pactl_change_volume.sh

#!/bin/bash
# Increments or decrements current output volume based on uint input parameter $1 (% volume)
# Each % increase in volume is a 655 units increase in the unitless volume value informed by pactl get-sink-volume
change_vol() {
    local rel_inc=$1 # Relative increment (input, can be negative)
    local abs_inc=$((rel_inc * 655)) # Absolute increment
    local max_vol=65536 # Exact 100%
    local min_vol=0
    local cur_vol=$(pactl get-sink-volume @DEFAULT_SINK@)
    cur_vol=${cur_vol##*left: } #
    cur_vol=${cur_vol%% /*}     # Pattern-matching to get the current volume value
    local new_vol=$((cur_vol + abs_inc))

    if   [ "$new_vol" -gt "$max_vol" ]; then
        pactl set-sink-volume @DEFAULT_SINK@ $max_vol
    elif [ "$new_vol" -lt "$min_vol" ]; then
        pactl set-sink-volume @DEFAULT_SINK@ $min_vol
    else
        pactl set-sink-volume @DEFAULT_SINK@ $new_vol
    fi
}

change_vol $1