[python] the ways to call external programs

import os
os.system( 'ls -al' )
a = os.popen( 'ls -al' ).read()
print a

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Here's a summary of the ways to call external programs and the advantages and disadvantages of each:

os.system("some_command with args") passes the command and arguments to your system's shell. This is nice because you can actually run multiple commands at once in this manner and set up pipes and input/output redirection. For example,

os.system("some_command < input_file another_command > output_file")

However, while this is convenient, you have to manually handle the escaping of shell characters such as spaces, etc. On the other hand, this also lets you run commands which are simply shell commands and not actually external programs.

http://docs.python.org/lib/os-process.html

stream = os.popen("some_command with args") will do the same thing as os.system except that it gives you a file-like object that you can use to access standard input/output for that process. There are 3 other variants of popen that all handle the i/o slightly differently. If you pass everything as a string, then your command is passed to the shell; if you pass them as a list then you don't need to worry about escaping anything.
http://docs.python.org/lib/os-newstreams.html

The Popen class of the subprocess module. This is intended as a replacement for os.popen but has the downside of being slightly more complicated by virtue of being so comprehensive. For example, you'd say

print Popen("echo Hello World", stdout=PIPE, shell=True).stdout.read()
instead of
print os.popen("echo Hello World").read()

but it is nice to have all of the options there in one unified class instead of 4 different popen functions.
http://docs.python.org/lib/node528.html

The call function from the subprocess module. This is basically just like the Popen class and takes all of the same arguments, but it simply wait until the command completes and gives you the return code. For example:
return_code = call("echo Hello World", shell=True)
http://docs.python.org/lib/node529.html

The os module also has all of the fork/exec/spawn functions that you'd have in a C program, but I don't recommend using them directly.

The subprocess module should probably be what you use.