通配符是我们在shell环境中不知不觉中都会用到的，有时甚至都不会考虑到去探究其实现过程，因为使用得太普遍了。而清晰地理解每一个过程，将有助于我们的分析和调试。
说白了，通配符就是在 shell 环境下用来选择匹配给定模式的文件簇的特殊字符。下面是一段相对正式的定义，援引自https://bash.cyberciti.biz/guide/Wildcards。
Wildcards is one of the most important features of Bash shell. It allows you to select a group of files. For example you can select all C programming files in a GUI file manager with mouse. To select all C programming files in a Bash shell you use wildcards. In short wildcards are nothing but special characters that allows you to select a group of files that matches certain pattern.通配符一般只出现在命令的参数中，当 shell 在参数中遇到了通配符时，会将其当做路径或文件名区在磁盘上搜寻最可能的匹配，若符合要求的匹配存在，则进行代换，否则就将该通配符作为一个普通字符传递给命令，然后由命令进行处理。
如下图1-1所示为 shell 进行命令行参数解析时的全过程，非常繁琐但很重要。
shell 常见通配符如表1所示。
而除了有通配符之外，还有一系列特有的特殊元字符，如表2所示。这些元字符基本是作用在命令上面，用作多命令分割或参数分割。要注意这里的 {} ，因为表1里面也有类似的字符，实际上他们的作用范围并不相同，因此不会出现混淆。
metacharacter A character that, when unquoted, separates words. One of the following: | & ; ( ) < > space tab另外对于表1和表2中的特殊字符，如果我们想让其变成普通字符，就需要使用转义符，shell中提供三种转义符，如表3所示。
There are three quoting mechanisms: the escape character, single quotes, and double quotes. A non-quoted backslash () is the escape character. It preserves the literal value of the next character that follows, with the exception of <newline>. If a <newline>
pair appears, and the backslash is not itself quoted, the <newline> is treated as a line continuation (that is, it is removed from the input stream and effectively ignored).Enclosing characters in single quotes preserves the literal value of each character within the quotes. A single quote may not occur between single quotes, even when preceded by a backslash. Enclosing characters in double quotes preserves the literal value of all characters within the quotes, with the exception of $, `, \, and, when history expansion is enabled, !.
The characters $ and ` retain their special meaning within double quotes. The backslash retains its special meaning only when followed by one of the following characters: $, `, ", \, or <newline>. A double quote may be quoted within double quotes by preceding it with a backslash. If enabled, history expansion will be performed unless an ! appearing in double quotes
is escaped using a backslash. The backslash preceding the ! is not removed.其中单引号与双引号的区别如下：
- 单引号属于强引用，它会忽略所有被引起来的字符的特殊处理，被引用起来的字符会被原封不动地使用，只是引号范围内不允许再引用单引号。
- 双引号属于弱引用，它会对其中几个被引用起来的字符做特殊处理，其它的则忽略。例如：
- 其中 $ 加变量名可以取变量的值；
- 反引号和 $() 括号里面的字符会被当做命令执行后替换原来的字符；
- 需要使用 $ ` " 时需要通过反斜杠转义。
参考资料
[1] shell脚本【命令解析过程】