IP address with ifconfig and sed

Well, my new favorite approach is to pipe ifconfig into a single sed command 🙂

So, how does it work? Well, there are two filters (aka scripts, ie the parameters after the -e flags). The first one, s/:127\.0\.0\.1 //g', simply strips out all occurrences of the local loopback address (127.0.0.1) – this can be left out if you want to include the loopback address in the results. And the second filter, 's/ *inet addr:\([0-9.]\+\).*/\1/gp' matches all lines with IP addresses, strips all but the IP address itself, and prints the matching line (note the ‘p‘ at the end of the filter, which works in with the -nflag at the stared of the sed command).

Obviously that explanation is only meant to be a brief summary. You should consult the sed manual is you’re still unsure of how / why this works… or post a question below 🙂

Just for comparison, here’s some examples of how others do the same thing:

• Read UNIX/Linux system IP address in a shell script
  ifconfig | grep ‘inet addr:’| grep -v ‘127.0.0.1’ |
  cut -d: -f2 | awk ‘{ print $1}’

  Notice that this one uses an incorrect loopback filter ('127.0.0.1' should be ':127\.0\.0\.1 ') and yet my new favorite is still shorter 😉

• Parsing IP Address from ifconfig
  ifconfig eth0 | grep “inet” | awk ‘{print $2}’ | awk -F: ‘{print $2}’

  Pretty reasonable, but two more pipes than mine 🙂

• Setting up a 6to4 IPv6 tunnel with Debian
  ifconfig $EXTIF | grep ‘inet addr’ | cut -d : -f2 | cut -d ‘ ‘ -f1

  This is almost identical to how I used to do it – before I became familiar with sed ;)

• Advance (sic) Linux commands

  ifconfig | grep -vw inet6 | grep -w inet | cut -d : -f 2 | cut -d \ -f 1

  Ok, but a little bit redundant.

• ifconfig eth0 and awk and grep

  ifconfig eth0 | head -n 2 \
  | sed ‘N;s/\n/ /;N;s/\n/ /’ | awk ‘{print $7 ” ” $9 ” ” $5}’ \
  | sed -e ‘s/addr://g’ -e ‘s/Mask://g’