Why does simple echo in subprocess not working

Question

I'm trying to perform simple echo operation using subprocess:

import subprocess
import shlex

cmd = 'echo $HOME'
proc = subprocess.Popen(shlex.split(cmd), shell=True, stdout=subprocess.PIPE)
print proc.communicate()[0]

But it prints nothing. Even if I change the command to echo "hello, world" it still prints nothing. Any help is appreciated.

Answer 1

On Unix shell=True implies that 2nd and following arguments are for the shell itself, use a string to pass a command to the shell:

import subprocess

cmd = 'echo $HOME'
proc = subprocess.Popen(cmd, shell=True, stdout=subprocess.PIPE)
print proc.communicate()[0],

You could also write it as:

import subprocess

cmd = 'echo $HOME'
print subprocess.check_output(cmd, shell=True),

From the subprocess' docs:

On Unix with shell=True, the shell defaults to /bin/sh. If args is a string, the string specifies the command to execute through the shell. This means that the string must be formatted exactly as it would be when typed at the shell prompt. This includes, for example, quoting or backslash escaping filenames with spaces in them. If args is a sequence, the first item specifies the command string, and any additional items will be treated as additional arguments to the shell itself. That is to say, Popen does the equivalent of:

Popen(['/bin/sh', '-c', args[0], args[1], ...])

Answer 2

You are confusing the two different invocations of Popen. Either of these will work:

proc=subprocess.Popen(['/bin/echo', 'hello', 'world'], shell=False, stdout=subprocess.PIPE)

or

proc=subprocess.Popen('echo hello world', shell=True, stdout=subprocess.PIPE)

When passing shell=True, the first argument is a string--the shell command line. When not using the shell, the first argument is a list. Both produce this:

print proc.communicate()
('hello world\n', None)