pcomplete: Generate completions from --help messages

* lisp/pcomplete.el (pcomplete-from-help): New function (and hash
table) to get pcomplete candidates from help messages.
(pcomplete-here-using-help): Helper function to define pcomplete for
simple commands
(pcomplete-completions-at-point): Provide annotation-function and
company-docsig properties.
* lisp/pcmpl-git.el: New file, provides pcomplete for Git.
* lisp/pcmpl-gnu.el: Add pcomplete for awk, gpg and gdb, emacs and
emacsclient.
* lisp/pcmpl-linux.el: Add pcomplete for systemctl and journalctl.
* lisp/pcmpl-rpm.el: Add pcomplete for dnf.
* lisp/pcmpl-unix.el: Add pcomplete for sudo and most commands found
in GNU Coreutils.
* lisp/pcmpl-x.el: Add pcomplete for tex, pdftex, latex, pdflatex,
rigrep and rclone.
* test/lisp/pcomplete-tests.el (pcomplete-test-parse-gpg-help,
pcomplete-test-parse-git-help): Tests for the new functions.

8 files changed, 1004 insertions, 24 deletions

diff --git a/lisp/pcmpl-git.el b/lisp/pcmpl-git.el
new file mode 100644
index 00000000000..3584fa06732
--- /dev/null
+++ b/lisp/pcmpl-git.el
@@ -0,0 +1,110 @@
+;;; pcmpl-git.el --- Completions for Git -*- lexical-binding: t -*-
+
+;; Copyright (C) 2022 Free Software Foundation, Inc.
+
+;; Package: pcomplete
+
+;; This file is part of GNU Emacs.
+
+;; GNU Emacs is free software: you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; GNU Emacs is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;; This library provides completion rules for the Git program.
+
+;;; Code:
+
+(require 'pcomplete)
+(require 'vc-git)
+
+(defun pcmpl-git--expand-flags (args)
+  "In the list of ARGS, expand arguments of the form --[no-]flag."
+  (mapcan (lambda (arg) (if (string-search "[no-]" arg)
+                            (list (string-replace "[no-]" "" arg)
+                                  (string-replace "[no-]" "no-" arg))
+                          (list arg)))
+          args))
+
+(defun pcmpl-git--tracked-file-predicate (&rest args)
+  "Return a predicate function determining the Git status of a file.
+Files listed by `git ls-files ARGS' satisfy the predicate."
+  (when-let ((files (mapcar #'expand-file-name
+                            (ignore-errors
+                              (apply #'process-lines
+                                     vc-git-program "ls-files" args)))))
+    (lambda (file)
+      (setq file (expand-file-name file))
+      (if (string-suffix-p "/" file)
+          (seq-some (lambda (f) (string-prefix-p file f))
+                    files)
+        (member file files)))))
+
+(defun pcmpl-git--remote-refs (remote)
+  "List the locally known Git revisions from REMOTE."
+  (delq nil
+        (mapcar
+         (let ((re (concat "\\`" (regexp-quote remote) "/\\(.*\\)")))
+           (lambda (s) (when (string-match re s) (match-string 1 s))))
+         (vc-git-revision-table nil))))
+
+;;;###autoload
+(defun pcomplete/git ()
+  "Completion for the `git' command."
+  (let ((subcommands (pcomplete-from-help `(,vc-git-program "help" "-a")
+                                          :margin "^\\( +\\)[a-z]"
+                                          :argument "[[:alnum:]-]+")))
+    (while (not (member (pcomplete-arg 1) subcommands))
+      (if (string-prefix-p "-" (pcomplete-arg))
+          (pcomplete-here (pcomplete-from-help `(,vc-git-program "help")
+                                               :margin "\\(\\[\\)-"
+                                               :separator " | "
+                                               :description "\\`"))
+        (pcomplete-here (completion-table-merge
+                         subcommands
+                         (when (string-prefix-p "-" (pcomplete-arg 1))
+                           (pcomplete-entries))))))
+    (let ((subcmd (pcomplete-arg 1)))
+      (while (pcase subcmd
+               ((guard (string-prefix-p "-" (pcomplete-arg)))
+                (pcomplete-here
+                 (pcmpl-git--expand-flags
+                  (pcomplete-from-help `(,vc-git-program "help" ,subcmd)
+                                       :argument
+                                       "-+\\(?:\\[no-\\]\\)?[a-z-]+=?"))))
+               ;; Complete modified tracked files
+               ((or "add" "commit" "restore")
+                (pcomplete-here
+                 (pcomplete-entries
+                  nil (pcmpl-git--tracked-file-predicate "-m"))))
+               ;; Complete all tracked files
+               ((or "mv" "rm" "grep" "status")
+                (pcomplete-here
+                 (pcomplete-entries nil (pcmpl-git--tracked-file-predicate))))
+               ;; Complete revisions
+               ((or "branch" "merge" "rebase" "switch")
+                (pcomplete-here (vc-git-revision-table nil)))
+               ;; Complete revisions and tracked files
+               ;; TODO: diff and log accept revision ranges
+               ((or "checkout" "reset" "show" "diff" "log")
+                (pcomplete-here
+                 (completion-table-in-turn
+                  (vc-git-revision-table nil)
+                  (pcomplete-entries nil (pcmpl-git--tracked-file-predicate)))))
+               ;; Complete remotes and their revisions
+               ((or "fetch" "pull" "push")
+                (pcomplete-here (process-lines vc-git-program "remote"))
+                (pcomplete-here (pcmpl-git--remote-refs (pcomplete-arg 1)))))))))
+
+(provide 'pcmpl-git)
+;;; pcmpl-git.el ends here
diff --git a/lisp/pcmpl-gnu.el b/lisp/pcmpl-gnu.el
index 3c9bf1ec9d2..cdfde5640a7 100644
--- a/lisp/pcmpl-gnu.el
+++ b/lisp/pcmpl-gnu.el
@@ -394,6 +394,40 @@ Return the new list."
     (while (pcomplete-here (pcomplete-dirs) nil #'identity))))
 
