You can use subprocess.call, with the stdout keyword argument:

import subprocess

cmd = ['ls', '-l']

with open('output.txt', 'w') as out:
    return_code = subprocess.call(cmd, stdout=out)

os.system() just runs the process, it doesn't capture the output:

If command generates any output, it will be sent to the interpreter standard output stream.

The return value is the exit code of the process:

On Unix, the return value is the exit status of the process encoded in the format specified for wait().

You'll need to use something like subprocess.check_output() or subprocess.Popen() directly to capture the output.

>>> arch = subprocess.check_output("uname -a | awk '{print $9}'", shell=True);
>>> arch
'x86_64\n'

import subprocess, sys, shlex

try:
    process = (subprocess.Popen(shlex.split(command), 
            stdout=subprocess.PIPE, 
            stderr=subprocess.PIPE, 
            shell=True, cwd="Pass desired directory in as string variable")

  stdout, stderr = process.communicate()

use the shlex to build your command, and subprocess to execute it in the command prompt.

subprocess.Popen Allows you to set the current working directory and set a pipe up for IPC.

<(...) is ksh syntax also recognised by zsh and bash, though when used as the target of a redirection, it's only supported by zsh and bash.

In any case, that is not sh syntax. Python's os.system() and os.popen() do run sh to interpret the given command line. And each invocation of those commands runs a new shell, so one variable defined in one will not be available in a the next shell invocation. Also python variables do not automagically become shell variables.

Here, you could do:

os.system("""
  sar -u 1 1 |
    awk -v t="$(date +%m-%d-%y-%T)" -v OFS=, '
      NR==4 {
        print t, $4, $5, $6, $7, $8, $9
      }' > outputfile.txt""")

Though it feels silly to invoke date and awk (or even a shell) from within python while python is very well capable of doing their jobs itsef.

os.system executes the command in a subshell and returns the command's exit code. It does not provide any mean to capture the outputs of the command ("outputs" => what the command prints to it's stdout/stderr streams).

To capture the command's outputs you'll have to use some of the subprocess module's feature, the most obvious here being subprocess.check_output

# ...
import subprocess
# ...
# NB : you may want to catch subprocess.CalledProcessError here
out = subprocess.check_output(['adb',  'devices', '-l'])
msg = "{t}\nChecking for connected devices:\n{out}".format(t=t, out=out)
with open('logfile.txt', 'w') as f:
    f.write(msg)