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Do shell functions and aliases fork child processes? Or are they both executed within the shell process?

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No.

An alias is a simple substitution of one (or more) words for some string before the line is parsed to tokens. There is no shell context change needed.

From bash manual on Shell Functions:

Shell functions are executed in the current shell context; no new process is created to interpret them.

Unless the code that compose the function does fork a subprocess, like in bash with (…) (not in ksh). A function could be defined with parenthesis instead of (or additionally to) curly braces.

Test:

#!/bin/bash
func(){ echo "$BASHPID"; }
fork()( echo "$BASHPID"; )
echo "$BASHPID"
func
fork

On execution:

$ ./script
8731
8731
8753

Understand the fork function as:

fork(){
        ( echo "BASHPID" )
      }
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    Or more to the point, a function is just giving a name to a command (though in bash contrary to most other Bourne-like shells), it's limited to compound commands (including, but not limited to (...) and {...;}). Invoking the function is like invoking the corresponding compound command (with the added benefits that you can pass arguments that will be available as $1, $2... in the command and with some shells have local scope for variables or options inside). So if the command forks a subshell, that will fork a subshell and if not, not.
    – Stéphane Chazelas
    Commented Jan 31, 2018 at 16:12
  • Note that subshell and forking a child process are not necessarily the same thing. Most shells (including bash) implement subshells by forking, but they don't have to. ksh93 for instance doesn't.
    – Stéphane Chazelas
    Commented Jan 31, 2018 at 16:15

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