Fetch IPv4 address from ifconfig with awk shell script.

fetch_ip.sh

#!/bin/sh

# Examples:
#   ./fetch_ip.sh "" docker
#   ./fetch_ip.sh "" lo
#   ./fetch_ip.sh 192.168
#   ./fetch_ip.sh 192.168 eth1

ifconfig | awk '{
   if ($0 ~ /Link encap:/ && $0 ~ /'$2'/) {
       if ((getline tmp) > 0) {
           where = match(tmp, /inet addr:[0-9\.]*/);
           if (where) {
               if (substr(tmp, where + 10, RLENGTH - 10) ~ /'$1'/) {
                   print substr(tmp, where + 10, RLENGTH - 10);
                   f=1; exit;
               }
           }
       }
   }
}
END { exit !f; }';

# single line, no script
# ifconfig | awk -v ip="192" -v interface="eth2" '{ if ($0 ~ /Link encap:/ && $0 ~ interface) { if ((getline tmp) > 0) { where = match(tmp, /inet addr:[0-9\.]*/); if (where) { if (substr(tmp, where + 10, RLENGTH - 10) ~ ip) { print substr(tmp, where + 10, RLENGTH - 10); f=1; exit; }}}}} END { exit !f; }'

# Test exit status.
# echo $?;

real 0m0.005s
user 0m0.003s
sys 0m0.002s