{ "type": "command", "text": "~/.config/fastfetch/fastfetch-nixos-generation.sh", "key": "Generation" },

fastfetch-nixos-generation.sh

#!/usr/bin/env bash

pushd "/nix/var/nix/profiles" > /dev/null

function gen-to-num() {
	if [[ "$1" =~ system-([0-9]+)-link ]]; then
		echo "${BASH_REMATCH[1]}"
	fi
}

booted=$(readlink /run/booted-system)
current=$(readlink /run/current-system)

gens=(echo system-*-link)
for gen in "${gens[@]}"; do

	target=$(readlink -f "$gen")

	[ "$target" = "$booted" ] && boot=$(gen-to-num "$gen")
	[ "$target" = "$current" ] && curr=$(gen-to-num "$gen")
done

system=$(gen-to-num $(readlink system))
default=$([[ "$curr" != "$system" ]] && echo " (default: $system)")
booted_from=$([[ "$curr" != "$boot" ]] && echo " (booted from: $boot)")

first=$(gen-to-num "${gens[1]}")
last=$(gen-to-num "${gens[-1]}")

echo "Generation $curr$booted_from$default, available: $first..$last"