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Comment by aexaey | Hacker News Reader

How does function sound for a syntactic sugar?

  $ today() { date +%F; }
  $ echo Today, $(today) is a great day!
  Today, 2019-01-08 is a great day!

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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.

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.)

It's not as nice, "cat $today" is easier to type than "cat $(today)" and would give better completion, just declared matching variables instead of matching functions, files and executables.

On the plus side, TIL the subshell syntax plays well with eval/expand shortcut (ctrl+alt+e).

It's not obvious that just expanding a variable runs code though.

Wouldn't "cat $today" result with "cat: No such file or directory: 2019-01-08"? Did you mean echo instead of cat?

Only in the case when no such file exists in the current directory :)

My real life use case for these dynamic variables would be more like "cat/vim/cp $log" to get today's log file which would expand to something like /somedir/logs/201901/09/product.log. Handy when you have a large matrix of products/environments.

* "cat $today" is easier to type than "cat $(today)"*

Except you should really be in the habit of typing "cat ${today}" ;)

Except you should really be in the habit of typing `cat "${today}"` ;) Quote everything!

Why? The {} doesn't prevent glob expansion or field separation.

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