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

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.

Answer 2

As far as I know, the -a option alone does not have any practical relevance, but I think it's a plus for readability when declaring arrays. It becomes more interesting when it is combined with other options to generate arrays of a special type.
For example:
# Declare an array of integers
declare -ai int_array
int_array=(1 2 3)
# Setting a string as array value fails
int_array[0]="I am a string"
# Convert array values to lower case (or upper case with -u)
declare -al lowercase_array
lowercase_array[0]="I AM A STRING"
lowercase_array[1]="ANOTHER STRING"
echo "${lowercase_array[0]}"
echo "${lowercase_array[1]}"
# Make a read only array
declare -ar readonly_array=(42 "A String")
# Setting a new value fails
readonly_array[0]=23

Answer 3

declare -f allows you to list all defined functions (or sourced) and their contents.
Example of use:
[ ~]$ cat test.sh
#!/bin/bash
f(){
echo "Hello world"
}
# print 0 if is defined (success)
# print 1 if isn't defined (failure)
isDefined(){
declare -f "$1" >/dev/null && echo 0 || echo 1
}
isDefined f
isDefined g
[ ~]$ ./test.sh
0
1
[ ~]$ declare -f
existFunction ()
{
declare -f "$1" > /dev/null && echo 0 || echo 1
}
f ()
{
echo "Hello world"
}
However as smartly said gniourf_gniourf below : it's better to use declare -F to test the existence of a function.