how to modify /etc/hosts from shell scripts?

Question

I need to change some hosts entries - for example, there's a desire to update hosts to have active.vm host match the IP of currently booted VM (with some testing appliance). Guest software within VM reports machine's address back to host, and I need to update /etc/hosts based on that information - there's always some time for writing another daemon/cronjob, but wanted to use something existing for fixing hosts entries with a dumb syntax of hostname current_ip.

Someone had recommended me the bind-tools package, but - its nsupdate|host don't suit the task at all. ..

Upd: still looking for something from bind suite, but at least dynamic regeneration seems to be the best solution, and augtool is nice too.

Upd: due to my laziness, dynamic regeneration is not planned to use right now, so started using augtool at the cronjob, that reads webserver's access logs for latest IP - at the VMs which run Windows, whole monitoring solution is just a webbrowser which loads specific url from host machine and has a small reload interval

Solution 1

Take a look at augeas. It has a ready-made parser/serializer for the hosts file (that's even in their quick tour guide) and comes with a simple tool to make changes (augtool).

Solution 2

I'd rather not touch /etc/hosts in terms of parsing and such. I'd simply create a directory called /etc/hosts.d/ and make up some script that takes:

• ip
• name
• aliases

creates one (1) file with one (1) line. The filename will be <ip>.host and then cat /etc/hosts.d/*host > /tmp/hosts.new && mv /tmp/hosts.new /etc/hosts. Maybe add an incron event that watches the /etc/hosts.d/ directory and regenerates /etc/hosts whenever something happens, that way you'll get rid of old entries too.

Comments

[kagali-san]

it's a very nice idea, with complete regeneration, sir - at least it saves from having empty hosts file

[kagali-san]

hm, it's in Gentoo portage tree, so will try it.. but somehow I feel that syntax of augtool> set /files/etc/hosts/*[alias[1] = 'active.vm']/ipaddr 1.1.1.1 will not be tolerated by it. Will test..

[kagali-san]

it actually worked, but had to use alias = instead of addressing alias[1] or alias[2].. it didn't supported them, so editing the entry by other than first name was impossible (not a problem, but will ask for a correct syntax on a list)

[BJladu4]

When you use alias in this expression, it matches any alias subnode, not just the firs the first one.

[chovy]

there's even a brew for it.