Bash hostname completion

As part of its programmable completion suite, Bash includes hostname completion. This completion mode reads hostnames from a file in hosts(5) format to find possible completions matching the current word. On Unix-like operating systems, it defaults to reading the file in its usual path at /etc/hosts.

For example, given the following hosts(5) file in place at /etc/hosts:

127.0.0.1      localhost
192.0.2.1      web.example.com www
198.51.100.10  mail.example.com mx
203.0.113.52   radius.example.com rad

An appropriate call to compgen would yield this output:

$ compgen -A hostname
localhost
web.example.com
www
mail.example.com
mx
radius.example.com
rad

We could then use this to complete hostnames for network diagnostic tools like ping(8):

$ complete -A hostname ping

Typing ping we and then pressing Tab would then complete to ping web.example.com. If the shopt option hostcomplete is on, which it is by default, Bash will also attempt host completion if completing any word with an @ character in it. This can be useful for email address completion or for SSH username@hostname completion.

We could also trigger hostname completion in any other Bash command line (regardless of complete settings) with the Readline shortcut Alt+@ (i.e. Alt+Shift+2). This works even if hostcomplete is turned off.

However, with DNS so widely deployed, and with system /etc/hosts files normally so brief on internet-connected systems, this may not seem terribly useful; you’d just end up completing localhost, and (somewhat erroneously) a few IPv6 addresses that don’t begin with a digit. It may seem even less useful if you have your own set of hosts in which you’re interested, since they may not correspond to the hosts in the system’s /etc/hosts file, and you probably really do want them looked up via DNS each time, rather than maintaining static addresses for them.

There’s a simple way to make host completion much more useful by defining the HOSTFILE variable in ~/.bashrc to point to any other file containing a list of hostnames. You could, for example, create a simple file ~/.hosts in your home directory, and then include this in your ~/.bashrc:

# Use a private mock hosts(5) file for completion
HOSTFILE=$HOME/.hosts

You could then populate the ~/.hosts file with a list of hostnames in which you’re interested, which will allow you to influence hostname completion usefully without messing with your system’s DNS resolution process at all. Because of the way the Bash HOSTFILE parsing works, you don’t even have to fake an IP address as the first field; it simply scans the file for any word that doesn’t start with a digit:

# Comments with leading hashes will be excluded
external.example.com
router.example.com router
github.com
google.com
...

You can even include other files from it with an $include directive!

$include /home/tom/.hosts.home
$include /home/tom/.hosts.work

Author’s note: This really surprised me when reading the source, because I don’t think /etc/hosts files generally support that for their usual name resolution function. I would love to know if any systems out there actually do support this.

The behaviour of the HOSTFILE variable is a bit weird; all of the hosts from the HOSTFILE are appended to the in-memory list of completion hosts each time the HOSTFILE variable is set (not even just changed), and host completion is attempted, even if the hostnames were already in the list. It’s probably sufficient just to set the file once in ~/.bashrc.

This setup allows you to set hostname completion as the default method for all sorts of network-poking tools, falling back on the usual filename completion if nothing matches with -o default:

$ complete -A hostname -o default curl dig host netcat ping telnet

You could also use hostname completions for ssh(1), but to account for hostname aliases and other ssh_config(5) tricks, I prefer to read Host directives values from ~/.ssh/config for that.

If you have machine-readable access to the complete zone data for your home or work domain, it may even be worth periodically enumerating all of the hostnames into that file, perhaps using rndc dumpdb -zones for a BIND9 setup, or using an AXFR request. If you have a locally caching recursive nameserver, you could even periodically examine the contents of its cache for new and interesting hosts to add to the file.