Linux Bash之通配符

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：

1. 1. 其中  $  加变量名可以取变量的值；
   2. 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；
   3. 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】

[2] Linux Shell 通配符、元字符、转义符使用实例介绍