How to Use the Command 'Complete' in Shell Scripting (with Examples)

The complete command in shell scripting is a powerful tool that enhances the shell’s functionality by providing argument autocompletion for commands. Autocompletion is a feature that helps users fill in the rest of a word after typing just a few letters, improving both speed and convenience when working with commands in a terminal. This command is particularly useful for developers and system administrators who want to streamline the command-line interface experience by customizing how autocompletion behaves for certain commands.

Use Case 1: Apply a Function that Performs Autocompletion to a Command

Code:

complete -F _myfunc mycommand

Motivation:

Imagine you frequently use a custom command, mycommand, for which autocompleting arguments can greatly enhance speed and accuracy. By using the complete command with a function, you can define versatile and dynamic autocompletion rules tailored specifically to your needs.

Explanation:

• -F _myfunc: The -F option tells the shell to use a function for autocompletion. The function name follows it, in this case, _myfunc. The function should be defined in the shell, and its purpose is to provide a list of words or paths that could follow the incomplete command or arguments.
• mycommand: This is the target command that you want autocompletion to be applied to. Every time you type mycommand and press tab, the shell will invoke _myfunc to provide autocompletion suggestions.

Example Output:

When typing mycommand and pressing the tab key, if _myfunc is set up to list valid options or paths, those elements will appear as suggestions to complete the command.

Use Case 2: Apply a Command that Performs Autocompletion to Another Command

Code:

complete -C autocomplete_script.sh mycommand

Motivation:

When dealing with commands that have complex or context-dependent arguments, a script can be used to dynamically generate valid autocompletion possibilities. This setup is ideal when the logic for determining completions is best encapsulated in a standalone script.

Explanation:

• -C autocomplete_script.sh: The -C option designates an external command or script for autocompletion. Here, autocomplete_script.sh is a script that outputs possible completions to the command line.
• mycommand: As before, this is the specific command that you want to enhance with autocompletion through an external script.

Example Output:

Executing mycommand and pressing tab will call autocomplete_script.sh, which could generate options based on current context, such as file lists or command dependencies, providing those as completion options.

Use Case 3: Apply Autocompletion Without Appending a Space to the Completed Word

Code:

complete -o nospace -F _myfunc mycommand

Motivation:

In certain scenarios, automatically appending a space after an autocompleted word can be undesirable, especially if the next character expected is another option rather than a space. Using the nospace option offers more flexibility, particularly in command-sensitive contexts.

Explanation:

• -o nospace: This option modifies autocompletion behavior so that a space is not automatically appended after a word is completed. This can be crucial for commands where subsequent characters or actions follow directly on from the completed word.
• -F _myfunc: As previously explained, this option uses a specified function to determine what gets suggested for autocompletion.
• mycommand: This is the command you wish to affect. The absence of an appended space can be particularly useful in shell scripts or command-line tools that interpret subsequent input immediately after a completed word.

Example Output:

When typing mycommand and selecting an option from _myfunc suggestions, the shell will not insert a space afterward, allowing more fluid entry of further command-line options or arguments.

Conclusion:

The complete command enriches the user experience by customizing how the shell handles command autocompletion. Through defining functions or leveraging external scripts, you can tailor autocompletion to reflect the intricacies of specific commands you frequently use, saving time and minimizing errors during command input. Moreover, the flexibility to control spacing behavior after autocompletion further augments its utility in sophisticated scripting scenarios.