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## 引言
shell这门语言，作为与Linux交互效率最高的工具，我相信每个code monkey在工作中或多或少都会用到；我今天要讲的是这门语言中最基本的部分——变量。shell中的变量与类C语言差异较大，相信大家看完后都会有所收获。在shell中，我们可以使用FOO=BAR这样的方式声明变量（注意，这里不能有空格），当使用这种方式声明变量时，变量是没有类型的，或者说变量的类型可以根据上下文自己转换。比如：
a=2334 # Integer.let "a += 1"echo "a = $a " # a = 2335echo # Integer, still.b=${a/23/BB} # Substitute "BB" for "23". # This transforms $b into a string.echo "b = $b" # b = BB35declare -i b # Declaring it an integer doesn't help.echo "b = $b" # b = BB35let "b += 1" # BB35 + 1echo "b = $b" # b = 1echo # Bash sets the "integer value" of a string to 0.c=BB34echo "c = $c" # c = BB34d=${c/BB/23} # Substitute "23" for "BB". # This makes $d an integer.echo "d = $d" # d = 2334let "d += 1" # 2334 + 1echo "d = $d" # d = 2335 我们可以使用shell内置的declare声明变量：
| Option | Meaning |
|---|---|
| -a | Variable is an array. |
| -f | Use function names only. |
| -i | The variable is to be treated as an integer; arithmetic evaluation is performed when the variable is assigned a value (see Section 3.4.6). |
| -p | Display the attributes and values of each variable. When -p is used, additional options are ignored. |
| -r | Make variables read-only. These variables cannot then be assigned values by subsequent assignment statements, nor can they be unset. |
| -t | Give each variable the trace attribute. |
| -x | Mark each variable for export to subsequent commands via the environment. |
shell支持三种数据类型：字符串、整型、数组。
字符串类型是shell声明一个变量时默认的类型，当我们执行
JAVA_HOME=/usr/default/java
时，JAVA_HOME这个变量的类型是string，
| 语法 | 说明 |
|---|---|
| ${parameter:-defaultValue} | Get default shell variables value |
| ${parameter:=defaultValue} | Set default shell variables value |
| ${parameter:?”Error Message”} | Display an error message if parameter is not set |
| ${#var} | Find the length of the string |
| ${var%pattern} | Remove from shortest rear (end) pattern |
| ${var%%pattern} | Remove from longest rear (end) pattern |
| ${var:num1:num2} | Substring |
| ${var#pattern} | Remove from shortest front pattern |
| ${var##pattern} | Remove from longest front pattern |
| ${var/pattern/string} | Find and replace (only replace first occurrence) |
| ${var//pattern/string} | Find and replace all occurrences |
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