python - Hiding console output produced by os.system

Question

I am calling this piece of code, but it produces some output in the console where I ran the python script (due to tee command):

os.system("echo 3 | sudo tee /proc/sys/vm/drop_caches")

This version does not produce console output but is there another way?

os.system('sudo bash -c "echo 3 > /proc/sys/vm/drop_caches"')

Answer 1

To answer the question based on its title in the most generic form:

To suppress all output from os.system(), append >/dev/null 2>&1 to the shell command, which silences both stdout and stderr; e.g.:

import os
os.system('echo 3 | sudo tee /proc/sys/vm/drop_caches >/dev/null 2>&1')

Note that os.system() by design passes output from the calling process' stdout and stderr streams through to the console (terminal) - your Python code never sees them.

Also, os.system() does not raise an exception if the shell command fails and instead returns an exit code; note that it takes additional work to extract the shell command's true exit code: you need to extract the high byte from the 16-bit value returned, by applying >> 8 (although you can rely on a return value other than 0 implying an error condition).

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Given the above limitations of os.system(), it is generally worthwhile to use the functions in the subprocess module instead:

For instance, subprocess.check_output() could be used as follows:

import subprocess
subprocess.check_output('echo 3 | sudo tee /proc/sys/vm/drop_caches', shell=True) 

The above will:

• capture stdout output and return it (with the return value being ignored in the example above)
• pass stderr output through; passing stderr=subprocess.STDOUT as an additional argument would also capture stderr.
• raise an error, if the shell command fails.

Note: Python 3.5 introduced subprocess.run(), a more flexible successor to both os.system() and subprocess.check_output() - see https://docs.python.org/3.5/library/subprocess.html#using-the-subprocess-module

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Note:

• The reason that the OP is employing tee in the first place - despite not being interested in stdout output - is that a na?ve attempt to use > ... instead would be interpreted before sudo is invoked, and thus fail, because the required privileges to write to /proc/sys/... haven't been granted yet.
• Whether you're using os.system() or a subprocess function, stdin is not affected by default, so if you're invoking your script from a terminal, you'll get an interactive password prompt when the sudo command is encountered (unless the credentials have been cached).