 ;;;###autoload
-(defalias 'pcomplete/gdb 'pcomplete/xargs)
+(defun pcomplete/awk ()
+  "Completion for the `awk' command."
+  (pcomplete-here-using-help "awk --help"
+                             :margin "\t"
+                             :separator "  +"
+                             :description "\0"
+                             :metavar "[=a-z]+"))
+
+;;;###autoload
+(defun pcomplete/gpg ()
+  "Completion for the `gpg` command."
+  (pcomplete-here-using-help "gpg --help" :narrow-end "^ -se"))
+
+;;;###autoload
+(defun pcomplete/gdb ()
+  "Completion for the `gdb' command."
+  (while
+      (cond
+       ((string= "--args" (pcomplete-arg 1))
+        (funcall pcomplete-command-completion-function)
+        (funcall (or (pcomplete-find-completion-function (pcomplete-arg 1))
+                     pcomplete-default-completion-function)))
+       ((string-prefix-p "-" (pcomplete-arg 0))
+        (pcomplete-here (pcomplete-from-help "gdb --help")))
+       (t (pcomplete-here (pcomplete-entries))))))
+
+;;;###autoload
+(defun pcomplete/emacs ()
+  "Completion for the `emacs' command."
+  (pcomplete-here-using-help "emacs --help" :margin "^\\(\\)-"))
+
+;;;###autoload
+(defun pcomplete/emacsclient ()
+  "Completion for the `emacsclient' command."
+  (pcomplete-here-using-help "emacsclient --help" :margin "^\\(\\)-"))
 
 ;;; pcmpl-gnu.el ends here
diff --git a/lisp/pcmpl-linux.el b/lisp/pcmpl-linux.el
index 7c072f3d40c..023c655a2a8 100644
--- a/lisp/pcmpl-linux.el
+++ b/lisp/pcmpl-linux.el
@@ -30,6 +30,7 @@
 (provide 'pcmpl-linux)
 
 (require 'pcomplete)
+(eval-when-compile (require 'rx))
 
 ;; Functions:
 
@@ -111,4 +112,71 @@ Test is done using `equal'."
        (pcomplete-uniquify-list points)
        (cons "swap" (pcmpl-linux-mounted-directories))))))
 
+;;; systemd
+
+(defun pcmpl-linux--systemd-units (&rest args)
+  "Run `systemd list-units ARGS' and return the output as a list."
+  (with-temp-buffer
+    (apply #'call-process
+           "systemctl" nil '(t nil) nil
+           "list-units" "--full" "--legend=no" "--plain" args)
+    (goto-char (point-min))
+    (let (result)
+      (while (re-search-forward (rx bol (group (+ (not space)))
+                                    (+ space) (+ (not space))
+                                    (+ space) (group (+ (not space)))
+                                    (+ space) (+ (not space))
+                                    (+ space) (group (* nonl)))
+                                nil t)
+        (push (match-string 1) result)
+        (put-text-property 0 1 'pcomplete-annotation
+                           (concat " " (match-string 2))
+                           (car result))
+        (put-text-property 0 1 'pcomplete-description
+                           (match-string 3)
+                           (car result)))
+      (nreverse result))))
+
+;;;###autoload
+(defun pcomplete/systemctl ()
+  "Completion for the `systemctl' command."
+  (let ((subcmds (pcomplete-from-help
+                  "systemctl --help"
+                  :margin (rx bol "  " (group) alpha)
+                  :argument (rx (+ (any alpha ?-)))
+                  :metavar (rx (group (+ " " (>= 2 (any upper "[]|."))))))))
+    (while (not (member (pcomplete-arg 1) subcmds))
+      (if (string-prefix-p "-" (pcomplete-arg 0))
+          (pcomplete-here (pcomplete-from-help "systemctl --help"
+                                               :metavar "[^ ]+"
+                                               :separator " \\(\\)-"))
+        (pcomplete-here subcmds)))
+    (let ((subcmd (pcomplete-arg 1))
+          (context (if (member "--user" pcomplete-args) "--user" "--system")))
+      (while (pcase subcmd
+               ((guard (string-prefix-p "-" (pcomplete-arg 0)))
+                (pcomplete-here
+                 (pcomplete-from-help "systemctl --help")))
+               ;; TODO: suggest only relevant units to each subcommand
+               ("start"
+                (pcomplete-here
+                 (pcmpl-linux--systemd-units context "--state" "inactive,failed")))
+               ((or "restart" "stop")
+                (pcomplete-here
+                 (pcmpl-linux--systemd-units context "--state" "active")))
+               (_ (pcomplete-here
+                   (completion-table-in-turn
+                    (pcmpl-linux--systemd-units context "--all")
+                    (pcomplete-entries)))))))))
+
+;;;###autoload
+(defun pcomplete/journalctl ()
+  "Completion for the `journalctl' command."
+  (while (if (string-prefix-p "-" (pcomplete-arg 0))
+             (pcomplete-here (pcomplete-from-help "journalctl --help"
+                                                  :metavar "[^ ]+"
+                                                  :separator " \\(\\)-"))
+           (pcomplete-here (mapcar (lambda (s) (concat s "="))
+                                   (process-lines "journalctl" "--fields"))))))
+
 ;;; pcmpl-linux.el ends here
diff --git a/lisp/pcmpl-rpm.el b/lisp/pcmpl-rpm.el
index f7925d9d9ec..ebb6b72600c 100644
--- a/lisp/pcmpl-rpm.el
+++ b/lisp/pcmpl-rpm.el
@@ -21,7 +21,8 @@
 
 ;;; Commentary:
 
-;; These functions provide completion rules for the `rpm' command.
+;; These functions provide completion rules for the `rpm' command and
+;; related tools.
 
