====== Program execution from Python ======

The right module to use is **[[http://docs.python.org/library/subprocess.html|subprocess]]**. Older modules and functions that should be avoided are:

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

===== Run a command =====

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

<code python>
retcode = subprocess.call(["mycmd", "myarg"])
</code>

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

<code python>
retcode = subprocess.call("mycmd myarg", shell=True)
</code>

===== Get the output of a program =====

Assign the output to a variable:

<code python>
output = subprocess.Popen(["mycmd", "myarg"], stdout=subprocess.PIPE).communicate()[0].decode('utf-8')
</code>

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

<code python>
subproc = subprocess.Popen(["mycmd", "myarg"], stdin=None, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
output, stderr = subproc.communicate()
retcode = subproc.returncode
for line in output.decode('utf-8').splitlines():
    print(line)
</code>

===== Redirect output to a file =====

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

===== Run two commands in a pipe =====

<code python>
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]
</code>

===== Write to command standard input =====

<code python>
cmd_input = []
cmd_input.append('line input one')
cmd_input.append('line input two')

p = subprocess.Popen('command', stdin=subprocess.PIPE)
p.communicate(os.linesep.join(cmd_input))
</code>