Why are "declare -f" and "declare -a" needed in bash scripts?

Question

Sorry for the innocent question - I'm just trying to understand...

For example - I have:

$ cat test.sh
#!/bin/bash
declare -f testfunct

testfunct () {
echo "I'm function"
}

testfunct

declare -a testarr

testarr=([1]=arr1 [2]=arr2 [3]=arr3)

echo ${testarr[@]}

And when I run it I get:

$ ./test.sh
I'm function
arr1 arr2 arr3

So here is a question - why do I have to (if I have to ...) insert declare here? With it - or without it it works the same...

I can understand for example declare -i var or declare -r var. But for what is -f (declare function) and -a (declare array)?

Answer 1

declare -f functionname is used to output the definition of the function functionname, if it exists, and absolutely not to declare that functionname is/will be a function. Look:

$ unset -f a # unsetting the function a, if it existed
$ declare -f a
$ # nothing output and look at the exit code:
$ echo $?
1
$ # that was an "error" because the function didn't exist
$ a() { echo 'Hello, world!'; }
$ declare -f a
a () 
{ 
    echo 'Hello, world!'
}
$ # ok? and look at the exit code:
$ echo $?
0
$ # cool :)

So in your case, declare -f testfunct will do nothing, except possibly if testfunct exists, it will output its definition on stdout.