what is difference in `declare -r` and `readonly` in bash?

Question

In bash, what is the difference in declare -r and readonly?

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

I'm not sure which to choose.

Answer 1

tl;dr readonly uses the default scope of global even inside functions. declare uses scope local when in a function (unless declare -g).

At first glance, no difference.

Examining using declare -p

$ 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

# 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, readonly may be used to change a locally declared variable property to readonly, not affecting scope.

$ 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  

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Note: adding -g flag to the declare statement (e.g. declare -rg a="a1") makes the variable scope global. (thanks @chepner).

Note: readonly is a "Special Builtin". If Bash is in POSIX mode then readonly (and not declare) has the effect "returning an error status will not cause the shell to exit".