Are those really POSIX or just bash? Namely the $() example. I was under the impression that only `` was POSIX, and that $() was bash/zsh-specific (maybe ksh too).

Also,

  ${CC:=gcc}

is only more readable than:

  [ -z "$CC" ] || CC=gcc

if you know what that syntax means. There are a ton of syntax in bash/zsh to modify varables inside of the ${} construct. I usually have to look them up every time that I see them, even when I was the one the wrote the original code. I would say that more people know the 'test' syntax than the ${:=} syntax.

  > 3. You can assign one variable to another without quoting

This is somewhat annoying, because something like this:

  command="ls -l"
  $command /path/to/dir

doesn't work. It tries to run a command called "ls -l" which probably doesn't exist. Passing that to exec will work, though. A more common example of this issue would be trying to build a command-line in the shell script before executing it, you might have base level of options:

  base_options="-l -d --debug=1"
  command $base_options $opt1 $opt2

But $base_options gets passed as a single parameter to command.