[_kst_]

Not as good. I use $today a lot in file and directory names, The overhead of having to type $(today) rather than $today would be, for me, significant.

I do have a workaround for this particular case:

    PROMPT_COMMAND='today=$(printf "%(%F)T\n" -1)'

but it only works in bash 4.2 and later.

I could use

    PROMPT_COMMAND='today=$(date +%F)'

but I'm trying to avoid executing an external command on every prompt. (Maybe the overhead is low enough that it's not worth worrying about.)

My thoughts are (a) if user-defined special variables like this were a shell feature, I could find other uses for them and (b) it seems neater to make such a feature available to users rather than restricting it to three special-case built-in variables.

On the other hand, it might have been cleaner for $RANDOM, $EPOCHSECONDS, and $EPOCHREALTIME to be implemented as built-in functions rather than as special variables.

[_kst_]

I have a better (at least cleaner IMHO) workaround:

    PROMPT_COMMAND='printf -v today "%(%F)T" -1'

printf's "-v" option was added in bash 4.0.

printf's "%(...)T" format was added in bash 4.2.

The "-1" argument became optional in bash 4.3.

So here's what I now have in my .bashrc :

    if [[ "$BASH_VERSION" > "4.2" ]] ; then
        PROMPT_COMMAND='printf -v today "%(%F)T" -1'
    else
        PROMPT_COMMAND='today=$(date +%F)'
    fi

(The "> 4.2" test is true for bash 4.2 and later, since the value of $BASH_VERSION for 4.2 is "4.2.0(1)-release". The ">" does a string comparison. I'll have to revisit this when and if there's a bash version 10.0, probably using $BASH_VERSINFO.)