wip

These should be shell commands, but I only have helpers for non-shell.
The error handling has to be beefed up, and is it safe to run shell
commands?

src/scriv/config.py

@@ -13,6 +13,7 @@
 from .exceptions import ScrivException
 from .literals import find_literal
 from .optional import tomllib
+from .shell import run_command
 
 logger = logging.getLogger(__name__)
 
@@ -363,6 +364,7 @@ def resolve_value(self, value: str) -> str:
         Prefixes:
             "file:" read the content from a file.
             "literal:" read a literal string from a file.
+            "command:" read the output of a shell command.
 
         """
         value = value.replace("${config:format}", self._options.format)
@@ -392,6 +394,14 @@ def resolve_value(self, value: str) -> str:
                     + f"{value!r}"
                 )
             value = found
+        elif value.startswith("command:"):
+            cmd = value.partition(":")[2].strip()
+            ok, out = run_command(cmd)
+            if not ok:
+                raise ScrivException(f"Command {cmd!r} failed:\n{out}")
+            if out.count("\n") == 1:
+                out = out.rstrip("\n")
+            value = out
         return value
 
     def read_file_value(self, file_name: str) -> str:

tests/test_config.py

@@ -344,3 +344,19 @@ def test_no_toml_installed_no_settings(self, temp_dir):
         with without_module(scriv.config, "tomllib"):
             config = Config.read()
         assert config.categories[0] == "Removed"
+
+
+def test_command_running(fake_run_command):
+    # Any setting can be the output of a command.
+    fake_run_command.patch_module("scriv.config")
+    fake_run_command.add_handler(
+        "showtext", lambda argv: (True, " ".join(argv[1:]))
+    )
+    text = Config(output_file="command: showtext Xyzzy 2 3").output_file
+    assert text == "Xyzzy 2 3"
+
+
+def test_nosuch_command(fake_run_command):
+    # Any setting can be the output of a command.
+    text = Config(output_file="command: xyzzyplugh").output_file
+    assert text == "Xyzzy 2 3"