在bash中`declare -r`和`readonly`有什么区别？

Question

在bash,就是在差异declare -r和readonly？

$ declare -r a="a1"
$ readonly b="b1"

我不知道该选哪个.

Answer 1

答:范围

乍一看,没有区别.

使用declare -p检查

$ declare -r a=a1
$ readonly b=b1
$ declare -p a b
declare -r a="a1"
declare -r b="b1"

# variable a and variable b are the same

但是,范围是不同的

# define variables inside function A
$ function A() {
      declare -r x=x1
      readonly y=y1
      declare -p x y
  }

$ A
declare -r x="x1"
declare -r y="y1"

# ***calling function A again will incur an error because variable y
#    was defined using readonly so y is in the global scope***

$ A
-bash: y: readonly variable
declare -r x="x1"
declare -r y="y1"

# after call of function A, the variable y is still defined

$ declare -p x y
bash: declare: x: not found
declare -r y="y1"

因此,一个区别是readonly将变量范围设为全局. declare使变量范围成为本地(这是预期的).

注意:declare -g在readonly语句中添加标志(例如-g)会使变量范围成为全局变量.(谢谢@chepner).

注意:declare是" 特殊建筑 ".如果Bash处于declare -rg a="a1"模式,那么readonly(而不是POSIX)具有" 返回错误状态将不会导致shell退出 "的效果.