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we have a script that needs to take action on a finite list of hosts. but every time we add or remove a host to the /etc/hosts file, we end up having to update this script.

basically, say my hosts file looks like:

192.168.100.1     hostip_1
192.168.100.2     hostip_2
192.168.100.10    hostip_3
192.168.100.20    hostip_5

and my script (bash) does something like:

callmyfunction hostip_1
callmyfunction hostip_2
callmyfunction hostip_3
callmyfunction hostip_5

if i want to add hostip_4 to the list of hosts, i now have to go in and edit my script and add it to the list. while it's a small edit, it is still a step that can be forgotten in the process (especially if someone new to the system comes in).

is there a way i can test to see if 'hostip_1' is a valid hostname within the system (without pinging the host or grepping the /etc/hosts file)? we may use multiple hosts files, and different configurations may have different filenames, so i can't rely on trying to grep a single file. i need the system to do that work for me.

any clues?

share|improve this question
have you tried nslookup hostip_1 (or the more modern dig hostip_1)? sounds like what you really need is to set up your own local DNS server... – kfmfe04 Jan 24 at 16:19
both of those need DNS. i ended up answering my own question. thanks for looking at this though. i appreciate the help. – jasonmclose Jan 24 at 16:34

3 Answers

up vote 2 down vote

first, my statement about things not being in the hosts file is wrong. that is exactly where they are. dumb on my part.

but the answer is: 

getent hosts

that will get it to print everything out, and i can do a lookup from there.

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i didn't figure this out. a coworker helped me on this. – jasonmclose Jan 24 at 16:33
1  
Actually, you can get getent to do the lookup for you: getent hosts www.stackoverflow.com – thkala Jan 24 at 17:34
you're right. i was doing a getent hosts | grep -c myhost1_ip thing, but just shortened it to getent hosts myhost1_ip. check to see if you get a blank string or not (result), and you've got the answer. – jasonmclose Jan 24 at 19:24
You don't even need to check the result as a string: getent returns non-zero if the host is not found -- if getent hosts $name; then echo "$name is in hosts"; else echo "$name not found"; fi – glenn jackman Jan 24 at 21:48
1  
Why not : for name in `getent hosts | grep 'host_ip[[:digit:]]\+' | awk '{ print $2 }'` ; do callmyfunction $name ; done – ydroneaud Jan 25 at 9:29
up vote 0 down vote

As you are populating the /etc/hosts file, I am assuming that you are not using DNS. So below solution wont fit your use case. But it will still get you some pointers.

In a working DNS environment, you can check the host name to its corresponding IP with below command

# host host_name

This is will give the IP address of the host. In case the host name does not exists, then it will give you corresponding host not found message.

You can parse the output of above command and can deduce whether a give host name exists.

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sorry, these are internal IPs not tied into DNS. should've specified that. – jasonmclose Jan 24 at 16:34
up vote 0 down vote

If all the targets are on the same subnet (same network), use arping, it will check that hosts are available using ARP.

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this is a less than desirable solution, because i was trying to do so without creating any extra network traffic. but it's a solution nonetheless, and thanks for the contribution. – jasonmclose Jan 24 at 19:25
ARP traffic is very limited compared to ping traffic. And ARP is required for ping and any other network exchange involving Ethernet and IPv4. But getent hosts is the way to go to use /etc/hosts as your list of target. – ydroneaud Jan 25 at 9:26

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