Python subprocess.call function does not redirect output

Question

I am trying to run a shell script called nn.sh (which constantly runs a Linux command over time), from within a python file. I am using the following piece of code:

from subprocess import call, Popen, PIPE
call(['/bin/sh', 'nn.sh', '172.20.125.44', '10', '>>', 'log.txt'])

This code is supposed to run nn.sh with inputs 172.20.125.44 and 10 and stores the result in the file log.txt. When I run this Python script, it only shows the results of running nn.sh on the screen and it does not save them in the fill log.txt. However, if I type

/bin/sh nn.sh 172.20.125.44 10 >> log.txt

in the command line, it is correctly saving all the data into the file log.txt. Any ideas on what goes wrong?

subprocess.call is doing you the favor of escaping the sequence for you. Consider the situation where the output file name is given by the user, and the user gives you the file name "out.txt; rm -rf *". If subprocess.call just dumped that as-is into a shell, you'd have a disaster on your hands. The correct solution is outlined by Anton Savin below. But, for your information, you could force subprocess.call to not escape anything by passing it shell = True, though this is a security issue and considered poor practice. — Ryan Haining
– Ryan Haining, Commented Aug 26, 2014 at 2:47

Anton Savin · Accepted Answer · 2014-08-26 00:46:44Z

4

You can't use >> in subprocess calls, instead use stdout parameter:

with open("log.txt", "at") as log:
    call(['/bin/sh', 'nn.sh', '172.20.125.44', '10'], stdout = log)

answered Aug 26, 2014 at 0:46

Anton Savin

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Ryan Haining Over a year ago

use with/as for the log file

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