Unexport CMD_DURATION

[bhamiltoncx]

Valid uses of the CMD_DURATION environment variable don't seem to include passing
it to subsequent child processes.

I work on the Buck build system at Facebook (http://facebook.github.io/buck/). One of Buck's features is to discard its cache if anything in the calling environment has changed since the last run:

https://github.com/facebook/buck/blob/master/src/com/facebook/buck/parser/Parser.java#L950

For all shells except fish, this is no problem. However, since fish exports CMD_DURATION to all sub-processes with a different value each time, Buck always discards its cache between runs, so it performs terribly.

I confirmed the variable is still available inside the shell by running:

function fish_prompt
    echo "duration [$CMD_DURATION] "
end

duration [0] sleep 2
duration [2002]

[zanchey]

This will need a note in the release notes, although it seems unlikely that anyone is depending on this behaviour.

[xfix]

In theory, someone could have a fish prompt implemented in different programming language using environment variable, but I would consider it unlikely (quick search on GitHub has shown one prompt not written in fish shell using CMD_DURATION, but even in this case the program generating a prompt gets explicit CMD_DURATION argument. Besides, even if it's a case, a parameter can be passed explicitly.