bash bash中的`declare -r`和`readonly`有什么区别？
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what is difference in `declare -r` and `readonly` in bash?
提问by JamesThomasMoon1979
In bash, what is the difference in declare -rand readonly?
在bash，就是在差异declare -r和readonly？
$ declare -r a="a1"
$ readonly b="b1"
I'm not sure which to choose.
我不确定该选择哪个。
回答by JamesThomasMoon1979
tl;drreadonlyuses the default scope of globaleven inside functions. declareuses scope localwhen in a function (unless declare -g).
tl;dr甚至在函数内部readonly使用global的默认范围。 在函数中declare使用局部作用域（除非declare -g）。
At first glance, no difference.
乍一看，没什么区别。
Examining using declare -p
使用声明 -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
Now review the difference when defined within a function
现在查看在函数中定义时的差异
# 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"
To add more nuance, readonlymay be used to change a locally declared variable property to readonly, not affecting scope.
要添加更多细微差别，readonly可用于将本地声明的变量属性更改为readonly，而不影响范围。
$ function A() {
declare a="a1"
declare -p a
readonly a
declare -p a
}
$ A
declare -- a="a1"
declare -r a="a1"
$ declare -p a
-bash: declare: a: not found
Note: adding -gflag to the declarestatement (e.g. declare -rg a="a1") makes the variable scope global. (thanks @chepner).
注意：-g在declare语句中添加标志（例如declare -rg a="a1"）会使变量作用域成为global。（感谢@chepner）。
Note: readonlyis a "Special Builtin". If Bash is in POSIXmode then readonly(and not declare) has the effect "returning an error status will not cause the shell to exit".
注意：readonly是“特殊内置”。如果 Bash 处于POSIX模式，则readonly（而不是declare）具有“返回错误状态不会导致外壳退出”的效果。
