Making Zsh Tab‑Completion Work for Your Own Scripts

Problem: Your script ignores Tab

You have written a handy command‑line tool, mytool, that accepts options like --input FILE and --mode (fast|slow). When you type mytool -- and press Tab, Zsh falls back to filename completion instead of showing the expected arguments. This breaks the flow and forces you to remember the exact spelling of each flag.

Thesis: Enable programmable completion with compinit and bind a simple completion function

Zsh’s completion system is turned on by autoloading and initializing compinit. Once active, it reads completion definitions from directories listed in $fpath. You can add your own definition without touching the core code by writing a shell function and associating it with the command via compdef.

1. Ensure the completion system is loaded

Start a fresh Zsh session (or source your .zshrc) and run:

# Run once per session; typically placed in .zshrc
autoload -Uz compinit   # load the compinit function
compinit                 # initialize it

This populates $fpath with the standard completion directories and prepares the zcompdump cache.

2. Write a minimal completion function for mytool

Create a file (e.g., ~/.zfunc/_mytool) that contains the function. The function name must start with an underscore.

# ~/.zfunc/_mytool
# Arguments: $1 = service, $2 = word being completed, $3 = word before cursor
_mytool() {
  local -a opts
  opts=(
    '--input[Specify input file]:_files'
    '--mode[Operation mode]:(fast slow)'
    '--help[Display help]' 
  )
  _describe 'command' opts
}
# Tell Zsh to use this function for the command 'mytool'
compdef _mytool mytool

Explanation:

• _files triggers filename completion for the argument after --input.
• The syntax (fast slow) offers a fixed set of values for --mode.
• _describe formats the options for the completion menu.

3. Make the function discoverable

Add the directory containing the function to $fpath before calling compinit. A common place is ~/.zfunc.

# In .zshrc, before compinit
fpath+=~/.zfunc
autoload -Uz compinit
compinit

After saving the changes, start a new shell or run source ~/.zshrc.
4. Verify the completion works
In the same session, type:
mytool --Tab

You should see a list: --help, --input, --mode. Pressing Tab again cycles through the options if you have enabled menu selection:
zstyle ':completion:*' menu select

Trade‑off and limitation
Enabling many completions or using complex zstyle patterns can increase shell startup latency, especially when $fpath points to large directories containing hundreds of files. The completion cache (~/.zcompdump) mitigates this by storing the parsed definitions, but the cache must be regenerated whenever you modify a completion function.
Practical way to check the cache:
ls -l ~/.zcompdump   # shows timestamp and size

After editing _mytool, delete the cache and restart the shell to force a rebuild:
rm -f ~/.zcompdump
exec zsh   # or start a new terminal

If a completion function is poorly written (e.g., an infinite loop inside the function), Zsh may appear to hang when you press Tab. Test new completions in a temporary shell (zsh -f) before adding them to your main configuration.
Actionable closing
To get reliable Tab completion for your own scripts:

  
Add fpath+=~/.zfunc before compinit in ~/.zshrc.
  
Write a completion function named _<command> that uses _describe and helpers like _files.
  
Link it with compdef _<command> <command>.
  
Reload the shell, verify with Tab, and optionally enable zstyle ':completion:*' menu select for a navigable list.
  
Keep an eye on startup time; if it grows, prune unused directories from $fpath or rely on the ~/.zcompdump cache.

With these steps, your custom commands will behave like built‑ins, offering contextual suggestions that save typing and reduce errors.