Use 'grealpath' if installed for realpath fallback (#3374)

* Use 'grealpath' if installed for realpath fallback

See discussion in #3370

* fish_realpath: filter out dangerous options

Per feedback do not use aliases to declare wrapped functions.

share/functions/realpath.fish

@@ -6,8 +6,41 @@
 # If we see that there is an external command by that name we just return. That will cause fish to
 # run the external command. On the other hand, if an external command isn't found we define a
 # function that will provide fallback behavior.
-if not type -q -P realpath
+if not command -s realpath >/dev/null
-    function realpath --description 'fallback realpath implementation'
+
-        builtin fish_realpath $argv[-1]
+    if command -s grealpath >/dev/null
+        function realpath -w grealpath -d "print the resolved path [grealpath]"
+            grealpath $argv
+        end
+    else
+        function realpath -w fish_realpath -d "get an absolute path without symlinks [fish_realpath]"
+            if test -z $argv
+                printf "usage: %s%s%s %sfile%s …\n" (set_color -o) $_ (set_color normal) (set_color -u) (set_color normal)
+                echo "       resolves files as absolute paths without symlinks"
+                return 1
+            end
+
+            for arg in $argv
+                switch $arg
+                    # These - no can do our realpath
+                    case -s --strip --no-symlinks -z --zero --relative-base\* --relative-to\*
+                        __fish_print_help fish_realpath
+                        return 2
+
+                    case -h --help --version
+                        __fish_print_help fish_realpath
+                        return 0
+
+                        # Service commands called with these arguments by
+                        # dropping the arguments to protext fish_realpath from them
+                        # There are no sure things here
+                    case -e --canonicalize-existing --physical -P -q --quiet -m --canonicalize-missing
+                        continue
+
+                    case "*"
+                        fish_realpath $argv
+                end
+            end
+        end
     end
 end