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Execute shell commands in Python

Solutons:


You can use os.system(), like this:

import os
os.system('ls')

Or in your case:

os.system('echo 1 > /proc/sys/net/ipv4/ip_forward')
os.system('iptables -t nat -A PREROUTING -p tcp --destination-port 80 -j REDIRECT --to-port 8080')

Better yet, you can use subprocess’s call, it is safer, more powerful and likely faster:

from subprocess import call
call('echo "I like potatos"', shell=True)

Or, without invoking shell:

call(['echo', 'I like potatos'])

If you want to capture the output, one way of doing it is like this:

import subprocess
cmd = ['echo', 'I like potatos']
proc = subprocess.Popen(cmd, stdout=subprocess.PIPE, stderr=subprocess.PIPE)

o, e = proc.communicate()

print('Output: ' + o.decode('ascii'))
print('Error: '  + e.decode('ascii'))
print('code: ' + str(proc.returncode))

I highly recommend setting a timeout in communicate, and also to capture the exceptions you can get when calling it. This is a very error-prone code, so you should expect errors to happen and handle them accordingly.

https://docs.python.org/3/library/subprocess.html

The first command simply writes to a file. You wouldn’t execute that as a shell command because python can read and write to files without the help of a shell:

with open('/proc/sys/net/ipv4/ip_forward', 'w') as f:
    f.write("1")

The iptables command is something you may want to execute externally. The best way to do this is to use the subprocess module.

import subprocess
subprocess.check_call(['iptables', '-t', 'nat', '-A',
                       'PREROUTING', '-p', 'tcp', 
                       '--destination-port', '80',
                       '-j', 'REDIRECT', '--to-port', '8080'])

Note that this method also does not use a shell, which is unnecessary overhead.

The quickest way:

import os
os.system("your command here")

This isn’t the most flexible approach; if you need any more control over your process than “run it once, to completion, and block until it exits”, then you should use the subprocess module instead.

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