Bash Special Variables
######################
.. list-table:: Bash Special Variables
   :widths: 20 80
   :header-rows: 1

   * - Variable
     - Description
   * - ``$?``
     - The exit status of the last executed command. ``0`` indicates success, and any non-zero value indicates an error.
   * - ``$$``
     - The process ID (PID) of the current shell.
   * - ``$!``
     - The process ID (PID) of the last background command executed.
   * - ``$#``
     - The number of positional parameters (arguments) passed to the script or function.
   * - ``$@``
     - Expands to all positional parameters passed to the script or function as separate entities.
   * - ``$*``
     - Expands to all positional parameters as a single string, separated by the first character of the ``IFS``.
   * - ``$0``
     - The name of the shell script or function.
   * - ``$1, $2, ...``
     - Positional parameters representing the arguments passed to the script or function, where ``$1`` is the first argument, ``$2`` is the second, and so on.

Example Usage
*************

Here is an example demonstrating the usage of these variables:

.. code-block:: bash

    echo "Script Name: $0"
    echo "Number of Arguments: $#"
    echo "All Arguments (individually): $@"
    echo "All Arguments (as a single string): $*"
    echo "First Argument: $1"
    echo "Exit Status of Last Command: $?"
    echo "Process ID of Current Shell: $$"

    # Simulate a background process
    sleep 5 &
    echo "Process ID of Last Background Command: $!"

Execute Parsed bash command
***************************

To execute a bash command after some shell parsing

Say we have want to manually run a command from our crontab

.. code-block:: bash

	crontab -l | tail -n 1

To execute that in bash we would do as follows

.. code-block:: bash

	bash -c "$(crontab -l | tail -n 1)"

Awk Guide for Substrings
************************

    substr(string, start, length)

- **string**: The input string.
- **start**: The starting position (1-based index).
- **length**: (Optional) The number of characters to extract. If omitted, extracts to the end of the string.

.. role:: bash(code)
   :language: bash

Examples
--------

.. list-table:: Common `awk` Substring Examples
   :widths: auto
   :header-rows: 1

   * - Command
     - Output
     - Explanation
   * - :bash:`echo Hello, World! | awk '{print substr($0, 1, 5)}'`
     - ``Hello``
     - First 5 characters
   * - :bash:`echo Hello, World! | awk '{print substr($2, 2, 2)}'`
     - ``or``
     - Use space as delim, Next 2 chars from second field
   * - :bash:`echo Hello, World! | awk '{print substr($0, 8)}'`
     - ``World!``
     - Char 8 to the end of the line.
   * - :bash:`echo "apple,banana,cherry" | awk -F, '{print substr($2, 1, 3)}'`.
     - ``ban``
     - Use comma as delim, 3 chars from second field (Corresponds to 3)
   * - :bash:`echo "abcdef" | awk '{print substr($0, 1, 3) "-" substr($0, 4, 3)}'`
     - ``abc-def``
     - Combine substrings formed by awk

Notes
-----

- The default field separator is a space. Use the ``-F`` option to specify a custom delimiter.
- Conditions can be added to extract substrings only when specific criteria are met, e.g., :bash:`if ($2 > 100)`.