 ;;; Code:
 
@@ -378,6 +379,46 @@
        (t
         (error "You must select a mode: -q, -i, -U, --verify, etc"))))))
 
+;;; DNF
+
+(defvar pcmpl-rpm-dnf-cache-file "/var/cache/dnf/packages.db"
+  "Location of the DNF cache.")
+
+(defun pcmpl-rpm--dnf-packages (status)
+  (when (and (file-exists-p pcmpl-rpm-dnf-cache-file)
+             (executable-find "sqlite3"))
+    (with-temp-message
+        "Getting list of packages..."
+      (process-lines "sqlite3" "-batch" "-init" "/dev/null"
+                     pcmpl-rpm-dnf-cache-file
+                     (pcase-exhaustive status
+                       ('available "select pkg from available")
+                       ('installed "select pkg from installed")
+                       ('not-installed "\
+select pkg from available where pkg not in (select pkg from installed)"))))))
+
+;;;###autoload
+(defun pcomplete/dnf ()
+  "Completion for the `dnf' command."
+  (let ((subcmds (pcomplete-from-help "dnf help"
+                                      :margin "^\\(\\)[a-z-]+  "
+                                      :argument "[a-z-]+")))
+    (while (not (member (pcomplete-arg 1) subcmds))
+      (pcomplete-here (completion-table-merge
+                       subcmds
+                       (pcomplete-from-help "dnf help"))))
+    (let ((subcmd (pcomplete-arg 1)))
+      (while (pcase subcmd
+               ((guard (pcomplete-match "\\`-" 0))
+                (pcomplete-here
+                 (pcomplete-from-help `("dnf" "help" ,subcmd))))
+               ((or "downgrade" "reinstall" "remove")
+                (pcomplete-here (pcmpl-rpm--dnf-packages 'installed)))
+               ((or "install" "mark" "reinstall" "upgrade")
+                (pcomplete-here (pcmpl-rpm--dnf-packages 'not-installed)))
+               ((or "builddep" "changelog" "info" "list" "repoquery" "updateinfo")
+                (pcomplete-here (pcmpl-rpm--dnf-packages 'available))))))))
+
 (provide 'pcmpl-rpm)
 
 ;;; pcmpl-rpm.el ends here
diff --git a/lisp/pcmpl-unix.el b/lisp/pcmpl-unix.el
index 8774f091c83..0c32f814d0e 100644
--- a/lisp/pcmpl-unix.el
+++ b/lisp/pcmpl-unix.el
@@ -25,7 +25,7 @@
 
 (require 'pcomplete)
 
-;; User Variables:
+;;; User Variables
 
 (defcustom pcmpl-unix-group-file "/etc/group"
   "If non-nil, a string naming the group file on your system."
@@ -56,7 +56,7 @@ being via `pcmpl-ssh-known-hosts-file'."
   :group 'pcmpl-unix
   :version "24.1")
 
-;; Functions:
+;;; Shell builtins and core utilities
 
 ;;;###autoload
 (defun pcomplete/cd ()
@@ -69,34 +69,38 @@ being via `pcmpl-ssh-known-hosts-file'."
 ;;;###autoload
 (defun pcomplete/rmdir ()
   "Completion for `rmdir'."
-  (while (pcomplete-here (pcomplete-dirs))))
+  (while (if (string-prefix-p "-" (pcomplete-arg))
+             (pcomplete-here (pcomplete-from-help "rmdir --help"))
+           (pcomplete-here (pcomplete-dirs)))))
 
 ;;;###autoload
 (defun pcomplete/rm ()
-  "Completion for `rm'."
-  (let ((pcomplete-help "(fileutils)rm invocation"))
-    (pcomplete-opt "dfirRv")
-    (while (pcomplete-here (pcomplete-all-entries) nil
-                           #'expand-file-name))))
+  "Completion for the `rm' command."
+  (pcomplete-here-using-help "rm --help"))
 
 ;;;###autoload
 (defun pcomplete/xargs ()
   "Completion for `xargs'."
   (while (string-prefix-p "-" (pcomplete-arg 0))
-    (pcomplete-here (funcall pcomplete-default-completion-function)))
+    (pcomplete-here (pcomplete-from-help "xargs --help"))
+    (when (pcomplete-match "\\`-[adEIiLnPs]\\'") (pcomplete-here)))
   (funcall pcomplete-command-completion-function)
   (funcall (or (pcomplete-find-completion-function (pcomplete-arg 1))
                pcomplete-default-completion-function)))
 
-;; FIXME: Add completion of sudo-specific arguments.
-(defalias 'pcomplete/sudo #'pcomplete/xargs)
-
 ;;;###autoload
-(defalias 'pcomplete/time 'pcomplete/xargs)
+(defun pcomplete/time ()
+  "Completion for the `time' command."
+  (pcomplete-opt "p")
+  (funcall pcomplete-command-completion-function)
+  (funcall (or (pcomplete-find-completion-function (pcomplete-arg 1))
+               pcomplete-default-completion-function)))
 
 ;;;###autoload
 (defun pcomplete/which ()
   "Completion for `which'."
+  (while (string-prefix-p "-" (pcomplete-arg 0))
+    (pcomplete-here (pcomplete-from-help "which --help")))
   (while (pcomplete-here (funcall pcomplete-command-completion-function))))
 
 (defun pcmpl-unix-read-passwd-file (file)
@@ -129,24 +133,454 @@ documentation), this function returns nil."
       (pcmpl-unix-read-passwd-file pcmpl-unix-passwd-file)))
 
