diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 2fe2e13..2520907 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -10,7 +10,7 @@ Prerequisites:
 # Arch Linux
 pacman -S uv just
 # optional
-pacman -S just-lsp yamlfmt shellcheck typos
+pacman -S just-lsp yamlfmt shellcheck typos git-delta
 ```
 
 The development environment and testing requires multiple specific Python versions; they can be installed with (inside the pulled git project directory):
diff --git a/justfile b/justfile
index 869c7e7..a298867 100644
--- a/justfile
+++ b/justfile
@@ -67,7 +67,7 @@ ci-gen: docs
   uv run --only-group=check ruff format --diff .
 
   echo "running formatter; yamlfmt (YAML)"
-  uv run yamlfmt -lint -quiet -output_format default -formatter indent=2,indentless_arrays=true,retain_line_breaks_single=true,trim_trailing_whitespace=true .
+  scripts/yamlfmt-diff.sh . | delta
 
 # fix formatting
 [group('dev')]
@@ -121,7 +121,7 @@ ci-gen: docs
 [group('dev')]
 @schema-check:
   echo "running schema checker (dbyaml config)"
-  uv run scripts/generate_schema.py | diff - src/dbyaml/resources/dbyaml.schema.json
+  uv run scripts/generate_schema.py | diff src/dbyaml/resources/dbyaml.schema.json -
 
 # generate all docs
 [group('dev')]
diff --git a/src/dbyaml/cli.py b/src/dbyaml/cli.py
index d86f18c..7e118c0 100644
--- a/src/dbyaml/cli.py
+++ b/src/dbyaml/cli.py
@@ -163,11 +163,24 @@ else:
   def main(ctx, filename, config, format_type, quiet, verbose, debug):
     """The uncompromising db(yaml) utility."""
     if ctx.invoked_subcommand is None:
-      ctx.invoke(list)
+      ctx.invoke(lst)
 
-  @main.command
+  @main.command("list")
+  @click.option(
+    "-k",
+    "--keys",
+    is_flag=True,
+    help="List keys.",
+  )
   @pass_context
-  def list(ctx):
+  def lst(ctx, keys):
     data = load(ctx.obj["filename"])
-    res = dump(data, format_type=ctx.obj["format_type"])
+    if keys:
+      if isinstance(data, dict):
+        df = list(data.keys())
+      elif isinstance(data, list):
+        df = len(data)
+    else:
+      df = data
+    res = dump(df, format_type=ctx.obj["format_type"])
     print(res)
diff --git a/src/dbyaml/db.pyi b/src/dbyaml/db.pyi
index 246d53f..951483b 100644
--- a/src/dbyaml/db.pyi
+++ b/src/dbyaml/db.pyi
@@ -1,3 +1,3 @@
 def load(filename: str) -> list | dict: ...
-def dump(data: list | dict, format_type: str) -> str: ...
+def dump(data: object, format_type: str) -> str: ...
 def run_shebang(script: str) -> None: ...
diff --git a/src/dbyaml/resources/dbyaml.schema.json b/src/dbyaml/resources/dbyaml.schema.json
index f45c2b0..dcbe3aa 100644
--- a/src/dbyaml/resources/dbyaml.schema.json
+++ b/src/dbyaml/resources/dbyaml.schema.json
@@ -31,7 +31,13 @@
     "list": {
       "type": "object",
       "additionalProperties": false,
-      "properties": {}
+      "properties": {
+        "keys": {
+          "type": "boolean",
+          "description": "List keys.",
+          "default": false
+        }
+      }
     }
   }
 }
diff --git a/tests/test_cli.py b/tests/test_cli.py
index 5807181..8e0c808 100644
--- a/tests/test_cli.py
+++ b/tests/test_cli.py
@@ -7,7 +7,7 @@ from dbyaml.cli import pass_context, uses_decorator  # ty: ignore[unresolved-imp
 
 def test_uses_decorator_cli():
   assert uses_decorator("main", "pass_context", dbyaml.cli.__file__)
-  assert uses_decorator("list", "pass_context", dbyaml.cli.__file__)
+  assert uses_decorator("lst", "pass_context", dbyaml.cli.__file__)
 
 
 def test_uses_decorator_wo():