Subprocess confuses when passing argument to the bash

Question

Actually i am trying to pass below code to the bash. But i think %s of tshark and pythonic %s are mixing somehow. It gives not enough argument format string error.

 def traffic_dest_port(self,filepcap):
    cmd = "tshark -o column.format:'\"Source\", \"%s\", \"Destination\", \"%d\", \"dstport\", \"%uD\"' -r %s |sort|uniq " %filepcap
    subprocess.call(cmd,shell=True)

Can you help me out on this?

Thanks.

Answer 1

You are right in your guess. Python's % operator will try to replace every % specifier in the string, not just some of them. Since it finds 4 % specifiers, and only one value, it will say TypeError: not enough arguments for format string.

But, even if it didn't, how did you expect it to guess which one you wanted it to replace out of those four? I'm a human, and even I'm having trouble guessing, so if I guessed wrong, hopefully you can figure out how to adjust the answer to fit your needs.

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One way around this is to escape the percents you don't want Python to process by doubling them:

cmd = "tshark -o column.format:'\"Source\", \"%%s\", \"Destination\", \"%%d\", \"dstport\", \"%%uD\"' -r %s |sort|uniq " %filepcap

As described in the documentation on printf-style String Formatting, the specifier %% means "No argument is converted, results in a '%' character in the result."

And if you print out the result (which you always should when trying to debug stuff like this!), you'll see that the %s has been replaced by the filepcap value, but the %%s has turned into %s.

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However, a much easier solution is to just use a different formatting mechanism that doesn't require %, like str.format:

cmd = "tshark -o column.format:'\"Source\", \"%s\", \"Destination\", \"%d\", \"dstport\", \"%uD\"' -r {} |sort|uniq ".format(filepcap)