 ;;;###autoload
+(defun pcomplete/cat ()
+  "Completion for the `cat' command."
+  (pcomplete-here-using-help "cat --help"))
+
+;;;###autoload
+(defun pcomplete/tac ()
+  "Completion for the `tac' command."
+  (pcomplete-here-using-help "tac --help"))
+
+;;;###autoload
+(defun pcomplete/nl ()
+  "Completion for the `nl' command."
+  (pcomplete-here-using-help "nl --help"))
+
+;;;###autoload
+(defun pcomplete/od ()
+  "Completion for the `od' command."
+  (pcomplete-here-using-help "od --help"))
+
+;;;###autoload
+(defun pcomplete/base32 ()
+  "Completion for the `base32' and `base64' commands."
+  (pcomplete-here-using-help "base32 --help"))
+;;;###autoload
+(defalias 'pcomplete/base64 'pcomplete/base32)
+
+;;;###autoload
+(defun pcomplete/basenc ()
+  "Completion for the `basenc' command."
+  (pcomplete-here-using-help "basenc --help"))
+
+;;;###autoload
+(defun pcomplete/fmt ()
+  "Completion for the `fmt' command."
+  (pcomplete-here-using-help "fmt --help"))
+
+;;;###autoload
+(defun pcomplete/pr ()
+  "Completion for the `pr' command."
+  (pcomplete-here-using-help "pr --help"))
+
+;;;###autoload
+(defun pcomplete/fold ()
+  "Completion for the `fold' command."
+  (pcomplete-here-using-help "fold --help"))
+
+;;;###autoload
+(defun pcomplete/head ()
+  "Completion for the `head' command."
+  (pcomplete-here-using-help "head --help"))
+
+;;;###autoload
+(defun pcomplete/tail ()
+  "Completion for the `tail' command."
+  (pcomplete-here-using-help "tail --help"))
+
+;;;###autoload
+(defun pcomplete/split ()
+  "Completion for the `split' command."
+  (pcomplete-here-using-help "split --help"))
+
+;;;###autoload
+(defun pcomplete/csplit ()
+  "Completion for the `csplit' command."
+  (pcomplete-here-using-help "csplit --help"))
+
+;;;###autoload
+(defun pcomplete/wc ()
+  "Completion for the `wc' command."
+  (pcomplete-here-using-help "wc --help"))
+
+;;;###autoload
+(defun pcomplete/sum ()
+  "Completion for the `sum' command."
+  (pcomplete-here-using-help "sum --help"))
+
+;;;###autoload
+(defun pcomplete/cksum ()
+  "Completion for the `cksum' command."
+  (pcomplete-here-using-help "cksum --help"))
+
+;;;###autoload
+(defun pcomplete/b2sum ()
+  "Completion for the `b2sum' command."
+  (pcomplete-here-using-help "b2sum --help"))
+
+;;;###autoload
+(defun pcomplete/md5sum ()
+  "Completion for checksum commands."
+  (pcomplete-here-using-help "md5sum --help"))
+;;;###autoload(defalias 'pcomplete/sha1sum 'pcomplete/md5sum)
+;;;###autoload(defalias 'pcomplete/sha224sum 'pcomplete/md5sum)
+;;;###autoload(defalias 'pcomplete/sha256sum 'pcomplete/md5sum)
+;;;###autoload(defalias 'pcomplete/sha384sum 'pcomplete/md5sum)
+;;;###autoload(defalias 'pcomplete/sha521sum 'pcomplete/md5sum)
+
+;;;###autoload
+(defun pcomplete/sort ()
+  "Completion for the `sort' command."
+  (pcomplete-here-using-help "sort --help"))
+
+;;;###autoload
+(defun pcomplete/shuf ()
+  "Completion for the `shuf' command."
+  (pcomplete-here-using-help "shuf --help"))
+
+;;;###autoload
+(defun pcomplete/uniq ()
+  "Completion for the `uniq' command."
+  (pcomplete-here-using-help "uniq --help"))
+
+;;;###autoload
+(defun pcomplete/comm ()
+  "Completion for the `comm' command."
+  (pcomplete-here-using-help "comm --help"))
+
+;;;###autoload
+(defun pcomplete/ptx ()
+  "Completion for the `ptx' command."
+  (pcomplete-here-using-help "ptx --help"))
+
+;;;###autoload
+(defun pcomplete/tsort ()
+  "Completion for the `tsort' command."
+  (pcomplete-here-using-help "tsort --help"))
+
+;;;###autoload
+(defun pcomplete/cut ()
+  "Completion for the `cut' command."
+  (pcomplete-here-using-help "cut --help"))
+
+;;;###autoload
+(defun pcomplete/paste ()
+  "Completion for the `paste' command."
+  (pcomplete-here-using-help "paste --help"))
+
+;;;###autoload
+(defun pcomplete/join ()
+  "Completion for the `join' command."
+  (pcomplete-here-using-help "join --help"))
+
+;;;###autoload
+(defun pcomplete/tr ()
+  "Completion for the `tr' command."
+  (pcomplete-here-using-help "tr --help"))
+
+;;;###autoload
+(defun pcomplete/expand ()
+  "Completion for the `expand' command."
+  (pcomplete-here-using-help "expand --help"))
+
+;;;###autoload
+(defun pcomplete/unexpand ()
+  "Completion for the `unexpand' command."
+  (pcomplete-here-using-help "unexpand --help"))
+
+;;;###autoload
+(defun pcomplete/ls ()
+  "Completion for the `ls' command."
+  (pcomplete-here-using-help "ls --help"))
+;;;###autoload(defalias 'pcomplete/dir 'pcomplete/ls)
+;;;###autoload(defalias 'pcomplete/vdir 'pcomplete/ls)
+
+;;;###autoload
+(defun pcomplete/cp ()
+  "Completion for the `cp' command."
+  (pcomplete-here-using-help "cp --help"))
+
+;;;###autoload
+(defun pcomplete/dd ()
+  "Completion for the `dd' command."
+  (let ((operands (pcomplete-from-help "dd --help"
+                                       :argument "[a-z]+="
+                                       :narrow-start "\n\n"
+                                       :narrow-end "\n\n")))
+    (while
+        (cond ((pcomplete-match "\\`[io]f=\\(.*\\)" 0)
+               (pcomplete-here (pcomplete-entries)
+                               (pcomplete-match-string 1 0)))
+              (t (pcomplete-here operands))))))
+
+;;;###autoload
+(defun pcomplete/install ()
+  "Completion for the `install' command."
+  (pcomplete-here-using-help "install --help"))
+
+;;;###autoload
+(defun pcomplete/mv ()
+  "Completion for the `mv' command."
+  (pcomplete-here-using-help "mv --help"))
+
+;;;###autoload
+(defun pcomplete/shred ()
+  "Completion for the `shred' command."
+  (pcomplete-here-using-help "shred --help"))
+
+;;;###autoload
+(defun pcomplete/ln ()
+  "Completion for the `ln' command."
+  (pcomplete-here-using-help "ln --help"))
+
+;;;###autoload
+(defun pcomplete/mkdir ()
+  "Completion for the `mkdir' command."
+  (pcomplete-here-using-help "mkdir --help"))
+
+;;;###autoload
+(defun pcomplete/mkfifo ()
+  "Completion for the `mkfifo' command."
+  (pcomplete-here-using-help "mkfifo --help"))
+
+;;;###autoload
+(defun pcomplete/mknod ()
+  "Completion for the `mknod' command."
+  (pcomplete-here-using-help "mknod --help"))
+
+;;;###autoload
+(defun pcomplete/readlink ()
+  "Completion for the `readlink' command."
+  (pcomplete-here-using-help "readlink --help"))
+
+;;;###autoload
 (defun pcomplete/chown ()
   "Completion for the `chown' command."
-  (unless (pcomplete-match "\\`-")
-    (if (pcomplete-match "\\`[^.]*\\'" 0)
-        (pcomplete-here* (pcmpl-unix-user-names))
-      (if (pcomplete-match "\\.\\([^.]*\\)\\'" 0)
-          (pcomplete-here* (pcmpl-unix-group-names)
-                           (pcomplete-match-string 1 0))
-        (pcomplete-here*))))
+  (while (pcomplete-match "\\`-" 0)
+    (pcomplete-here (pcomplete-from-help "chown --help")))
+  (if (pcomplete-match "\\`[^.]*\\'" 0)
+      (pcomplete-here* (pcmpl-unix-user-names))
+    (if (pcomplete-match "\\.\\([^.]*\\)\\'" 0)
+        (pcomplete-here* (pcmpl-unix-group-names)
+                         (pcomplete-match-string 1 0))
+      (pcomplete-here*)))
   (while (pcomplete-here (pcomplete-entries))))
 
