英文文本新增

Table of Contents
4.1. Variable Substitution
4.2. Variable Assignment
4.3. Bash Variables Are Untyped
4.4. Special Variable Types
Variables are how programming and scripting languages represent data. A variable is nothing more than a label, a name assigned to a location or set of locations in computer memory holding an item of data.

Variables appear in arithmetic operations and manipulation of quantities, and in string parsing.