sh-mode: highlight variables in strings

modules/module-sh.el

@@ -4,6 +4,38 @@
   (define-repl! sh-mode narf-inf-shell)
   (add-hook! sh-mode 'flycheck-mode)
 
+  (defun sh-extra-font-lock--is-in-double-quoted-string ()
+    "Non-nil if point in inside a double-quoted string."
+    (let ((state (syntax-ppss)))
+      (eq (nth 3 state) ?\")))
+
+  (defun sh-extra-font-lock--match-var-in-double-quoted-string (limit)
+    "Search for variables in double-quoted strings bounded by LIMIT."
+    (let (res)
+      (while
+          (and (setq res
+                     (re-search-forward
+                      "\\$\\({#?\\)?\\([[:alpha:]_][[:alnum:]_]*\\|[-#?@!]\\)"
+                      limit t))
+               (not (sh-extra-font-lock--is-in-double-quoted-string))))
+      res))
+
+  (defvar sh-extra-font-lock--keywords
+    '((sh-extra-font-lock--match-var-in-double-quoted-string
+       (2 font-lock-variable-name-face prepend))))
+
+  (defun sh-extra-font-lock-activate ()
+    "Activate sh-extra-font-lock."
+    (interactive)
+    (font-lock-add-keywords nil sh-extra-font-lock--keywords)
+    (if (fboundp 'font-lock-flush)
+        (font-lock-flush)
+      (when font-lock-mode
+        (with-no-warnings
+          (font-lock-fontify-buffer)))))
+
+  (add-hook 'sh-mode-hook 'sh-script-extra-font-lock-activate)
+
   ;; [pedantry intensifies]
   (defadvice sh-mode (after sh-mode-rename-modeline activate)
     (setq mode-name "sh"))