 ;;;###autoload
 (defun pcomplete/chgrp ()
   "Completion for the `chgrp' command."
-  (unless (pcomplete-match "\\`-")
-    (pcomplete-here* (pcmpl-unix-group-names)))
+  (while (pcomplete-match "\\`-" 0)
+    (pcomplete-here (pcomplete-from-help "chgrp --help")))
+  (pcomplete-here* (pcmpl-unix-group-names))
   (while (pcomplete-here (pcomplete-entries))))
 
+;;;###autoload
+(defun pcomplete/chmod ()
+  "Completion for the `chmod' command."
+  (pcomplete-here-using-help "chmod --help"))
+
+;;;###autoload
+(defun pcomplete/touch ()
+  "Completion for the `touch' command."
+  (pcomplete-here-using-help "touch --help"))
+
+;;;###autoload
+(defun pcomplete/df ()
+  "Completion for the `df' command."
+  (pcomplete-here-using-help "df --help"))
+
+;;;###autoload
+(defun pcomplete/du ()
+  "Completion for the `du' command."
+  (pcomplete-here-using-help "du --help"))
+
+;;;###autoload
+(defun pcomplete/stat ()
+  "Completion for the `stat' command."
+  (pcomplete-here-using-help "stat --help"))
+
+;;;###autoload
+(defun pcomplete/sync ()
+  "Completion for the `sync' command."
+  (pcomplete-here-using-help "sync --help"))
+
+;;;###autoload
+(defun pcomplete/truncate ()
+  "Completion for the `truncate' command."
+  (pcomplete-here-using-help "truncate --help"))
+
+;;;###autoload
+(defun pcomplete/echo ()
+  "Completion for the `echo' command."
+  (pcomplete-here-using-help '("echo" "--help")))
+
+;;;###autoload
+(defun pcomplete/test ()
+  "Completion for the `test' command."
+  (pcomplete-here-using-help '("[" "--help")
+                             :margin "^ +\\([A-Z]+1 \\)?"))
+;;;###autoload(defalias (intern "pcomplete/[") 'pcomplete/test)
+
+;;;###autoload
+(defun pcomplete/tee ()
+  "Completion for the `tee' command."
+  (pcomplete-here-using-help "tee --help"))
+
+;;;###autoload
+(defun pcomplete/basename ()
+  "Completion for the `basename' command."
+  (pcomplete-here-using-help "basename --help"))
+
+;;;###autoload
+(defun pcomplete/dirname ()
+  "Completion for the `dirname' command."
+  (pcomplete-here-using-help "dirname --help"))
+
+;;;###autoload
+(defun pcomplete/pathchk ()
+  "Completion for the `pathchk' command."
+  (pcomplete-here-using-help "pathchk --help"))
+
+;;;###autoload
+(defun pcomplete/mktemp ()
+  "Completion for the `mktemp' command."
+  (pcomplete-here-using-help "mktemp --help"))
+
+;;;###autoload
+(defun pcomplete/realpath ()
+  "Completion for the `realpath' command."
+  (pcomplete-here-using-help "realpath --help"))
+
+;;;###autoload
+(defun pcomplete/id ()
+  "Completion for the `id' command."
+  (while (string-prefix-p "-" (pcomplete-arg 0))
+    (pcomplete-here (pcomplete-from-help "id --help")))
+  (while (pcomplete-here (pcmpl-unix-user-names))))
+
+;;;###autoload
+(defun pcomplete/groups ()
+  "Completion for the `groups' command."
+  (while (pcomplete-here (pcmpl-unix-user-names))))
+
+;;;###autoload
+(defun pcomplete/who ()
+  "Completion for the `who' command."
+  (pcomplete-here-using-help "who --help"))
+
+;;;###autoload
+(defun pcomplete/date ()
+  "Completion for the `date' command."
+  (pcomplete-here-using-help "date --help"))
+
+;;;###autoload
+(defun pcomplete/nproc ()
+  "Completion for the `nproc' command."
+  (pcomplete-here-using-help "nproc --help"))
+
+;;;###autoload
+(defun pcomplete/uname ()
+  "Completion for the `uname' command."
+  (pcomplete-here-using-help "uname --help"))
+
+;;;###autoload
+(defun pcomplete/hostname ()
+  "Completion for the `hostname' command."
+  (pcomplete-here-using-help "hostname --help"))
+
+;;;###autoload
+(defun pcomplete/uptime ()
+  "Completion for the `uptime' command."
+  (pcomplete-here-using-help "uptime --help"))
+
+;;;###autoload
+(defun pcomplete/chcon ()
+  "Completion for the `chcon' command."
+  (pcomplete-here-using-help "chcon --help"))
+
+;;;###autoload
+(defun pcomplete/runcon ()
+  "Completion for the `runcon' command."
+  (while (string-prefix-p "-" (pcomplete-arg 0))
+    (pcomplete-here (pcomplete-from-help "runcon --help"))
+    (when (pcomplete-match "\\`-[turl]\\'" 0) (pcomplete-here)))
+  (funcall pcomplete-command-completion-function)
+  (funcall (or (pcomplete-find-completion-function (pcomplete-arg 1))
+               pcomplete-default-completion-function)))
+
+;;;###autoload
+(defun pcomplete/chroot ()
+  "Completion for the `chroot' command."
+  (while (string-prefix-p "-" (pcomplete-arg 0))
+    (pcomplete-here (pcomplete-from-help "chroot --help")))
+  (pcomplete-here (pcomplete-dirs))
+  (funcall pcomplete-command-completion-function)
+  (funcall (or (pcomplete-find-completion-function (pcomplete-arg 1))
+               pcomplete-default-completion-function)))
+
+;;;###autoload
+(defun pcomplete/env ()
+  "Completion for the `env' command."
+  (while (string-prefix-p "-" (pcomplete-arg 0))
+    (pcomplete-here (pcomplete-from-help "env --help"))
+    (when (pcomplete-match "\\`-[uCS]\\'") (pcomplete-here)))
+  (while (pcomplete-match "=" 0) (pcomplete-here)) ; FIXME: Complete env vars
+  (funcall pcomplete-command-completion-function)
+  (funcall (or (pcomplete-find-completion-function (pcomplete-arg 1))
+               pcomplete-default-completion-function)))
+
+;;;###autoload
+(defun pcomplete/nice ()
+  "Completion for the `nice' command."
+  (while (string-prefix-p "-" (pcomplete-arg 0))
+    (pcomplete-here (pcomplete-from-help "nice --help"))
+    (pcomplete-here))
+  (funcall pcomplete-command-completion-function)
+  (funcall (or (pcomplete-find-completion-function (pcomplete-arg 1))
+               pcomplete-default-completion-function)))
+
+;;;###autoload
+(defun pcomplete/nohup ()
+  "Completion for the `nohup' command."
+  (while (string-prefix-p "-" (pcomplete-arg 0))
+    (pcomplete-here (pcomplete-from-help "nohup --help")))
+  (funcall pcomplete-command-completion-function)
+  (funcall (or (pcomplete-find-completion-function (pcomplete-arg 1))
+               pcomplete-default-completion-function)))
+
+;;;###autoload
+(defun pcomplete/stdbuf ()
+  "Completion for the `stdbuf' command."
+  (while (string-prefix-p "-" (pcomplete-arg 0))
+    (pcomplete-here (pcomplete-from-help "stdbuf --help"))
+    (when (pcomplete-match "\\`-[ioe]\\'") (pcomplete-here)))
+  (funcall pcomplete-command-completion-function)
+  (funcall (or (pcomplete-find-completion-function (pcomplete-arg 1))
+               pcomplete-default-completion-function)))
+
+;;;###autoload
+(defun pcomplete/timeout ()
+  "Completion for the `timeout' command."
+  (while (string-prefix-p "-" (pcomplete-arg 0))
+    (pcomplete-here (pcomplete-from-help "timeout --help"))
+    (when (pcomplete-match "\\`-[ks]\\'") (pcomplete-here)))
+  (pcomplete-here)                      ; eat DURATION argument
+  (funcall pcomplete-command-completion-function)
+  (funcall (or (pcomplete-find-completion-function (pcomplete-arg 1))
+               pcomplete-default-completion-function)))
+
+;;;###autoload
+(defun pcomplete/numfmt ()
+  "Completion for the `numfmt' command."
+  (pcomplete-here-using-help "numfmt --help"))
+
+;;;###autoload
+(defun pcomplete/seq ()
+  "Completion for the `seq' command."
+  (pcomplete-here-using-help "seq --help"))
+
+;;; Network commands
 
