Bash Programming Notes.
=======================
  Syntax Rules :
  --------------
    '#'   - Donates a comment, can be placed anywhere on a line. 
Everything 
after it will be ignored until a new line.
    '#!'  - Donates the interpreter (when on the first line).
    ';'   - Command separator.
    ';;'  - Case option terminator.
    '.'   - Equivalent to source command.
    '"'   - Allows special characters to be interpreted.
    '''   - String literal, no special character interpretation.
    ','   - Used to seperate mathamatical expressions all are executed but 
only the last is returned.
    '\x'  - Donates an escape/special character to be interpreted.
    '`'   - This allows a programs output to be assigned to a variable.
    ':'   - Null command, returns true always.
    '!'   - Reverse the value of a true|false value or expression.
    '*'   - Wild card operator. Or arithmetic op for multiplication.
    '**'  - Exponent op.
    '?'   - Single character wild card.
    '$'   - Variable substitution.
    '${}' - Parameter substitution.
    '()'  - Command group.
    '{}'  - Code block.
    '[]'  - Test construct.
    '[[]]'- Test Construct.
    '[]'  - Array element.
    '(())'- Integer expansion.
    '&&'  - AND logical operator.
    '||'  - OR logical operator.

  Scriptlets :
  ------------
    echo $((2#10101001)) - Base conversion.

  Special Shell Variables :
  -------------------------
    '$0' - Script name.
    '${n}' - Parameter.
    '$#' - Number of parameters.
    '$*' - All parameters as a single word.
    '$@' - All parameters as seperate strings.
    '$?' - Return value.
    '$$' - Script PID.
    '$PWD' - Current working directory.
    '$OLDPWD' - Old working directory.

  Comparison Operators :
  ----------------------
    '-eq' '=' '==' - Equality.
    '-ne' '!=' - Inequality.
    '-lt' '\<' '<' - Less than.
    '-le' '<=' - Less than or equal.
    '-gt' '\>' '>' - Greater than.
    '-ge' '>=' - Greater than or equal.
    '-z' - String is empty.
    '-n' - String is not empty.

  Test Operations (files) :
  -------------------------
    '-e' - File exists.
    '-s' - File is not zero size.
    '-f' - File is a regular file.
    '-d' - File is a directory.
    '-h' '-L' - File is a symlink.
    '-b' - File is a block device.
    '-c' - File is a character device.
    '-p' - File is a pipe.
    '-S' - File is a socket.
    '-r' - File has read permission.
    '-w' - File has write permission.
    '-x' - File has execute permission.
    '-t' - File has a terminal association.
    '-N' - File has been modified since last read.
    '-O' - File owned by executing UID.
    '-G' - File has same GID as executor.
    '-g' - SGID flag set.
    '-u' - SUID flag set.
    '-k' - Sticky bit set.
    '!'  - Reverses sense of file test operations.

  String Operations :
  -------------------
    ${#stringName} - Length of string.
    ${varName:position} - Extract sub-string at position.
    ${varName:position:length} - Extract sub-string of length.

  Flow Control :
  --------------
    If Statements :
    ---------------
      if [ expression ]
        then
          # Code to be executed.
        fi

      if [ expression ]
        then
          # Code to be executed.
        else
          # Code to be executed if expression not satisfied.
        fi

      if [ expression ]
        then
          # Code to be executed.
        elif [ expression ]
          then
            # Code to be executed.
        else
          # Code to be executed.
        fi

    Case Statement :
    ----------------
      case "$varName" in
        abc)
          echo "First case."
          ;;
        def)
          echo "Second case."
          ;;
        ghi)
          echo "Third case."
          ;;
        jkl)
          echo "Forth case."
          ;;
      esac

    For Loop :
    ----------
      for [currentItr] in [itrList]
        do
          [ command ... ]
        done