[reptation]

the difference is that the author's version backs up every command to a file as it happens. If you have multiple terminals open, you do not get the same output running "history".

[SEJeff]

something other than:

    shopt -s histappend

Isn't that exactly what that shell option does?

[LukeShu]

That does make multiple terminals play nice with eachother, but it's still different, as OP's still flushes to HISTFILE after every command. But, then we can do that with

    PROMPT_COMMAND='history -a'