 ;; ssh support by Phil Hagelberg.
 ;; https://www.emacswiki.org/cgi-bin/wiki/pcmpl-ssh.el
@@ -239,6 +673,18 @@ Includes files as well as host names followed by a colon."
   (pcomplete-opt "xl(pcmpl-unix-user-names)")
   (pcmpl-unix-complete-hostname))
 
+;;; Miscellaneous
+
+;;;###autoload
+(defun pcomplete/sudo ()
+  "Completion for the `sudo' command."
+  (while (string-prefix-p "-" (pcomplete-arg 0))
+    (pcomplete-here (pcomplete-from-help "sudo --help"))
+    (when (pcomplete-match "\\`-[CDghpRtTUu]\\'") (pcomplete-here)))
+  (funcall pcomplete-command-completion-function)
+  (funcall (or (pcomplete-find-completion-function (pcomplete-arg 1))
+               pcomplete-default-completion-function)))
+
 (provide 'pcmpl-unix)
 
 ;;; pcmpl-unix.el ends here
diff --git a/lisp/pcmpl-x.el b/lisp/pcmpl-x.el
index 261a3d4e27b..1ede867c5fb 100644
--- a/lisp/pcmpl-x.el
+++ b/lisp/pcmpl-x.el
@@ -28,6 +28,22 @@
 (eval-when-compile (require 'cl-lib))
 (require 'pcomplete)
 
+;;; TeX
+
+;;;###autoload
+(defun pcomplete/tex ()
+  "Completion for the `tex' command."
+  (pcomplete-here-using-help "tex --help"
+                             :margin "^\\(?:\\[-no\\]\\)?\\(\\)-"))
+;;;###autoload(defalias 'pcomplete/pdftex 'pcomplete/tex)
+;;;###autoload(defalias 'pcomplete/latex 'pcomplete/tex)
+;;;###autoload(defalias 'pcomplete/pdflatex 'pcomplete/tex)
+
+;;;###autoload
+(defun pcomplete/luatex ()
+  "Completion for the `luatex' command."
+  (pcomplete-here-using-help "luatex --help"))
+;;;###autoload(defalias 'pcomplete/lualatex 'pcomplete/luatex)
 
 ;;;; tlmgr - https://www.tug.org/texlive/tlmgr.html
 
@@ -142,6 +158,12 @@
         (unless (pcomplete-match "^--" 0)
           (pcomplete-here* (pcomplete-dirs-or-entries)))))))
 
+;;; Grep-like tools
+
+;;;###autoload
+(defun pcomplete/rg ()
+  "Completion for the `rg' command."
+  (pcomplete-here-using-help "rg --help"))
 
