Add fish debugging examples

doc_src/cmds/fish.rst

@@ -13,7 +13,7 @@ Synopsis
 Description
 -----------
 
-``fish`` is a command-line shell written mainly with interactive use in mind. This page briefly describes the options for invoking fish. The :ref:`full manual <intro>` is available in HTML by using the :ref:`help <cmd-help>` command from inside fish, and in the `fish-doc(1)` man page. The :ref:`tutorial <tutorial>` is available as HTML via ``help tutorial`` or in `fish-tutorial(1)`.
+fish is a command-line shell written mainly with interactive use in mind. This page briefly describes the options for invoking fish. The :ref:`full manual <intro>` is available in HTML by using the :ref:`help <cmd-help>` command from inside fish, and in the `fish-doc(1)` man page. The :ref:`tutorial <tutorial>` is available as HTML via ``help tutorial`` or in `fish-tutorial(1)`.
 
 The following options are available:
 
@@ -21,9 +21,9 @@ The following options are available:
 
 - ``-C`` or ``--init-command=COMMANDS`` evaluate the specified commands after reading the configuration, before running the command specified by ``-c`` or reading interactive input
 
-- ``-d`` or ``--debug=CATEGORY_GLOB`` enables debug output and specifies a glob for matching debug categories (like ``fish -d``). Defaults to empty.
+- ``-d`` or ``--debug=DEBUG_CATEGORIES`` enable debug output and specify a pattern for matching debug categories. See :ref:`Debugging <debugging-fish>` below for details.
 
-- ``-o`` or ``--debug-output=path`` Specify a file path to receive the debug output, including categories and ``fish_trace``. The default is stderr.
+- ``-o`` or ``--debug-output=DEBUG_FILE`` specify a file path to receive the debug output, including categories and ``fish_trace``. The default is stderr.
 
 - ``-i`` or ``--interactive`` specify that fish is to run in interactive mode
 
@@ -46,3 +46,20 @@ The following options are available:
 - ``-f`` or ``--features=FEATURES`` enables one or more :ref:`feature flags <featureflags>` (separated by a comma). These are how fish stages changes that might break scripts.
 
 The fish exit status is generally the :ref:`exit status of the last foreground command <variables-status>`.
+
+.. _debugging-fish:
+
+Debugging
+---------
+
+While fish provides extensive support for :ref:`debugging fish scripts <debugging>`, it is also possible to debug and instrument its internals. Debugging can be enabled by passing the ``--debug`` option. For example, the following command turns on debugging for background IO thread events, in addition to the default categories, i.e. *debug*, *error*, *warning*, and *warning-path*::
+
+    > fish --debug=iothread
+
+Available categories are listed by ``fish --print-debug-categories``. The ``--debug`` option accepts a comma-separated list of categories, and supports glob syntax. The following command turns on debugging for *complete*, *history*, *history-file*, and *profile-history*, as well as the default categories::
+
+    > fish --debug='complete,*history*'
+
+Debug messages output to stderr by default. Note that if ``fish_trace`` is set, execution tracing also outputs to stderr by default. You can output to a file using the ``--debug-output`` option::
+
+    > fish --debug='complete,*history*' --debug-output=/tmp/fish.log --init-command='set fish_trace on'

doc_src/cmds/fish_indent.rst

@@ -28,7 +28,7 @@ The following options are available:
 
 - ``--html`` outputs HTML, which supports syntax highlighting if the appropriate CSS is defined. The CSS class names are the same as the variable names, such as ``fish_color_command``.
 
-- ``-d`` or ``--debug=CATEGORY_GLOB`` enables debug output and specifies a glob for matching debug categories (like ``fish -d``). Defaults to empty.
+- ``-d`` or ``--debug-level=DEBUG_LEVEL`` enables debug output and specifies a verbosity level. Defaults to 0.
 
 - ``-D`` or ``--debug-stack-frames=DEBUG_LEVEL`` specify how many stack frames to display when debug messages are written. The default is zero. A value of 3 or 4 is usually sufficient to gain insight into how a given debug call was reached but you can specify a value up to 128.
 

share/completions/fish.fish

@@ -6,7 +6,7 @@ complete -c fish -s n -l no-execute -d "Only parse input, do not execute"
 complete -c fish -s i -l interactive -d "Run in interactive mode"
 complete -c fish -s l -l login -d "Run as a login shell"
 complete -c fish -s p -l profile -d "Output profiling information to specified file" -r
-complete -c fish -s d -l debug -d "Specify verbosity level" -x -a "0\t'Warnings silenced'
+complete -c fish -s d -l debug -d "Specify debug categories" -x -a "0\t'Warnings silenced'
 1\t'Default'
 2\t'Basic debug output'
 3\t'More debug output'