shell编程---替换运算符

替换运算符1、{ }运算符规则1、若未定义该变量或者该变量值为空则将用运算符后面的值赋值给该变量2、变量值不为空不更新变量值。#! /bin/bash PRINTRemind some import things echo $PRINT echo ${STRING1:hello} STRING1string1 echo ${STRING1:hello} STRING1 echo ${STRING1:hello}2、{}运算符规则1、若未定义该变量或者该变量值为空退出程序并报错2、变量值不为空不更新变量值。#! /bin/bash PRINTRemind some import things echo $PRINT STRING2string2 echo ${STRING2:?2} echo ${STRING1:?}3、{}运算符规则1、若未定义该变量或者该变量值为空定义该变量但该变量值为空2、已定义该变量且变量值不为空更新变量值。#! /bin/bash PRINTRemind some import things echo $PRINT STRING2string2 echo ${STRING2:2} echo ${STRING1:} echo ${STRING1:2}4、{-}运算符规则1、若未定义该变量或者该变量值为空定义该变量且更新变量值2、已定义该变量且变量值不为空不更新变量值。#! /bin/bash PRINTRemind some import things echo $PRINT STRING2string2 echo ${STRING2:-2} echo ${STRING1:-} echo ${STRING1:-2}Tips运算符{}和{-}功能一样。5、 实例#! /bin/bash echo Hi man !! # test operator {: } echo ${VAR1: 1} echo $VAR1 # test operator {:- } VAR2${VAR2:- 2} echo $VAR2 VAR3${VAR:- 5} echo $VAR3