#!/bin/bash
set -e

CUR_FILE=/var/tmp/current_volume

set +e
CUR="$(cat "$CUR_FILE" || echo 50)"
set -e

case "${1:0:1}" in
    '')
        exit 1
        ;;
    '+'|'-')
        NEW="$((CUR + "$1"))"
        ;;
    *)
        NEW="$1"
        ;;
esac

[ "$NEW" -lt 0 ] && NEW=0
[ "$NEW" -gt 100 ] && NEW=100

pactl set-sink-volume 0 "$NEW"%
pactl set-sink-mute 0 0

echo "$NEW"