compgen

Install

# compgen is a Bash builtin. It is available whenever you are using bash.

What it does

compgen prints possible completions for a given prefix or category. It is the workhorse behind many programmable completion functions, and it is also useful on its own for listing things Bash “knows about” (commands, aliases, variables, users, etc.).

How it works (mechanical)

• Runs inside Bash and queries built-in tables for commands, variables, functions, aliases, etc.
• Can generate filesystem matches (files/directories) using Bash’s glob logic.
• Can generate matches from a word list (-W) or from a function used by complete.
• Most often used inside completion functions to populate COMPREPLY.
• Output is plain text — you can pipe it into tools like sort, grep, fzf, etc.

Quick Start

# List all available commands in PATH + shell builtins
compgen -c | head

10 Practical Examples

# 1) List commands (PATH + builtins) matching a prefix
compgen -c sys

# 2) List only builtins
compgen -b

# 3) List only aliases
compgen -a

# 4) List shell functions
compgen -A function

# 5) List variables (environment + shell vars)
compgen -v | head

# 6) List users from /etc/passwd (handy for admin scripting)
compgen -u | head

# 7) List groups
compgen -g | head

# 8) List files matching a prefix (filesystem completion)
compgen -f /etc/ho

# 9) Generate completions from a custom word list
compgen -W "start stop status restart reload" -- st

# 10) Use compgen inside a completion function (minimal example)
_mytool_complete() {
  local cur="${COMP_WORDS[COMP_CWORD]}"
  COMPREPLY=( $(compgen -W "start stop status restart" -- "$cur") )
}
complete -F _mytool_complete mytool

Notes & Gotchas

• Because it is a Bash builtin, it won’t work if you’re in sh or another shell.
• Some categories use -A (like -A function), others have short flags (-c, -u, etc.).
• Output can be large; pipe to head, less, sort -u, etc.
• When used in completion scripts, always quote $cur and use -- before it.
• For real completions, pair it with complete and optional bash-completion package.

Historical Context

Bash introduced programmable completion to move beyond simple filename completion. compgen became the low-level generator that completion scripts use to list valid candidates efficiently.

Modern Equivalent

Bash’s completion ecosystem is still very active (especially on servers). Other shells like zsh and fish provide different completion frameworks, but compgen remains central for Bash.

Related Commands

• complete — register completion rules for commands.
• compopt — adjust completion behavior from inside a completion function.
• bind — Readline key bindings (Tab invokes completion).
• type — identify how a command name is resolved (builtin/alias/file/function).
• declare — inspect variables/functions that compgen can list.