if statement, that calls a function and compares values to variables

Question

I use i3 and want to control the volume of my sink using the volume keys, that call pactl. Since pactl does not support a parameter for max volume, but is adviced to use with pipewire, the script should get the current volume and compare, if the next volume_increment step would result in a higher value than a certain max_volume. My confusion is now related to an if statement, with the combination of calculating the difference between the volume_increment and max_volume, and compare it to the outcome of the current_volume function. So the volume will be raised, until the certain value, where the increment would result in a higher volume then the maximum value

#!/bin/bash

volume_increment=$1
max_volume=$2

function get_volume() {
pactl get-sink-volume @DEFAULT_SINK@ | head -n1 | cut -d/ -f2 | tr -d ' %'
}
 
if (( get_volume >= $max_volume - $volume_increment )) ; then
    pactl set-sink-volume @DEFAULT_SINK@ +$volume_increment%
else
    pactl set-sink-volume @DEFAULT_SINK@ $max_volume%
fi

I've googled a lot and tried several variations of brackets in the if statement. I've managed to get it to work at some intermediate step. But then I've read that (()) brackets are not universal for POSIX shells.

Now I'm very confused and can't separate these various aspects in my head. Would someone be so kind to explain the logic of calling a function in a statement and compare it to a difference of variables? Preferably for universal POSIX usage, nut bash specific.

Answer 1

Note that in arithmetic context, variables are implicitly expaned. Therefore, ((var)) uses $var automatically.

In your statement

(( get_volume >= $max_volume - $volume_increment ))

the $ is not needed - you could equally well have written max_volume - volume_increment , but you are refering to a variable get_volume and never set it. An unset variable is treated as zero, and therefore get_volume is always 0. Assuming that you can caluclate this value from the function current_volume , you could do a

get_volume=$(current_volume)

or run this function directly inside the arithmetic expression, ie

(( $(current_volume) >= $max_volume - $volume_increment ))

Of course, all this requires that you are restricting yourself to integer arithmetic in your script.