In a shell script how to validate if a given host name belongs to localhostname family - bash

Question

In a shell script how to validate if a given host name is localhost
for example :-
localhost
127.0.0.1
myhostname.com
::1
all belong to same machine name, Is there any way to identify that a given host name belongs to localhostname family

Answer 1

Usually all local host names are in /etc/hosts file:
grep -c machine_name /etc/hosts
if the machine name is among the localhost the command above returns 1 or greater, otherwise is 0.
for example:
grep -c myhostname.com /etc/hosts
1
grep -c google.com /etc/hosts
0
Not sure if this is exactly you're looking for but I hope it will help.
Beware of partial match, for example if you have 'myhost' in /etc/hosts grep -c host will return 1 as well. In this case you'll need to use regular expressions or parse /etc/hosts file with cut, awk and similar tools. Or use the following command:
grep -c '\bmachine name\b'
To skip the comments use the command below:
grep -v '^#.*' /etc/hosts | grep -c machine_name
so full command is
grep -v '^#.*' /etc/hosts | grep -c '\bmachine_name\b'

Answer 2

You could check
sysctl kernel.hostname
i.e
sysctl kernel.hostname | grep -c "my_hostname"

Answer 3

I use the following to check whether a supplied hostname is the same as localhost:
hostname_ip(){
host "$1" | sed -e 's/.* \([^ ]*[^ .]\)\.*$/\1/'
}
normalize_hostname(){
local normalized="$1"
grep -q "^\(\([0-9]{1,3}\)\.\)\{3\}\([0-9]{1,3}\)$" <<< "$normalized" || normalized="$(hostname_ip "$normalized")"
normalized="$(hostname_ip "$normalized")"
echo "$normalized"
}
myname="$(normalize_hostname "$(hostname)")"
argname="$(normalize_hostname "$1")"
if [[ "$myname" == "$argname" || "$argname" == "localhost" ]]; then
...
First, normalize the supplied parameter into format set by host utility by running it twice. If IP address is supplied -- checked by regex -- run it only once.
Then compare the value to normalized value of hostname utility or to string "localhost".