Autocompletion in Bash for a simple CLI

🤐

Currently, I’ve been developing a CLI forDEV Community in Go. During this journey I’ve learned a lot of really cool stuffs :sunglasses:. After having thedevto almost completed, I started wondering, “How can I make autocompletion possible?”, so I started looking for information related. Here I’ll try to explain how to achieve this in Bash, no idea how would be the case forcmd orPowerShell :thinking:.

CLI commands

Now,devto-cli has these commands:

articles
auth
comments
followers
listings
organizations
podcasts
reading_lists
tags
webhooks

and other sub commands as well, but for simplicity I’ll focus on how to achieve the autocompletion with these commands only.

Wanted behavior

The idea is to give the user some feedback after runningdevto <TAB>, for example in this case, the user would see the following suggestions:

articles auth comments followers listings organizations podcasts reading_lists tags webhooks

Which are all the commands available, but if the user start typingdevto a<TAB>, the suggestions would be the commands:articles auth

Given that these two are the only ones that start with the lettera.

The general idea is that the completion is triggered after<TAB> been pressed.

The programmable completion feature in Bash

One of the many cool features in bash is theprogrammable completion, bash have twobuiltins that will help me, one iscomplete and the other one iscompgen. The documentation of these two commands can be found inman bash.

Jumping to code

After reading several articles, including the mentioned previously, I could manage to adapt that information to my problem. In other words, I made use of the ancestral technique ofCopy, Paste and Adapt. The third step of this technique is crucial, given that it is on this step, when you really start understanding other people code. Never skip this part, even if yourCopy and Paste works at the first.

Get prepare to read other people code, here we go:

_complete_devto () {    # word that is been typed    local word=${COMP_WORDS[COMP_CWORD]}    local line=${COMP_LINE}    # local variable to store the list of commands    # that are going to be passed to compgen    local xpat    # List of all the commands available    local commands="articles auth comments followers listings organizations podcasts reading_lists tags webhooks"    # simple switch case when program is devto    case "$1" in     devto)        case "$line" in        *)            xpat="$commands"            ;;        esac    esac    # commands to reply to the user    COMPREPLY=($(compgen -W "$xpat" -- "${word}"))}complete -F _complete_devto devto

Several variables in detail here:

COMP_WORDS> An array variable consisting of the individual words in the current command line.
COMP_LINE> The current command line.
COMP_CWORD> An index into${COMP_WORDS} of the word containing the current cursor position.
COMPREPLY> An array variable from which bash reads the possible completions generated by a shell function invoked by the programmable
> completion facility.

All the definitions taken fromman bash.

Now just one final step, store that code on a file, let’s call itdevto.sh and source the file.

source devto.sh

Just try to typedevto a<Tab>. This should give youarticles andauth as suggestions.

That’s all folks👋

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