When substituting interpreter variables, allow '$' to be part of

the variable expresstion only within '[]' or '()'. That lets
forms like "$a$b" to be treated as two substitutions, not one.

src/frontend/variable.c

@@ -789,7 +789,7 @@ cp_getvar(char *name, enum cp_types type, void *retval, size_t rsize)
  * isset is TRUE if the variable is being set, FALSE if unset.  Also
  * required is a routine cp_enqvar(name) which returns a struct
  * variable *, which allows the host program to provide values for
- * non-cshpar variables. See line 855 */
+ * non-cshpar variables.  */
 
 /* Non-alphanumeric characters that may appear in variable names. < is very
  * special...
@@ -806,10 +806,11 @@ cp_getvar(char *name, enum cp_types type, void *retval, size_t rsize)
  * Return value
  * Address of the first character after the variable name.
  */
-char *span_var_expr(char *t)
+char *span_var_expr(char *s)
 {
-    int parenthesis = 0;
-    int brackets = 0;
+    char *t = s;
+    int   parenthesis = 0;
+    int   brackets = 0;
 
     while (*t && (isalnum_c(*t) || strchr(VALIDCHARS, *t)))
         switch (*t++)
@@ -832,6 +833,14 @@ char *span_var_expr(char *t)
             if (--parenthesis <= 0)
                 return t;
             break;
+        case '$':
+            if (brackets <= 0 && parenthesis <= 0) {
+                if (t == s + 1) // Special case: "$$".
+                    return t;
+                else
+                    return t-1;
+            }
+            break;
         default:
             break;
         }
@@ -1000,6 +1009,8 @@ wordlist *vareval(/* NOT const */ char *string)
         if (eq(v->va_name, string))
             break;
     if (!v && isdigit_c(*string)) {
+        /* Treat $i for some integer i as argv[i] - positional parameters. */
+
         for (v = variables; v; v = v->va_next) {
             if (eq(v->va_name, "argv")) {
                 break;