Shell Integration

Overview

Shell integration covers the APIs and patterns for building tools that extend or interact with Unix shells. This includes completion systems, prompt hooks, key bindings, terminal control, and plugin distribution across Zsh, Bash, and Fish.

When to use: Building CLI tool completions, shell plugins, prompt customizations, terminal UI, dotfile managers, installation scripts, or native binary wrappers.

When NOT to use: General-purpose scripting unrelated to shell extension (use POSIX scripting reference for standalone scripts), GUI applications, or web server development.

Quick Reference

Pattern	Shell	Key Points
Completion function	Zsh	`compdef`, `compadd`, `zstyle` for matcher configuration
Completion function	Bash	`complete`, `compgen`, `COMP_WORDS`, `COMP_CWORD`, `COMPREPLY`
Completion function	Fish	`complete -c cmd -a args`, condition flags, subcommand patterns
ZLE widget	Zsh	`zle -N widget func`, `bindkey` to map keys
Prompt hook	Zsh	`precmd`, `preexec`, `chpwd` via `add-zsh-hook`
Prompt hook	Bash	`PROMPT_COMMAND` (string or array in Bash 5.1+)
Event handler	Fish	`--on-event`, `--on-variable`, `--on-signal`
Abbreviation	Fish	`abbr -a name expansion`, `--function` for dynamic
Parameter expansion	Zsh	`${(s.:.)var}`, `${var:=default}`, flags and modifiers
Terminal control	All	ANSI/CSI escape sequences, `tput`, `stty`
Signal handling	All	`trap` builtin, cleanup patterns, `EXIT`/`INT`/`TERM`
Process management	All	Job control (`&`, `wait`, `bg`, `fg`), subshells, coprocesses
Plugin installation	All	Sourcing strategies, version detection, `ZDOTDIR` loading order

Common Mistakes

Mistake	Correct Pattern
Using `echo -e` for escape sequences portably	Use `printf` or `tput` for portability across shells and OSes
Modifying `PROMPT_COMMAND` with `=` in Bash	Append with `+=` to avoid overwriting other tools
Defining Fish event handlers in lazy-loaded functions	Place event handlers in config.fish or source them explicitly
Hardcoding terminal capabilities	Query via `tput` which respects `TERM` and terminfo
Missing `emulate -L zsh` in Zsh functions	Always set local options to avoid polluting caller environment
Using `$COMP_LINE` splitting instead of `COMP_WORDS`	Use `COMP_WORDS[$COMP_CWORD]` for reliable word extraction
Not quoting `$@` in wrapper scripts	Always use `"$@"` to preserve argument boundaries
Assuming `/bin/sh` is Bash	Target POSIX sh for portable scripts, test with `dash`
Using `function` keyword in POSIX scripts	Use `name() { ... }` syntax for POSIX compatibility
Ignoring `EXIT` trap for cleanup	Always set `trap cleanup EXIT` for temp files and state

Delegation

Completion testing: Use Explore agent to verify completions interactively
Script review: Use Task agent for cross-shell compatibility audits
Code review: Delegate to code-reviewer agent

If the rust skill is available, delegate native binary compilation patterns to it. Shell wrappers often invoke Rust-compiled binaries for performance-critical operations. If the cli-power-tools skill is available, delegate modern CLI utility patterns to it. Many shell plugins wrap tools like fd, ripgrep, and fzf.