Bash – Setting a shell variable in a null coalescing fashion

Nested substitution is not available in any modern Bourne-like shells except zsh:

$ print -rl -- ${$(echo):-C}
C
$ print -rl -- ${$(echo 1):-C}
1

In other shells:

A=$(nc -l 443)
A=${A:-C}

The while instruction is followed by the command from which to evaluate the return code to determine if it should loop or not. It proceeds if the return value is 0.

Referring to an uninitialized variable is identical to having it initialized to ''.

Since running an empty command causes no error, it returns the corresponding return code (0):

$ foo=''
$ $foo
$ echo $?
0

That is normal behavior since the shell would otherwise give an error each time you press enter without typing anything.

Note that this differs from specifying an empty string which the shell identifies as an issue:

$ ''
-bash: : command not found
$ echo $?
127