BASH Autocompletion - Stuff… And Things…

BASH Autocompletion - Stuff… And Things…

A couple of posts back, I showed off some functions to pop up notifications when a host became pingable again or when a port became reachable. Today's (semi) quick tip is how to use BASH's autocomplete functionality add hostname autocompletion to those notifications functions.

BASH autocompletion is a system that provides tab completion of command arguments. You're familiar with it's default behavior which is to complete filenames and paths.

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~ ls enctypt<TAB> encrypt encrypt-only-sym.rb encrypt-time.rb encrypt-decrypt.rb encrypt-sym.rb encrypt.rb ~ ls encrypt

You can override this behavior by providing BASH with a list of possible completions. The list can be a literal list of words, or it can be a function that looks at the current environment ($PWD, user, time on day, etc) and generates context aware list.

So, what we want is a way to generate a list of hosts we know about. And, it just so happens we have such a list lying around. You know how the first time you SSH to a new server, your prompted to confirm it's identity? Well, that confirmation, along with the host's name is stored in ~/.ssh/known_hosts. From that file we can extract a list which should cover most of the servers we care about.

The simple approach is to build a list when you login. If you Google around you'll find lots of example scripts for pulling hostnames out of known_hosts, but the most common looks like:

1	echo `cat ~/.ssh/known_hosts | cut -f 1 -d ' ' | sed -e s/,.*//g | uniq | grep -v "\["`

The command that setups autocompletion is complete. When giving it a list, pass them in as an augument to the -W option:

1	complete -W "$(echo `cat ~/.ssh/known_hosts | cut -f 1 -d ' ' | sed -e s/,.*//g | uniq | grep -v "\["`;)" wait-for-host

I'm capturing the output of the command with $() and wrapping it in double quotes. The last argument is the name of the command (which can be also be a function or alias) that will use this autocompletion. Now we get this:

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