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passing a variable to cmd

Chris Angelico rosuav at gmail.com
Sun Nov 6 08:59:57 EST 2016 
On Sun, Nov 6, 2016 at 10:48 PM, SS <sami.strat at gmail.com> wrote:
> # cmd='dig @4.2.2.2 nbc.com ns +short'
> cmd="dig @4.2.2.2 %s ns +short", % (domname)
> proc=subprocess.Popen(shlex.split(cmd),stdout=subprocess.PIPE)
> out,err=proc.communicate()
> print(out)
>
> The line that is commented out works fine.  However, I want to substitute a variable for "nbc.com".  The command:

Then don't use a single string; use a list of strings:

cmd = ["dig", "@4.2.2.2", domname, "ns", "+short"]

This is what shlex.split does, and you're fighting against things to
try to force everything through that one channel. What happens if
someone puts a space in domname? It should come back with an error
from dig (you can't have spaces in domain names!), but with naive
string interpolation, you would be passing multiple separate
arguments.

Get used to using lists of arguments for all command execution. Python
isn't optimized for keyboard shorthands in running subcommands, the
way an interactive shell is; it's designed more to be reliable and
dependable. I wouldn't accept a shell that forced me to type commands
with all their parts quoted, but I also don't use single strings with
Python very often. This is the safer way.

ChrisA
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