Why doesn't vim recognize bash self-made functions

Question

Say, I defined a command in my ~/.bash_profile, (MacOS Sierra, Macbook Air, if it's relevant)

function sayHello(){
   echo Hello world!
}

and while using vim, intend to call

:! sayHello

It does not recognize, and tells me

/bin/bash: sayHello: command not found
shell returned 127

The actual situation is that I want compile something, so it would be handy to use external command. If not, then fine, I can open up another terminal, cd-ed on the same directory.

Answer 1

External commands are not executed in an interactive shell so your .bash_profile is not read. To make your custom command available you must turn it into an actual shell script, put it in your $PATH, and make it executable:

#!/usr/bin/env bash

echo "Hello world!"

But there's a much better way to "compile something": :help :make.