lsext

#!/usr/bin/env racket
#lang racket/base

(require racket/cmdline
         (only-in racket/file fold-files)
         (only-in racket/path path-get-extension)
         racket/set)

(define recursive #f)
(define dir (current-directory))
(define follow-links? #f)
(define exts
  (command-line
   #:usage-help "if you provide no extensions, then lsext will list all extensions in the directory; if you give extensions, then it'll list all files matching any of the provided extensions. DO NOT PREFIX THE EXTENSION WITH A PERIOD."
   #:once-each [("-r" "--recursive") "recurse through directory" (set! recursive #t)]
   [("-d" "--dir") d "set directory to a value other than current working directory" (set! dir (string->path d))]
   [("--follow-links") "" (set! follow-links? #t)]
   #:args extensions (list->set extensions)))

(unless (directory-exists? dir)
  (eprintf "directory ~a does not exist~n" dir)
  (exit 1))

(let ([s (fold-files (λ (path type s)
                       (let* ([fext (path-get-extension path)]
                              [fext-str (and fext (string-downcase (bytes->string/utf-8 (subbytes fext 1))))])
                         (values (if (and fext-str (eq? type 'file))
                                     (cond [(set-empty? exts) (set-add s fext-str)]
                                           [(and fext-str (set-member? exts fext-str)) (set-add s (path->string path))]
                                           [else s])
                                     s)
                        (and (member 'read (file-or-directory-permissions path)) (or recursive (eq? dir path))))))
            (set)
            dir
            follow-links?)])
(for-each displayln (sort (set->list s) string<=?)))