fqdns.sh

#!/bin/sh
#
#  Author:   Mitch Richling<http://www.mitchr.me/>
#  IP:       Copyright 1998 by Mitch Richling.  All rights reserved.
#  Key word: fqdns hostname unix
#  Notes:    Determining the fully qualified host name of a UNIX 
#            host from it's 'hostname' can be a bit of a challenge.
#            In fact, it is just about impossible to do from the shell
#            in a standards based way.  This script is a best
#            effort attempt, but if you really need it to work, then
#            use a Perl script or C program!
#
#            Most of the system software is designed to have the host
#            name be simple ('foo' and not 'foo.bar.com').  Nothing
#            breaks if it is fully qualified, and this is common
#            practice with modern system administrators.  This
#            script attempts to do the right thing in both cases and
#            it makes use of some non-traditional and non-standard
#            techniques that generally work relatively well in
#            practice.

PATH=/bin:/usr/bin:$PATH
export PATH

# If the hostname has a '.', then we assume it is FQDNS as the RFCs
# indicate that a simple host name hostname may not contain a '.'.
HOSTNAME=`hostname`
if echo $HOSTNAME | grep '\.' >/dev/null; then
   echo $HOSTNAME
   exit
fi

# We try to use nslookup on the host name.  If we get a result we
# print it out and quit.  Note that newer versions of nslookup print a
# warning.  You can use 'host' or 'dig' if this is the case on systems
# you support.
RESOUT=`nslookup $HOSTNAME| sed -n 's/Name: *//p'`
if echo $RESOUT | grep '\.' >/dev/null; then
   echo $RESOUT
   exit
fi

# Well, gee wiz, now we look in resolv.conf and try to find a domain
# name that way.  This may do very bad things if you have strange
# resolv.conf files in your network..
if test -e /etc/resolv.conf; then
   if grep domain /etc/resolv.conf; then
      echo `hostname`.`tr -s '[:space:]' ' ' </etc/resolv.conf  | sed -n 's/^domain *//p'`
      exit
   fi
fi

# Well, we couldn't figure it out.  We just return the simple name:
echo $HOSTNAME