Linux Bash之通配符

通配符是咱們在shell環境中不知不覺中都會用到的，有時甚至都不會考慮到去探究其實現過程，由於使用得太廣泛了。而清晰地理解每個過程，將有助於咱們的分析和調試。

說白了，通配符就是在 shell 環境下用來選擇匹配給定模式的文件簇的特殊字符。下面是一段相對正式的定義，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.

通配符通常只出如今命令的參數中，當 shell 在參數中遇到了通配符時，會將其當作路徑或文件名區在磁盤上搜尋最可能的匹配，若符合要求的匹配存在，則進行代換，不然就將該通配符做爲一個普通字符傳遞給命令，而後由命令進行處理。

以下圖1-1所示爲 shell 進行命令行參數解析時的全過程，很是繁瑣但很重要。

shell 常見通配符如表1所示。

而除了有通配符以外，還有一系列特有的特殊元字符，如表2所示。這些元字符基本是做用在命令上面，用做多命令分割或參數分割。要注意這裏的  {}  ，由於表1裏面也有相似的字符，實際上他們的做用範圍並不相同，所以不會出現混淆。

metacharacter     A character that, when unquoted, separates words. One of the following: |  & ; ( ) < > space tab

另外對於表1和表2中的特殊字符，若是咱們想讓其變成普通字符，就須要使用轉義符，shell中提供三種轉義符，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.

其中單引號與雙引號的區別以下：

• 單引號屬於強引用，它會忽略全部被引發來的字符的特殊處理，被引用起來的字符會被原封不動地使用，只是引號範圍內不容許再引用單引號。
• 雙引號屬於弱引用，它會對其中幾個被引用起來的字符作特殊處理，其它的則忽略。例如：

1. 1. 其中  $  加變量名能夠取變量的值；
   2. 反引號和  $()  括號裏面的字符會被當作命令執行後替換原來的字符；
   3. 須要使用  $ ` "  時須要經過反斜槓轉義。

參考資料

[1] shell腳本【命令解析過程】

[2] Linux Shell 通配符、元字符、轉義符使用實例介紹