How Bash Completion Works
How Bash completion works
Published October 6, 2019 by  Chris PatuzzoThis is the first of two parts on Bash completion. Part two is here.▲Bash completion for ‘The tuzz automation tool’Over the years I’ve developed a command-line tool I use for routine tasks such
as provisioning my machine, generating project templates and managing secrets.
The tool is written in Ruby and I invoke it with the zz command.Most of what it does is fairly
straightforward. The clever bits are usually delegated to something else. For
example, zz provision is really just a wrapper that installs and runs Chef,
while passing various options to it.Recently, I added Bash completion to my tool. I’ve wanted this for a while, but
decided to add it now in preparation for secrets
management. For example, I want to be able to type zz secret --readamaz<TAB><TAB> and have it complete to zz secret --read amazon/.  Perhaps
hitting <TAB> again will list all secrets under this path, e.g. username,
password, access_key, etc.The mechanicsIn Bash, completion is handled through the complete ‘built-in’:$ type completecomplete is a shell builtinThis command allows you to register a method of completion for a command. For
example, an rgb command might register its known colors:$ complete -W "red green blue yellow purple pink orange" color▲Setting a hardcoded list of completionsYou could then complete color names:$ color <TAB><TAB>blue    green   orange  pink    purple  red     yellow$ color p<TAB><TAB>pink    purple$ color pi<TAB> # completes to pinkThe -W switch configures a static list of completions that are printed in
alphabetical order. It’s just one of the many methods of completion.Listing completion methodsTo see which commands have completion methods, run complete without arguments:$ completecomplete -W 'red green blue yellow purple pink orange' colorcomplete -F _nodenv nodenvcomplete -F _rbenv rbenv▲Listing all registered completion methodsHere you can see nodenv and rbenv support completion. They use the -F
switch to specify functions to handle their completion, namely _nodenv and _rbenv. When you complete one of
these commands, their output is context-aware:$ rbenv install 2.5<TAB><TAB>2.5.0          2.5.0-rc1      2.5.1          2.5.2          2.5.3That’s helpful! rbenv has kindly listed which Ruby 2.5.x versions are available
to install. We could find this out from rbenv install --list but that’s
inefficient because we’d have to clear our current command then re-type it.How completion functions workWhen a function is registered as the method of completion with the -F switch,
it must comply with an ‘interface’ of sorts. When the function is called, Bash
sets some environment variables to be used by the completion function.They tell it the contents of the
command-line, the cursor position, etc. For example, $COMP_LINE contains the
full line that was typed, $COMP_WORDS is that same line broken into an array
of words and $COMP_POINT is the cursor’s index position.In return, the completion function should set $COMPREPLY to specify which
completions to print for the command.An exampleEverybody loves FizzBuzz, right? Let’s demonstrate Bash completion with a custom
function that magically completes the next term in the sequence:function _fizzbuzz () {  length=${#COMP_WORDS[@]}  number=$((length - 1))  if   ! ((number % 15)); then COMPREPLY=(fizzbuzz)  elif ! ((number % 3));  then COMPREPLY=(fizz)  elif ! ((number % 5));  then COMPREPLY=(buzz)  else                         COMPREPLY=($number)  fi}complete -F _fizzbuzz fizzbuzz▲Setting a Bash function to complete fizzbuzzOur command is called fizzbuzz so we name
our completion function _fizzbuzz, as per the convention. We first set the
length variable to the number of words on the command-line and number to
one less, since ‘fizzbuzz’ itself counts as a word.We’ve probably all seen FizzBuzz before so let’s skip the modulo logic. The
important part is to set $COMPREPLY - in this case, to an array of the next
term in the sequence.Now, if we type fizzbuzz <TAB><TAB>, Bash completion kicks in and as if by
magic the next term is appended to the current command-line.  Our fizzbuzz
command doesn’t actually exist but that doesn’t seem to matter!▲Using Bash completion to generate the FizzBuzz sequenceAs you can see, there’s plenty of fun to be had! In
part two we’ll implement Bash completion for my
automation tool and see how it works in practice.