell:

>>> import subprocess
>>> subprocess.Popen('echo "Hello world!"', shell=True)
Hello world!
<subprocess.Popen object at 0x...>

As you can see, this prints Hello world! to the standard output as before but the interactive console also displays that we have created an instance of the subprocess.Popen class.

If you save this as process_test.py and execute it on the command line you get the same result:

$ python process_test.py
Hello world!

So far so good.

You might we wondering which shell is being used. On Unix, the default shell is /bin/sh. On Windows, the default shell is specified by the COMSPEC environment variable. When you specify shell=True you can customise the shell to use with the executable argument.

>>> subprocess.Popen('echo "Hello world!"', shell=True, executable="/bin/bash")
Hello world!
<subprocess.Popen object at 0x...>

This example works the same as before but if you were using some shell-specific features you would notice the differences.

Let's explore some other features of using the shell:

Variable expansion:

>>> subprocess.Popen('echo $PWD', shell=True)
/home/james/Desktop
<subprocess.Popen object at 0x...>

Pipes and redirects:

subprocess.Popen('echo "Hello world!" | tr a-z A-Z 2> errors.txt', shell=True)
<subprocess.Popen object at 0x...>
>>> HELLO WORLD!

The errors.txt file will be empty because there weren't any errors. Interstingly on my computer, the Popen instance is displayed before the HELLO WORLD! message is printed to the standard output this time. Pipes and redirects clearly work anyway.

Here documents:

>>> subprocess.Popen("""
... cat << EOF > new.txt
... Hello World!
... EOF
... """, shell=True)
<subprocess.Popen object at 0xb7dbbe2c>

The new.txt file now exists with the content Hello World!.

Unsurprisingly the features that work with a command passed to the shell via the command line also work when passed to the shell from Python, including shell commands.

Strings or Argument Lists

While it is handy to be able to execute commands pretty much as you would on the command line, you often need to pass variables from Python to the commands you are using. Let's say we wanted to re-write this function to use echo:

def print_string(string):
    print string

You might do this:

def print_string(string):
    subprocess.Popen('echo "%s"'%string, shell=True)

This will work fine for the string Hello World!:

>>> print_string('Hello world!')
Hello world!

But not for this:

>>> print_string('nasty " example')
/bin/sh: Syntax error: Unterminated quoted string

The command being executed is echo "nasty " example" and as you can see, there is a problem with the escaping.

One approach is to deal with the escaping in your code but this can be cumbersome because you have to deal with all the possible escape characters, spaces etc.

Python can handle it for you but you have to avoid using the shell. Let's look at this next.

Without the Shell

Now let's try the same thing without the shell:

def print_string(string):
    subprocess.Popen(['echo', string], shell=False)

>>> print_string('Hello world!')
Hello world!
>>> print_string('nasty " example')
nasty " example

As you can see it works correctly with the correct escaping.

>>> subprocess.Popen('echo "Hello world!"', shell=False)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.5/subprocess.py", line 594, in __init__
    errread, errwrite)
  File "/usr/lib/python2.5/subprocess.py", line 1147, in _execute_child
    raise child_exception
OSError: [Errno 2] No such file or directory

Reading from Standard Output and Standard Error

When you run programs with Popen, output is just sent to stdout as usual which is why you've been able to see the output from all the examples to far.

If you want to be able to read the standard output from a program you have to set the stdout argument in the initial call to Popen to specify that a pipe should be opened to the process. The special value you set is subprocess.PIPE:

subprocess.PIPE

Special value that can be used as the stdin, stdout or stderr argument to Popen and indicates that a pipe to the standard stream should be opened.

Here's an example:

>>> process = subprocess.Popen(['echo', 'Hello World!'], shell=False, stdout=subprocess.PIPE)

To read the output from the pipe you use the communicate() method:

>>> print process.communicate()
('Hello World!\n', None)

The value returned from a call to communicate() is a tuple with two values, the first is the data from stdout and the second is the data from stderr.

Here's a program we can use to test the stdout and stderr behaviour. Save it as test1.py:

import sys
sys.stdout.write('Message to stdout\n')
sys.stderr.write('Message to stderr\n')

Let's execute this:

>>> process = subprocess.Popen(['python', 'test1.py'], shell=False, stdout=subprocess.PIPE)
Message to stderr
>>> print process.communicate()
('Message to stdout\n', None)

Notice that the message to stderr gets displayed as it is generated but the message to stdout is read via the pipe. This is because we only set up a pipe to stdout. Let's set up one to stderr too.

>>> process = subprocess.Popen(['python', 'test1.py'], shell=False, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
>>> print process.communicate()
('Message to stdout\n', 'Message to stderr\n')

This time both stdout and stderr can be accessed from Python.

