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+==============================
+``grep`` system 'grep' command
+==============================
+
+Overview
+========
+
+The ``grep`` command searches for lines matching a regular expression in
+text files (or standard input). It uses the NuttX libc regular expression
+library (``regcomp`` / ``regexec``), so it supports real regular
+expressions including anchors (``^``, ``$``) and character classes
+(``[...]``), not just fixed substring matching.
+
+By default ``grep`` reads from ``stdin`` when no file is given, which
+makes it useful in NSH pipelines. When more than one file is searched (or
+when recursing into directories), the matching file name is prefixed to
+each result line.
+
+Configuration
+=============
+
+Enable the command with ``CONFIG_SYSTEM_GREP``. This option depends on
+``CONFIG_LIBC_REGEX`` and is disabled by default.
+
+The following configuration options are available:
+
+``CONFIG_SYSTEM_GREP_PROGNAME``
+  Program name for the ``grep`` command. Default: ``grep``.
+
+``CONFIG_SYSTEM_GREP_PRIORITY``
+  Task priority for the ``grep`` command. Default: 100.
+
+``CONFIG_SYSTEM_GREP_STACKSIZE``
+  Stack size for the ``grep`` command. Default:
+  ``DEFAULT_TASK_STACKSIZE``.
+
+Usage
+=====
+
+.. code-block:: console
+
+   grep [-invrHh] <pattern> [<file>...]
+
+When no ``<file>`` is given, ``grep`` reads from ``stdin``.
+
+Options
+=======
+
+``-i``
+  Case-insensitive match.
+
+``-n``
+  Prefix each matching line with its 1-based line number.
+
+``-v``
+  Invert the match: select lines that do **not** match.
+
+``-r``
+  Recurse into subdirectories. When set, the file name is always prefixed
+  to each result.
+
+``-H``
+  Always print the file name prefix, even with a single input file.
+
+``-h``
+  Never print the file name prefix.
+
+Exit status
+===========
+
+``0``
+  At least one line matched.
+
+``1``
+  No line matched.
+
+``2``
+  An error occurred (invalid option, unreadable file, bad regex, ...).
+
+Examples
+========
+
+Search ``stdin`` for lines containing ``error``:
+
+.. code-block:: console
+
+   nsh> dmesg | grep error
+
+Case-insensitive search with line numbers:
+
+.. code-block:: console
+
+   nsh> grep -in error /tmp/log.txt
+
+Invert the match (print non-matching lines):
+
+.. code-block:: console
+
+   nsh> grep -v error /tmp/log.txt
+
+Regular expression anchor -- match lines starting with ``ERR``:
+
+.. code-block:: console
+
+   nsh> grep "^ERR" /tmp/log.txt
+
+Recurse a directory, always printing the file name:
+
+.. code-block:: console
+
+   nsh> grep -r -H TODO /data/src
+
+Pipe usage -- filter the output of another NSH command:
+
+.. code-block:: console
+
+   nsh> ps | grep work
+
+Notes
+=====
+
+- ``grep`` is built on the libc regex library; ``CONFIG_SYSTEM_GREP``
+  depends on ``CONFIG_LIBC_REGEX``, so both must be enabled to satisfy the
+  ``regcomp`` / ``regexec`` dependency.
+- The ``-r`` option implies multi-file mode, so the file name is always
+  prefixed regardless of ``-H`` / ``-h``.
+- Long lines are truncated at ``GREP_LINE_MAX`` (512 bytes); matching is
+  performed on the (newline-stripped) content of each line.