< Faux> I have a command which emits a number. This number is heading towards zero. I want to know when it will arrive at zero, and how close to zero it has got.

Damn right you can.

eta2zero () { A = $( eval ${ @ } ) while [ ${ A } -gt 0 ] do B = $( eval ${ @ } ) printf % $(( ${ A } - ${ B } )) s A = ${ B } sleep 1 done | pv -s ${ A } >/dev/null }

In action:

$ rm -rf /big/path & [ 1 ] 4895 $ eta2zero find /big/path \| wc -l 10 B 0 :00:14 [ 0 B/s ] [================================ > ] 90 % ETA 0 :00:10

(Sincere apologies for the lack of strace...)