Shell变量详解-优快云博客

## 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。

语法

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 = 2335
echo                     # Integer, still.


b=${a/23/BB}             # Substitute "BB" for "23".
                         # This transforms $b into a string.
echo "b = $b"            # b = BB35
declare -i b             # Declaring it an integer doesn't help.
echo "b = $b"            # b = BB35

let "b += 1"             # BB35 + 1
echo "b = $b"            # b = 1
echo                     # Bash sets the "integer value" of a string to 0.

c=BB34
echo "c = $c"            # c = BB34
d=${c/BB/23}             # Substitute "23" for "BB".
                         # This makes $d an integer.
echo "d = $d"            # d = 2334
let "d += 1"             # 2334 + 1
echo "d = $d"            # d = 2335

declare

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

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

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