ENH: Print usage help for all passed actions.

Extend the support for specific usage help to built-in actions.

tests/t2110-help-action.sh

@@ -0,0 +1,66 @@
+#!/bin/bash
+
+test_description='built-in actions help functionality
+
+This test checks listing the usage help of a built-in action.
+'
+. ./test-lib.sh
+
+test_todo_session 'nonexisting action help' <<'EOF'
+>>> todo.sh help doesnotexist
+TODO: No action "doesnotexist" exists.
+=== 1
+
+>>> todo.sh help hel
+TODO: No action "hel" exists.
+=== 1
+
+>>> todo.sh help h
+TODO: No action "h" exists.
+=== 1
+EOF
+
+test_todo_session 'single action help' <<'EOF'
+>>> todo.sh help shorthelp
+    shorthelp
+      List the one-line usage of all built-in and add-on actions.
+\
+EOF
+
+test_todo_session 'multiple actions help' <<'EOF'
+>>> todo.sh help shorthelp append
+    shorthelp
+      List the one-line usage of all built-in and add-on actions.
+\
+    append ITEM# "TEXT TO APPEND"
+    app ITEM# "TEXT TO APPEND"
+      Adds TEXT TO APPEND to the end of the task on line ITEM#.
+      Quotes optional.
+\
+EOF
+
+test_todo_session 'short and long form of action help' <<'EOF'
+>>> todo.sh help append
+    append ITEM# "TEXT TO APPEND"
+    app ITEM# "TEXT TO APPEND"
+      Adds TEXT TO APPEND to the end of the task on line ITEM#.
+      Quotes optional.
+\
+
+>>> todo.sh help app
+    app ITEM# "TEXT TO APPEND"
+      Adds TEXT TO APPEND to the end of the task on line ITEM#.
+      Quotes optional.
+\
+EOF
+
+test_todo_session 'mixed existing and nonexisting action help' <<'EOF'
+>>> todo.sh help shorthelp doesnotexist list
+    shorthelp
+      List the one-line usage of all built-in and add-on actions.
+\
+TODO: No action "doesnotexist" exists.
+=== 1
+EOF
+
+test_done