 ;;;; ack - https://betterthangrep.com
 
@@ -288,6 +310,8 @@ long options."
                                     (pcmpl-x-ag-options))))
       (pcomplete-here* (pcomplete-dirs-or-entries)))))
 
+;;; Borland
+
 ;;;###autoload
 (defun pcomplete/bcc32 ()
   "Completion function for Borland's C++ compiler."
@@ -321,5 +345,24 @@ long options."
 ;;;###autoload
 (defalias 'pcomplete/bcc 'pcomplete/bcc32)
 
+;;; Network tools
+
+;;;###autoload
+(defun pcomplete/rclone ()
+  "Completion for the `rclone' command."
+  (let ((subcmds (pcomplete-from-help "rclone help"
+                                      :margin "^  "
+                                      :argument "[a-z]+"
+                                      :narrow-start "\n\n")))
+    (while (not (member (pcomplete-arg 1) subcmds))
+      (pcomplete-here (completion-table-merge
+                       subcmds
+                       (pcomplete-from-help "rclone help flags"))))
+    (let ((subcmd (pcomplete-arg 1)))
+      (while (if (pcomplete-match "\\`-" 0)
+                 (pcomplete-here (pcomplete-from-help
+                                  `("rclone" ,subcmd "--help")))
+               (pcomplete-here (pcomplete-entries)))))))
+
 (provide 'pcmpl-x)
 ;;; pcmpl-x.el ends here
diff --git a/lisp/pcomplete.el b/lisp/pcomplete.el
index 0e3d1df7814..6fe29d9dcfb 100644
--- a/lisp/pcomplete.el
+++ b/lisp/pcomplete.el
@@ -119,6 +119,9 @@
 ;;; Code:
 
 (require 'comint)
+(eval-when-compile
+  (require 'cl-lib)
+  (require 'rx))
 
 (defgroup pcomplete nil
   "Programmable completion."
@@ -481,6 +484,14 @@ Same as `pcomplete' but using the standard completion UI."
           (when completion-ignore-case
             (setq table (completion-table-case-fold table)))
           (list beg (point) table
+                :annotation-function
+                (lambda (cand)
+                  (when (stringp cand)
+                    (get-text-property 0 'pcomplete-annotation cand)))
+                :company-docsig
+                (lambda (cand)
+                  (when (stringp cand)
+                    (get-text-property 0 'pcomplete-help cand)))
                 :predicate pred
                 :exit-function
                 ;; If completion is finished, add a terminating space.
@@ -1325,6 +1336,133 @@ If specific documentation can't be given, be generic."
       (pcomplete-read-hosts pcomplete-hosts-file 'pcomplete--host-name-cache
                    'pcomplete--host-name-cache-timestamp)))
 
+;;; Parsing help messages
+
+(defvar pcomplete-from-help (make-hash-table :test #'equal)
+  "Memoization table for function `pcomplete-from-help'.")
+
+(cl-defun pcomplete-from-help (command
+                               &rest args
+                               &key
+                               (margin (rx bol (+ " ")))
+                               (argument (rx "-" (+ (any "-" alnum)) (? "=")))
+                               (metavar (rx (? " ")
+                                            (or (+ (any alnum "_-"))
+                                                (seq "[" (+? nonl) "]")
+                                                (seq "<" (+? nonl) ">")
+                                                (seq "{" (+? nonl) "}"))))
+                               (separator (rx ", " symbol-start))
+                               (description (rx (* nonl)
+                                                (* "\n" (>= 9 " ") (* nonl))))
+                               narrow-start
+                               narrow-end)
+  "Parse output of COMMAND into a list of completion candidates.
+
+COMMAND can be a string to be executed in a shell or a list of
+strings (program name and arguments).  It should print a help
+message.
+
+A list of arguments is collected after each match of MARGIN.
+Each argument should match ARGUMENT, possibly followed by a match
+of METAVAR.  If a match of SEPARATOR follows, then more
+argument-metavar pairs are collected.  Finally, a match of
+DESCRIPTION is collected.
+
+Keyword ARGS:
+
+MARGIN: regular expression after which argument descriptions are
+  to be found.  Parsing continues at the end of the first match
+  group or, failing that, the entire match.
+
+ARGUMENT: regular expression matching an argument name.  The
+  first match group (failing that, the entire match) is collected
+  as the argument name.  Parsing continues at the end of the
+  second matching group (failing that, the first group or entire
+  match).
+
+METAVAR: regular expression matching an argument parameter name.
+  The first match group (failing that, the entire match) is
+  collected as the parameter name and used as completion
+  annotation.  Parsing continues at the end of the second
+  matching group (failing that, the first group or entire match).
+
+SEPARATOR: regular expression matching the separator between
+  arguments.  Parsing continues at the end of the first match
+  group (failing that, the entire match).
+
+DESCRIPTION: regular expression matching the description of an
+  argument.  The first match group (failing that, the entire
+  match) is collected as the parameter name and used as
+  completion help.  Parsing continues at the end of the first
+  matching group (failing that, the entire match).
+
+NARROW-START, NARROW-END: if non-nil, parsing of the help message
+  is narrowed to the region between the end of the first match
+  group (failing that, the entire match) of these regular
+  expressions."
+  (with-memoization (gethash (cons command args) pcomplete-from-help)
+    (with-temp-buffer
+      (let ((case-fold-search nil)
+            (default-directory (expand-file-name "~/"))
+            (command (if (stringp command)
+                         (list shell-file-name
+                               shell-command-switch
+                               command)
+                       command))
+            i result)
+        (apply #'call-process (car command) nil t nil (cdr command))
+        (goto-char (point-min))
+        (narrow-to-region (or (and narrow-start
+                                   (re-search-forward narrow-start nil t)
+                                   (or (match-beginning 1) (match-beginning 0)))
+                              (point-min))
+                          (or (and narrow-end
+                                   (re-search-forward narrow-end nil t)
+                                   (or (match-beginning 1) (match-beginning 0)))
+                              (point-max)))
+        (goto-char (point-min))
+        (while (re-search-forward margin nil t)
+          (goto-char (or (match-end 1) (match-end 0)))
+          (setq i 0)
+          (while (and (or (zerop i)
+                          (and (looking-at separator)
+                               (goto-char (or (match-end 1)
+                                              (match-end 0)))))
+                      (looking-at argument))
+            (setq i (1+ i))
+            (goto-char (seq-some #'match-end '(2 1 0)))
+            (push (or (match-string 1) (match-string 0)) result)
+            (when (looking-at metavar)
+              (goto-char (seq-some #'match-end '(2 1 0)))
+              (put-text-property 0 1
+                                 'pcomplete-annotation
+                                 (or (match-string 1) (match-string 0))
+                                 (car result))))
+          (when (looking-at description)
+            (goto-char (seq-some #'match-end '(2 1 0)))
+            (let ((help (string-clean-whitespace
+                         (or (match-string 1) (match-string 0))))
+                  (items (take i result)))
+              (while items
+                (put-text-property 0 1 'pcomplete-help help
+                                   (pop items))))))
+        (nreverse result)))))
+
+(defun pcomplete-here-using-help (command &rest args)
+  "Perform completion for a simple command.
+Offer switches and directory entries as completion candidates.
+The switches are obtained by calling `pcomplete-from-help' with
+COMMAND and ARGS as arguments."
+  (while (cond
+          ((string= "--" (pcomplete-arg 1))
+           (while (pcomplete-here (pcomplete-entries))))
+          ((pcomplete-match "\\`--[^=]+=\\(.*\\)" 0)
+           (pcomplete-here (pcomplete-entries)
+                           (pcomplete-match-string 1 0)))
+          ((string-prefix-p "-" (pcomplete-arg 0))
+           (pcomplete-here (apply #'pcomplete-from-help command args)))
+          (t (pcomplete-here (pcomplete-entries))))))
+
 (provide 'pcomplete)
 
