Shell: Count Number of Characters in a String

x="This is a test"
y="${x//[^s]}"
echo "$y"
echo "${#y}"

To match both 's' and 'S', enter:

x="This is a test. S"
y="${x//[^s|S]}"
echo "${#y}"

Multimidia: Adjust vol/gain on ffmpeg transcoding

ffmpeg -i input.mp4 -ab 192 -ar 44100 -vol 512 output.mp3

256, no change. 128, decrease twice. 512, increase twice.