Bash script; how to use vars and funcs defined aft

Question

How to use variables or functions that are defined after the command.

Variable

#!/bin/bash

echo Hello "$who"
who="World"

Function

#!/bin/bash

function_name

function_name() {
echo Hello World
}

I also heard there is a command to read entire bash script before executing any commands, this would work for my case. But it would be nice if there is a more pinpoint way.

More in-depth

#!/bin/bash

h=Hello

echo $h "$who"

var1=World

who=$(cat <<HEREDOC
You
Me
$var1
HEREDOC
)

Answer 1

Variables and functions always have to be defined before use. This is because function definitions are actually commands that assign the name in the current context, and not like in C where they merely provide an implementation for a name.

You can instead use control flow to ensure that the definitions execute before your code, regardless of their relative layout in the file:

main() {
  echo "Hello $var"
}

var="world"
main