fadeout.sh

#!/bin/sh
# fadeout.sh length [pid]...
# Slowly lower the volume along the half sine slope then kill
# some processes and restore original volume at the end.
# Requires sleepenh and amixer.
# klg, Jun 2013

CTL=PCM
LEN=${1:-0}
shift

A=$(amixer -- get "$CTL" |
  sed -n 's/^.*: Playback \([0-9][0-9]*\) (*\[.*)*$/\1/p' | sed 1q)
T=$({
  echo "a=${A:-0}; t=$LEN"
  echo ' /* arcsine */
  define r(x){
    if (x<0) return (a(sqrt(1-x*x)/x)+4*a(1));
    if (x>0) return (a(sqrt(1-x*x)/x));
    return (2*a(1));
  }'
  echo 'scale=10'
  echo 'for (x=a-1; x>=0; x--) { t*r(2*x/a-1)/a(1)/4 }'
} | bc -l)

trap 'amixer -- set "$CTL" playback $A > /dev/null; exit' 0 INT QUIT

AX=$A
T0=$(sleepenh 0)
for X in $T; do
  sleepenh "$T0" "$X"
  amixer -- set "$CTL" playback $((AX-=1))
done > /dev/null

amixer -- set "$CTL" playback 0 > /dev/null
test 0 -lt "$#" && kill "$@"
sleep 1