"trap DEBUG" as an alternative to $PS0

Prior to $PS0, you could use trap DEBUG:

Using trap DEBUG straightforwardly would echo your prompt more than once if you used a pipe or semicolon in your command. If you used a while or for loop, it could echo that prompt many times.
trap DEBUG can be made to work
- It's only a little circuitous.
- Using a combination of PROMPT_COMMAND and trap DEBUG is the way to do it with bash 4.3 and earlier.
Relatively minor downsides
- This has a slightly odd interaction with the bash vi-mode "v" command - it outputs some extraneous cruft, but otherwise seems to function.
- It's also been observed hanging on very old versions of macOS (formerly OS/X), when you try to use a pipe. I think it was macOS 10.3, but I could be wrong.
- The preexec output can be unintentionally redirected, unless you have your preexec write to /dev/tty (or perhaps better, to $(tty)) - which reduces portability a bit. This is an example of what you don't want to see:
- It interacts a little oddly with an interactive set -x.

BTW, hcm's "fancy terminal window" sets all this up for you automatically if you're on a bash that doesn't have $PS0.

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