How do I pass a variable to os.popen?

[Chris Rebert]

On Mon, Oct 31, 2011 at 1:16 PM, extraspecialbitter
<pauldavidmena at gmail.com> wrote:
> I'm trying to write a simple Python script to print out network
> interfaces (as found in the "ifconfig -a" command) and their speed
> ("ethtool <interface>").  The idea is to loop for each interface and
> print out its speed.  os.popen seems to be the right solution for the

os.popen() is somewhat deprecated. Use the subprocess module instead.

> ifconfig command, but it doesn't seem to like me passing the interface
> variable as an argument.  Code snippet is below:
>
> ============
>
> #!/usr/bin/python
>
> # Quick and dirty script to print out available interfaces and their
> speed
>
> # Initializations
>
> output = " Interface: %s Speed: %s"
>
> import os, socket, types
>
> fp = os.popen("ifconfig -a")
> dat=fp.read()
> dat=dat.split('\n')
> for line in dat:
>    if line[10:20] == "Link encap":
>       interface=line[:9]
>    cmd = 'ethtool %interface'

cmd will literally contain a percent-sign and the word "interface". If
your shell happens to use % as a prefix to indicate a variable, note
that Python variables are completely separate from and not accessible
from the shell. So either ethtool will get the literal string
"%interface" as its argument, or since there is no such shell
variable, after expansion it will end up getting no arguments at all.
Perhaps you meant:
cmd = "ethtool %s" % interface
Which could be more succinctly written:
cmd = "ethtool " + interface

>    print cmd
>    gp = os.popen(cmd)
>    fat=gp.read()

The subprocess equivalent is:
fat = subprocess.check_output(["ethtool", interface])

>    fat=fat.split('\n')
>
> =============
>
> I'm printing out "cmd" in an attempt to debug, and "interface" seems
> to be passed as a string and not a variable.  Obviously I'm a newbie,
> and I'm hoping this is a simple syntax issue.  Thanks in advance!

Cheers,
Chris
--
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