Learn Fzf - Fuzzy Completion

#Introduction

Episode 7 installed fzf into your shell via keybindings; episode 8 organized its configuration variables. Episode 9 opens up another side of the same integration script: fuzzy completion. If CTRL-T selects after you've typed the context, fuzzy completion works within the context — in the middle of the command you're typing. The simplest example: vim **<TAB>.

This feature is often called fzf's "killer feature" for one reason: it turns TAB, which previously only completed prefixes, into a full search engine. Instead of pressing TAB repeatedly to browse candidates, you type a few letters and the best result is selected directly.

#Concept: The **<TAB> Trigger

How to activate fuzzy completion: type two asterisks then press TAB:

vim **<TAB>

Once **<TAB> is pressed, fzf replaces the command segment behind it with fuzzily filtered candidates. This differs from CTRL-T, which inserts a path as is; completion understands the command context and picks a matching candidate list.

#Built-in Completion

The integration script provides completers for common contexts. The pattern is always the same — type a command, followed by **<TAB>:

#Files & Directories

cd **<TAB>

This is the completion version of ALT-C: the candidates are directories, the result is a path ready for the command. For files, just change the command context — fzf guesses whether files or directories are needed from the command in front of it.

#Processes

kill -9 **<TAB>

A favorite example: typing kill -9 **<TAB> shows the process list with their PIDs, pick one fuzzily, and fzf inserts its PID — not the process name. No need to run ps aux first to find the PID.

#SSH Hosts

ssh **<TAB>

This completer reads ~/.ssh/config and known_hosts, so a host like web-prod-01 can be selected without typing its full name. This is invaluable if you manage many servers.

#Variables, Aliases, and Commands

unset **<TAB>

Each context has its own candidate logic: unset shows env vars, unalias shows aliases, man shows commands registered in $PATH. What differs is not the syntax — always **<TAB> — but what the integration script sends as candidates.

#Customization: FZF_COMPLETION_OPTS & FZF_COMPLETION_TRIGGER

Two variables control completion behavior globally:

FZF_COMPLETION_OPTS works like FZF_DEFAULT_OPTS but specifically for completion invocations. You can add a preview, layout, or keybinding only for completion mode without affecting other fzf usage:

export FZF_COMPLETION_OPTS="--height=30% --preview='head -20 {}'"

#Custom Completion: Your Own Completer

The full power of completion appears when you add candidates of your own data. The integration script defines a _fzf_complete helper that is called from a completer function named _fzf_complete_<command>.

The basic structure is the same for bash and zsh:

_fzf_complete_todo() {
  _fzf_complete -- "$@" < <(
    grep '^@' "$HOME/todo.txt"
  )
}
complete -F _fzf_complete_todo -o bashdefault -o default todo

_fzf_complete_todo() {
  _fzf_complete -- "$@" < <(
    grep '^@' "$HOME/todo.txt"
  )
}
compdef _fzf_complete_todo todo

With the definitions above, typing todo **<TAB> shows all lines starting with @ from todo.txt, select one fuzzily, and it is inserted onto the command line. Let's break down the parts:

• _fzf_complete_todo — the function name must follow the _fzf_complete_<command> pattern so it's called automatically.
• _fzf_complete -- "$@" — calls the core helper; -- "$@" forwards the command-line arguments being typed (usable for --query, --multi, etc.).
• < <(...) — process substitution sends candidates to fzf's stdin without modifying the environment.
• complete -F (bash) / compdef (zsh) — registers the completer for the todo command.

#Controlling the Output

The candidates you send determine what can be selected — and also what is inserted. If you want to insert something different from what's displayed, use --accept-nth (episode 6) inside the _fzf_complete call:

_fzf_complete_kill_byservice() {
  _fzf_complete --accept-nth=1 -- "$@" < <(
    systemctl list-units --type=service --no-legend | awk '{print $1, $5}'
  )
}
complete -F _fzf_complete_kill_byservice -o bashdefault -o default kill-byservice

Here the list shows nama.service deskripsi, but only the first field is inserted (--accept-nth=1) — a practical candidate for passing on to the next command.

#Closing

Episode 9 brought fzf's "command-completing" side to life: the **<TAB> trigger for fuzzy completion, built-in completers for files, directories, processes, SSH hosts, env vars, aliases, and commands, global settings via FZF_COMPLETION_TRIGGER and FZF_COMPLETION_OPTS, and custom completers with the _fzf_complete helper.

The key takeaways:

• Type **<TAB> in the middle of a command to trigger fuzzy completion.
• kill -9 **<TAB>, ssh **<TAB>, cd **<TAB>, unset **<TAB> are the most productive examples.
• Build your own completer with _fzf_complete_<command> + complete -F (bash) or compdef (zsh).
• Candidates from process substitution determine what's displayed and inserted.

In episode 10, we will build one of the features that makes fzf feel "alive": the preview window — a second pane that shows file contents, images, even command output in real time as you navigate the list. See you in episode 10!