Provide a better error message for 'nvd list' on non-NixOS systems (fixes #21).

CHANGELOG.md

@@ -2,6 +2,9 @@
 
 ## 0.2.5 (unreleased)
 
+- Improve the error message when calling `nvd list`, and `/run/current-system`
+  doesn't exist (issue #21).
+
 ## 0.2.4 (2024-09-29)
 
 - Added an `nvd history` command for diffing successive generations of a

src/nvd

@@ -61,6 +61,8 @@ NIX_BIN_DIR = None
 USE_NIX_COMMAND_FEATURE = False  # Whether to explicitly enable the nix-command feature.
 USE_COLOUR = False
 
+LIST_DEFAULT_ROOT = "/run/current-system"
+
 SGR_RESET = 0
 SGR_BOLD = 1
 SGR_FG = 30
@@ -937,6 +939,12 @@ def run_list(
 
     if not path.exists():
         sys.stderr.write(f"Path does not exist: {path}\n")
+        if str(path) == LIST_DEFAULT_ROOT:
+            sys.stderr.write(
+                "Perhaps this isn't a NixOS system?\n"
+                "Pass -r/--root to change the target of 'nvd list'.\n"
+                "For example: nvd list -r ~/.nix-profile\n"
+            )
         sys.exit(1)
 
     path = path.resolve()
@@ -1233,8 +1241,8 @@ def parse_args():
     add_sort_arg(list_parser)
     list_parser.add_argument(
         "-r", "--root",
-        default="/run/current-system",
+        default=LIST_DEFAULT_ROOT,
-        help="The The Nix store path to work with.")
+        help=f"The Nix store path to work with (default {LIST_DEFAULT_ROOT}).")
     list_parser.add_argument(
         nargs="*",
         default=[],

src/nvd.1

@@ -216,6 +216,8 @@ package are present.
 For the
 .B list
 command, this is the root store path to use, or a link to the store path to use.
+The default is
+.B /run/current-system.
 .TP
 <name-pattern>
 For the