Now that all the messages have been printed, if we call communicate() again we get an error:

>>> print process.communicate()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.5/subprocess.py", line 668, in communicate
    return self._communicate(input)
  File "/usr/lib/python2.5/subprocess.py", line 1207, in _communicate
    rlist, wlist, xlist = select.select(read_set, write_set, [])
ValueError: I/O operation on closed file

The communicate() method only reads data from stdout and stderr, until end-of-file is reached.

Writing to Standard Input

Writing to a process is very similar. In order to write to the process you have to open a pipe to stdin. Once again this can be done by specifying stdin=subprocess.PIPE as an argument to Popen.

To test it let's write another program which simply prints Received: and then repeats the message you send it. It will repeat 3 messages before exiting. Call this test2.py:

import sys
input = sys.stdin.read()
sys.stdout.write('Received: %s'%input)

To send a message to stdin, you pass the string you want to send as the input argument to communicate(). Let's try the program:

>>> process = subprocess.Popen(['python', 'test2.py'], shell=False, stdin=subprocess.PIPE)
>>> print process.communicate('How are you?')
Received: How are you?(None, None)

Notice that the message generated in the test2.py process was printed to stdout and then the return value (None, None) was printed because no pipes were set up to stdout or stderr.

You can also specify stdout=subprocess.PIPE and stderr=subprocess.PIPE just as before to set up the pipes.

Accessing Return Values, poll() and wait()

When a program exits it can return an integer value to indicate the exit status. Zero is considered “successful termination” and any nonzero value is considered “abnormal termination” by shells and the like. Most systems require it to be in the range 0-127, and produce undefined results otherwise. Some systems have a convention for assigning specific meanings to specific exit codes, but these are generally underdeveloped; Unix programs generally use 2 for command line syntax errors and 1 for all other kind of errors.

You can access the return code from an exited child process via the .returncode attribute of a Popen instance. Here's an example:

>>> process = subprocess.Popen(['echo', 'Hello world!'], shell=False)
>>> process.poll()
>>> print process.returncode
None
>>> process.poll()
0
>>> print process.returncode
0

The returncode value is not ever set by the child process, it starts off with a default value of None, and remains None until you call a method in the subprocess such as poll() or wait(). Those methods set and then return returncode. As a result, if you want to know what the status of the child process is, you have to call either poll() or wait().

The poll() and wait() methods are subtley different:

Popen.poll()

Check if child process has terminated. Set and return .returncode attribute.

Popen.wait()

Wait for child process to terminate. Set and return .returncode attribute.

Convenience Functions

The subprocess module has some convenience functions to make executing a command in the simple case more straightforward. I don't use them though.

Understanding sys.argv

If you are writing a Python program to accept command line arguments, the arguments passed on the command line are accessed as sys.argv. Here's a simple example, save it as command.py:

#!/usr/bin/env python

if __name__ == '__main__':
    import sys
    print "Executable: %s"%sys.argv[0]
    for arg in sys.argv[1:]:
        print "Arg: %s"%arg

The if __name__ == '__main__': line ensures the code beneath it only gets run if the file is executed, not if it is imported. Make this file executable:

$ chmod 755 command.py

Here are some examples of this being run:

$ python command.py
Executable: command.py
$ python command.py arg1
Executable: command.py
Arg: arg1
$ python command.py arg1 arg2
Executable: command.py
Arg: arg1
Arg: arg2

Notice that sys.argv[0] always contains the name of the script which was executed, regardless of how Python was invoked. sys.argv[1] and onward are the command line arguments. You can also invoke the program using Python's -m switch to run it as a module import:

$ python -m command
Executable: /home/james/Desktop/command.py
$ python -m command arg1
Executable: /home/james/Desktop/command.py
Arg: arg1
$ python -m command arg1 arg2
Executable: /home/james/Desktop/command.py
Arg: arg1
Arg: arg2

Once again, Python works out that python -m command are all part of the invoking command so sys.argv[0] only contains the path of the Python script. Let's try executing it directly:

$ ./command.py
Executable: ./command.py
$ ./command.py arg1
Executable: ./command.py
Arg: arg1
$ ./command.py arg1 arg2
Executable: ./command.py
Arg: arg1
Arg: arg2

As you can see, sys.argv[0] still contains the Python script and sys.argv[1] and onwards represent each argument.

Further Reading

See also:

• http://www.doughellmann.com/PyMOTW/subprocess/ (and its O'Reilly copy here)

• http://docs.python.org/library/subprocess.html

• http://webpython.codepoint.net/cgi_shell_command

• http://www.artima.com/weblogs/viewpost.jsp?thread=4829 (About writing main() functions)

Topics for a future post:

• Send and receive signals between running processes

• Run a program in the background or the foreground