fahd.blog: Writing your own Bash Completion Function

fahd.blog: Writing your own Bash Completion Function

Bash programmable completion is a powerful feature which allows you to specify how arguments to commands should be completed. You do this using the complete command. For example, you can set completion up so that when you type the unzip command and hit the TAB key, it only shows completions for files ending with the .zip extension. Similarly, the completion for the ssh command would display hosts taken from your known_hosts file.

In this post, I will describe how you can write a custom completion function for a command foo. Bash will execute this function when foo [TAB][TAB] is typed at the prompt and will display possible completions.

Bash uses the following variables for completion:

• COMPREPLY: an array containing possible completions as a result of your function
• COMP_WORDS: an array containing individual command arguments typed so far
• COMP_CWORD: the index of the command argument containing the current cursor position
• COMP_LINE: the current command line

Therefore, if you want the current argument that you are trying to complete, you would index into the words array using: ${COMP_WORDS[COMP_CWORD]}.

So, how do you build the result array COMPREPLY? The easiest way is to use the compgen command. You can supply a list of words to compgen and a partial word, and it will show you all words that match it. Let's try it out:

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