(1) os.system
仅仅在一个子终端运行系统命令，而不能获取命令执行后的返回信息
system(command) -> exit_status
execute the command (a string) in a subshell.
如果再命令行下执行，结果直接打印出来
>>> os.system('ls')
04101419778.chm   bash      document    media      py-django   video
11.wmv            books     downloads   pictures  python
all-20061022      desktop   examples    project    tools
(2) os.popen
该方法不但执行命令还返回执行后的信息对象
popen(command [, mode='r' [, bufsize]]) -> pipe
open a pipe to/from a command returning a file object.
>>>tmp = os.popen('ls *.py').readlines()
>>>tmp
out[21]:
['dump_db_pickle.py ',
'dump_db_pickle_recs.py ',
'dump_db_shelve.py ',
'initdata.py ',
'__init__.py ',
'make_db_pickle.py ',
'make_db_pickle_recs.py ',
'make_db_shelve.py ',
'peopleinteract_query.py ',
'reader.py ',
'testargv.py ',
'teststreams.py ',
'update_db_pickle.py ',
'writer.py ']
好处在于：将返回的结果赋于一变量，便于程序的处理。
(3)  使用模块 subprocess
>>> import subprocess
>>> subprocess.call([cmd, arg1, arg2],shell=true)
获取返回和输出:
import subprocess
p = subprocess.popen('ls', shell=true, stdout=subprocess.pipe, stderr=subprocess.stdout)
for line in p.stdout.readlines():
    print line,
retval = p.wait()
(4)  使用模块 commands
>>> import commands
>>> dir(commands)
['__all__', '__builtins__', '__doc__', '__file__', '__name__', 'getoutput', 'getstatus','getstatusoutput', 'mk2arg', 'mkarg']
>>> commands.getoutput(date)
'wed jun 10 19:39:57 cst 2009'
>>>
>>> commands.getstatusoutput(date)
(0, 'wed jun 10 19:40:41 cst 2009')
注意： 当执行命令的参数或者返回中包含了中文文字，那么建议使用subprocess，如果使用os.popen则会出现下面的错误:
traceback (most recent call last):
  file ./test1.py, line 56, inmain()
  file ./test1.py, line 45, in main
    fax.sendfax()
  file ./mailfax/fax.py, line 13, in sendfax
    os.popen(cmd)
unicodeencodeerror: 'ascii' codec can't encode characters in position 46-52: ordinal not inrange(128)