Credit: Jürgen Hermann
One approach to system-administration scripts is to dig down into system internals, and Python supports this approach:
#! /usr/bin/env python
import fcntl, struct, sys
from socket import *
# Set some symbolic constants
SIOCGIFFLAGS = 0x8913
null256 = ' '*256
# Get the interface name from the command line
ifname = sys.argv[1]
# Create a socket so we have a handle to query
s = socket(AF_INET, SOCK_DGRAM)
# Call ioctl( ) to get the flags for the given interface
result = fcntl.ioctl(s.fileno( ), SIOCGIFFLAGS, ifname + null256)
# Extract the interface's flags from the return value
flags, = struct.unpack('H', result[16:18])
# Check "UP" bit and print a message
up = flags & 1
print ('DOWN', 'UP')[up]
# Return a value suitable for shell's "if"
sys.exit(not up)
This recipe shows how to call some of the low-level modules of
Python’s standard library, handling their results
with the struct
module. To really understand how
this recipe works, you need to take a look at the system includes. On
Linux, the necessary definitions are located in
/usr/include/linux/if.h
.
Though this code is certainly more complex than the traditional
scripting approach (i.e., running /sbin/ifconfig
and parsing its output), you get two positive effects in return.
Directly using the system calls avoids the overhead (albeit modest)
of spawning a new process for such a simple query, and you are not
dependent on the output format of ifconfig,
which might change over time (or from system to system) and break
your code. On the other hand, of course, you are dependent on the
format of the structure returned by ioctl
, which
may be a bit more stable than
ifconfig’s text output but no
more widespread. Win some, lose some. It is nice (and crucial) that
Python gives you a choice!