A portable statusbar script which can update its components at different time intervals, using minimal sub-processes

statusbar.sh

#!/bin/sh

set -ef

main() {
	trap 'kill 0' EXIT HUP INT
	emitter | collector
}

# All variables used here are dynamically assigned via `export`.
# Silence shellcheck warnings about the use of unassigned variables.
# shellcheck disable=2154
update() {
	fmt='D:%-5s L:%4s C:%3s%% T:%2sC M:%3s%% D:%10s U:%10s %s %s\n'
	set -- "${desktop:=-}" "$cpu_load" "$cpu_pct" "${cpu_temp%???}" \
		"$mem_usage" "$net_down" "$net_up" "$date" "$time"

	# To stay below 75 columns we use a variable as format string for
	# `printf`. Disable shellcheck warning about this.
	# shellcheck disable=2059
	printf "$fmt" "$@"
}

emitter() {
	every() { delta=${1:-1}; [ $((${round:=0} % delta)) -eq 0 ]; }

	# Event driven items should be added (and backgrounded) here.
	[ -n "$DISPLAY" ] &&
		desktops &

	while :; do
		# Shortest interval should come last, so that delta
		# (set in every()) has the right value.
		every 60 && {
			cpu_load
			date_time
		}
		every 4 && {
			cpu_pct
			cpu_temp
			mem_usage
			net down
			net up
		}

		printf update\\n

		round=$((round + delta))
		sleep "$delta"
	done
}

collector() {
	while IFS= read -r line; do
		case $line in
			update) update ;;
			*)
				# The expansion is intentional here.
				# shellcheck disable=2163
				export "$line"
			;;
		esac
	done
}

##########################################################################

cpu_load() {
	# We don't make use of load of the last 5 and 15 minutes.
	# shellcheck disable=2034
	read -r cpu_load cpu_load5 cpu_load15 x </proc/loadavg
	printf 'cpu_load=%s\n' "$cpu_load"
}

cpu_temp() {
	read -r cpu_temp </sys/class/thermal/thermal_zone0/temp
	printf 'cpu_temp=%s\n' "$cpu_temp"
}

# This function assigns the $prev_* variables using `eval`, silence the
# wrong warnings about those variables not being assigned.
# shellcheck disable=2154
cpu_pct() {
	vars="user nice system idle iowait irq softirq steal"

	# Store previous values.
	for var in $vars; do
		eval "prev${var}=\${$var:-0}"
	done

	# Read new values.
	# The expansion and word splitting of $vars is intentional.
	# shellcheck disable=2229,2086
	while read -r name $vars x </proc/stat; do
		[ "$name" = "cpu" ] && break
	done

	# Based on https://stackoverflow.com/a/23376195.
	PrevIdle=$((previdle + previowait))
	Idle=$((idle + iowait))

	PrevNonIdle=$((prevuser + prevnice + prevsystem
		+ previrq + prevsoftirq + prevsteal))
	NonIdle=$((user + nice + system + irq + softirq + steal))

	PrevTotal=$((PrevIdle + PrevNonIdle))
	Total=$((Idle + NonIdle))

	# Differentiate: actual value minus the previous one.
	totald=$((Total - PrevTotal))
	idled=$((Idle - PrevIdle))

	printf 'cpu_pct=%s\n' "$((100 * (totald - idled) / totald))"
}

date_time() {
	date=$(date +'%F %H:%M')
	printf 'date=%s\ntime=%s\n' "${date% *}" "${date#* }"
}

desktops() {
	desk_name_by_num() { shift "$(($1 + 1))"; printf %s\\n "$1"; }

	xprop -spy -root -notype \
		_NET_DESKTOP_NAMES \
		_NET_CURRENT_DESKTOP \
	| while read -r line; do
		case $line in
			_NET_DESKTOP_NAMES*)
				# Use `tr` instead of parameter expansion,
				# since this only ever runs once anyway.
				# Wordsplitting is intentional here.
				# shellcheck disable=2046
				set -- $(printf %s\\n "${line#* = }" \
					| tr -d \",)
			;;

			*)
				num=${line#* = }
				printf 'desktop=%s\nupdate\n' \
					"$(desk_name_by_num "$num" "$@")"
			;;
		esac
	done
}

mem_usage() {
	unset memtotal memavail

	# Loop until $memtotal and $memavail are set.
	# We want to discard the third column so we split it into $x. It
	# will remain unused, hence silence shellcheck.
	# shellcheck disable=2034
	while read -r name val x; do
		case $name in
			MemTotal:)
				memtotal=$val
				[ -n "${memavail:-}" ] && break
			;;

			MemAvailable:)
				memavail=$val
				[ -n "${memtotal:-}" ] && break
			;;
		esac
	done </proc/meminfo

	printf 'mem_usage=%s\n' \
		"$((100 * (memtotal - memavail) / memtotal))"
}

net() {
	net_interface=enp3s0

	case ${1:-down} in
		down)
			f=rx
			var_name=net_down
			prev=${rx:-x}
		;;

		up)
			f=tx
			var_name=net_up
			prev=${tx:-x}
		;;
	esac

	path=/sys/class/net/$net_interface/statistics/${f}_bytes
	# We either read "rx" or "tx" bytes. The expansion of $f is thus
	# intentional.
	# shellcheck disable=2229
	read -r "$f" <"$path"

	case $f in
		rx) bytes=$rx ;;
		tx) bytes=$tx ;;
	esac

	case $prev in
		x) prev=$bytes ;;
	esac

	bytes=$(((bytes - prev) / delta))

	if [ "${#bytes}" -gt 9 ]; then
		unit=GB
		value=${bytes%?????????}
	elif [ "${#bytes}" -gt 6 ]; then
		unit=MB
		value=${bytes%??????}
	elif [ "${#bytes}" -gt 3 ]; then
		unit=kB
		value=${bytes%???}
	else
		printf '%s=%d B/s\n' "$var_name" "$bytes"
		return
	fi

	value=$value.${bytes#$value}
	printf '%s=%.1f %s/s\n' "$var_name" "$value" "$unit"
}

main "$@"