1.${parameter-default}和${parameter:-default}

${parameter-default}，如果变量parameter没被声明，那么就使用默认值

${parameter:-default}，如果变量parameter没被声明，那么就使用默认值

 ${parameter-default}和${parameter:-default}在绝大多数的情况下，都是相同的，只有在parameter已经被声明，但是值为空的时候，这个额外的“:”才会产生不同的结果，即：其会使用默认值

2.实例

脚本实例：

#!/bin/bashvariable0=
echo "variable0 has been declared,but is set to null."
echo "variable0 = ${variable0-`whoami`}"
#不会有输出，variable0的输出结果为空echo
echo "variable1 has not been declared."
echo "variable1 = ${variable1-`whoami`}"
#variable1的输出结果为whoami命令的执行结果echo
variable2=
echo "variable2 has been declared,but is set to null."
echo "variable2 = ${variable2:-`whoami`}"
#variable2的输出结果为whoami命令的执行结果。因为“:-”会比“-”多一个条件测试。exit 0

结果：

variable0 has been declared,but is set to null.
variable0 = variable1 has not been declared.
variable1 = rootvariable2 has been declared,but is set to null.
variable2 = root