 ;;; pcomplete.el ends here
diff --git a/test/lisp/pcomplete-tests.el b/test/lisp/pcomplete-tests.el
new file mode 100644
index 00000000000..00a82502f30
--- /dev/null
+++ b/test/lisp/pcomplete-tests.el
@@ -0,0 +1,100 @@
+;;; pcomplete-tests.el --- Tests for pcomplete.el  -*- lexical-binding: t -*-
+
+;; Copyright (C) 2022 Free Software Foundation, Inc.
+
+;; This file is part of GNU Emacs.
+
+;; GNU Emacs is free software: you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; GNU Emacs is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;;; Code:
+
+(require 'ert)
+(require 'pcomplete)
+
+(ert-deftest pcomplete-test-parse-gpg-help ()
+  (cl-letf ((pcomplete-from-help (make-hash-table :test #'equal))
+            ((symbol-function 'call-process)
+             (lambda (&rest _) (insert "\
+gpg (GnuPG) 2.3.7
+
+Commands:
+
+ -s, --sign                         make a signature
+     --clear-sign                   make a clear text signature
+ -b, --detach-sign                  make a detached signature
+     --tofu-policy VALUE            set the TOFU policy for a key
+
+Options to specify keys:
+ -r, --recipient USER-ID            encrypt for USER-ID
+ -u, --local-user USER-ID           use USER-ID to sign or decrypt
+
+(See the man page for a complete listing of all commands and options)
+
+Examples:
+
+ -se -r Bob [file]          sign and encrypt for user Bob
+ --clear-sign [file]        make a clear text signature
+"))))
+    (should
+     (equal-including-properties
+      (pcomplete-from-help "gpg --help" :narrow-end "^ -se")
+      '(#("-s" 0 1 (pcomplete-help "make a signature"))
+        #("--sign" 0 1 (pcomplete-help "make a signature"))
+        #("--clear-sign" 0 1 (pcomplete-help "make a clear text signature"))
+        #("-b" 0 1 (pcomplete-help "make a detached signature"))
+        #("--detach-sign" 0 1 (pcomplete-help "make a detached signature"))
+        #("--tofu-policy" 0 1
+          (pcomplete-help "set the TOFU policy for a key" pcomplete-annotation " VALUE"))
+        #("-r" 0 1 (pcomplete-help "encrypt for USER-ID"))
+        #("--recipient" 0 1
+          (pcomplete-help "encrypt for USER-ID" pcomplete-annotation " USER-ID"))
+        #("-u" 0 1
+          (pcomplete-help "use USER-ID to sign or decrypt"))
+        #("--local-user" 0 1
+          (pcomplete-help "use USER-ID to sign or decrypt" pcomplete-annotation " USER-ID")))))))
+
+(ert-deftest pcomplete-test-parse-git-help ()
+  (cl-letf ((pcomplete-from-help (make-hash-table :test #'equal))
+            ((symbol-function 'call-process)
+             (lambda (&rest _) (insert "\
+usage: git [-v | --version] [-h | --help] [-C <path>] [-c <name>=<value>]
+           [--exec-path[=<path>]] [--html-path] [--man-path] [--info-path]
+           [-p | --paginate | -P | --no-pager] [--no-replace-objects] [--bare]
+           [--git-dir=<path>] [--work-tree=<path>] [--namespace=<name>]
+           [--super-prefix=<path>] [--config-env=<name>=<envvar>]
+           <command> [<args>]
+"))))
+    (should
+     (equal-including-properties
+      (pcomplete-from-help "git help"
+                           :margin "\\(\\[\\)-"
+                           :separator " | "
+                           :description "\\`")
+      '("-v" "--version" "-h" "--help"
+        #("-C" 0 1 (pcomplete-annotation " <path>"))
+        #("-c" 0 1 (pcomplete-annotation " <name>"))
+        #("--exec-path" 0 1 (pcomplete-annotation "[=<path>]"))
+        "--html-path" "--man-path" "--info-path"
+        "-p" "--paginate" "-P" "--no-pager"
+        "--no-replace-objects" "--bare"
+        #("--git-dir=" 0 1 (pcomplete-annotation "<path>"))
+        #("--work-tree=" 0 1 (pcomplete-annotation "<path>"))
+        #("--namespace=" 0 1 (pcomplete-annotation "<name>"))
+        #("--super-prefix=" 0 1 (pcomplete-annotation "<path>"))
+        #("--config-env=" 0 1 (pcomplete-annotation "<name>")))))))
+
+(provide 'pcomplete-tests)
+;;; pcomplete-tests.el ends here