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sh变量特性（2）-Linux菜鸟
source link: http://blog.51cto.com/lythjq/2073516
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变量设置方式说明示例${#变量}变量的长度A='Hello world!'echo ${#A}结果：12${变量:-word}如果变量为空或未定义，则变量展开为”word”；否则，展开为变量的值A='Hello world!'echo ${A:-OK}结果：Hello wold!unset A或 A=’’echo ${A:-OK}结果：OK${变量:+word}如果
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