The right module to use is subprocess. Older modules and functions that should be avoided are:

• os.system
• os.spawn*
• os.popen*
• popen2.*
• commands.*

Run command with arguments. Wait for command to complete, then return the returncode attribute.

retcode = subprocess.call(["mycmd", "myarg"])

Same as above, but run in a subshell (in this case the command may contain shell redirections):

retcode = subprocess.call("mycmd myarg", shell=True)

Assign the output to a variable:

output = subprocess.Popen(["mycmd", "myarg"], stdout=subprocess.PIPE).communicate()[0]

Get also the return code, stderr and iterate on the output:

subproc = subprocess.Popen(["mycmd", "myarg"], stdin=None, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
output, stderr = subproc.communicate()
retcode = subproc.returncode
for line in output.splitlines():
    print line

file = open("/tmp/cmd_output", "w")
subprocess.call(["mycmd", "myarg"], stdout=file)
file.close()

cmd1 = ["oggdec", "-Q", "-o", "-", src]
cmd2 = ["lame", "--preset", "cd", "-", dst]
p1 = subprocess.Popen(cmd1, stdout=subprocess.PIPE)
p2 = subprocess.Popen(cmd2, stdin=p1.stdout, stdout=subprocess.PIPE)
p1.stdout.close()
output = p2.communicate()[0]