The zsh-completions plugin doesn’t actually add new completion definitions; it provides a framework and a curated set of common completions that are then made available to Zsh’s completion system.
Let’s see this in action. Imagine you have git installed. If you type git and press Tab, Zsh will try to offer completions for git subcommands. If you have zsh-completions installed and sourced correctly, you’ll get a much richer set of options than you would with just the default Zsh completion system.
Consider this simple scenario: you want to complete a git checkout branch name. Without zsh-completions, you might get a basic list. With it, you’ll get a more intelligent list, potentially distinguishing between local and remote branches, or even showing stashed branches.
Here’s what the underlying mechanism looks like when you type git checkout and hit Tab:
# When you press Tab after 'git checkout '
# Zsh invokes its completion system.
# The system looks for registered completion functions.
# If zsh-completions is sourced, it provides a set of these functions.
# For git, zsh-completions includes a sophisticated _git function.
# This _git function knows about git commands, branches, tags, remotes, etc.
# It queries your local git repository to generate the actual completion candidates.
# Zsh then displays these candidates to you.
The core problem zsh-completions solves is the maintenance and distribution of a large, high-quality set of completion definitions for a vast number of command-line tools. Writing these from scratch for every tool you use would be incredibly tedious. zsh-completions bundles thousands of these definitions, covering everything from common system utilities (ls, grep) to more complex applications (docker, kubectl, terraform).
The mental model is that Zsh’s completion system is a powerful engine, and zsh-completions is a massive library of "drivers" or "maps" for that engine, specifically designed to navigate the command structures of most popular software. When you install and source zsh-completions, you’re essentially telling Zsh’s engine, "Here’s a huge collection of detailed guides on how to complete arguments for all these commands."
The exact levers you control are primarily through which completions are enabled or disabled, and how they are configured. For instance, you might find that a particular completion is too verbose or not behaving as expected. Many completion functions within zsh-completions accept arguments that modify their behavior. For example, the _git completion might have options to control whether it shows remote branches by default or how it handles detached HEAD states. You’d typically find these options documented within the completion function file itself (e.g., _git in the zsh-completions’s Completion/Git directory) or in the zsh-completions documentation.
The real magic is how these completions are loaded. Zsh’s compinit function is the gatekeeper. When compinit runs, it scans directories for completion definition files (usually starting with _). zsh-completions places its files in a structured way (e.g., site-functions or within a plugin manager’s structure), and compinit discovers them. The system then registers these as available completion functions.
Most users simply install zsh-completions via a plugin manager like zplug, antigen, or oh-my-zsh, which handles the sourcing and compinit integration automatically. The configuration of specific completions is often handled by setting environment variables or by adding specific calls to zstyle within your .zshrc that target the completion function in question. For example, to customize how git completions behave, you might see something like:
# Example of zstyle configuration for git completions
zstyle ':completion:*:git:*' remote-branches yes
zstyle ':completion:*:git:*' stash-branches yes
These zstyle commands fine-tune the behavior of the _git completion function. The first line tells the git completion to include remote branches in its suggestions. The second line similarly enables suggestions for stashed branches. Without these, the default behavior might be more limited.
The next concept you’ll likely run into is custom completion functions. While zsh-completions is extensive, there will always be specific tools or custom scripts you write that don’t have a pre-built completion definition. Learning to write your own _ functions is the natural progression to extend Zsh’s completion capabilities beyond what’s provided by default or by zsh-completions.