testsAndMisc/.github/skills/efficient-polling-scripts/SKILL.md

name	description
efficient-polling-scripts	Use BEFORE writing any shell or Python script that runs on a timer, per-tick status bar (i3blocks/waybar/polybar), cron-like loop, or any repeated invocation. Prevents fork-storm anti-patterns that can consume many CPU-hours per day from tiny polling scripts.

Efficient Polling & Status-Bar Scripts

When this applies

Any script that runs frequently — per second or per few seconds — especially:

• i3blocks / waybar / polybar / xmobar / tmux status-line scripts
• cron / systemd-timer jobs with intervals < 1 min
• watcher loops invoked by another process every tick
• Python CLIs invoked from a shell hot loop

A single fork pipeline running once per second will consume ~30–50 CPU-minutes per day per forked helper. Five such scripts with 3–8 helpers each turn into days of CPU-time lost per day and tens of thousands of forked processes showing up in atop.

The rules

R1. Zero forks in the hot path when possible

Every $(...), backtick, and | in a shell script forks a process. Favor bash builtins:

Instead of	Use
$(cat /proc/loadavg)	$(</proc/loadavg) or read -r one _ < /proc/loadavg
echo "$x" | awk '{print $1}'	read -r first _ <<< "$x" or arr=($x); first=${arr[0]}
echo "$x" | tr -d '%'	${x//%/}
echo "$x" | grep -Po '\d+%'	[[ $x =~ ([0-9]+)% ]] && vol=${BASH_REMATCH[1]}
echo "$a < $b" | bc -l	(( a_times_100 < b_times_100 )) (scale decimals to ints)
sensors | awk ...	read -r milli < /sys/class/hwmon/hwmonN/temp1_input
acpi -b | awk ...	read -r cap < /sys/class/power_supply/BAT0/capacity
free -h | awk ...	parse /proc/meminfo with while read -r
df -h / | awk ...	stat -f builtin? No: use a long-lived reader, or accept one fork at low frequency
lspci | grep -i nvidia	check /sys/bus/pci/devices/*/vendor (0x10de == NVIDIA)

R2. Read from /sys and /proc directly

The kernel exposes structured data without forking anything. Useful paths:

• CPU load: /proc/loadavg
• CPU per-core stat: /proc/stat
• Memory: /proc/meminfo
• Temps / fans / voltages: /sys/class/hwmon/hwmon*/
  • CPU on AMD: name=k10temp, temp1_input = Tctl (milli-°C, divide by 1000)
  • CPU on Intel: name=coretemp
  • Motherboard Super-I/O: name=nct* / it87* / f71*
  • AMD GPU: name=amdgpu, plus /sys/class/drm/card*/device/gpu_busy_percent
• Battery: /sys/class/power_supply/BAT*/ (capacity, status, energy_now, power_now)
• Backlight: /sys/class/backlight/*/brightness
• Network link: /sys/class/net/*/operstate, /sys/class/net/*/statistics/*_bytes

NVIDIA is the unfortunate exception — there is no sysfs utilization interface, so nvidia-smi is required. Mitigate with R4 (long-lived producer).

R3. Integer arithmetic, never bc in a hot loop

bc forks a process. For decimal comparisons, multiply out:

# "1.23" → 123, "0.45" → 45; compare against threshold ×100.
load_x100=$((10#${one//./}))
(( load_x100 < 150 )) && echo 'normal'

Bash's ((…)) and [[ … ]] are builtins — free.

R4. Prefer event-driven / long-lived producers over polling + sleep

When an update needs to happen often, replace "poll + sleep + exit" with one of:

• i3blocks interval=persist: script runs forever, prints one block per update. Block on an event stream with read — no sleep, no busy-wait.
• pactl subscribe: event stream for PulseAudio/PipeWire volume/mute changes.
• udevadm monitor: hardware / power-supply / backlight events.
• inotifywait -m: file/dir changes.
• dbus-monitor: session-wide events (network, media keys, NetworkManager).
• journalctl -f: new log lines.
• nvidia-smi --loop=N / nvidia-smi dmon -d N: one long-lived nvidia-smi emitting rows instead of forking every N seconds. Tail its stdout with while read.
• mpstat N, iostat N, vmstat N: same pattern for CPU/IO.

Canonical persist skeleton:

#!/bin/bash
set -u
emit() { printf '%s\n' "$1"; }

emit "$(initial_value)"
producer_command | while read -r line; do
  # `read` blocks on I/O — no CPU, no sleep, no poll.
  [[ $line matches relevant event ]] || continue
  emit "$(compute_new_value)"
done

R5. One-shot scripts must still be cheap

Even with interval=5, 1728 invocations/day × 3 forks = 5k forks/day. Make the single-invocation path fork-free when possible. Profile with:

strace -f -e trace=%process -c ./myscript.sh

The clone / execve counts are your fork count.

R6. Python called from a hot loop is an anti-pattern

CPython startup is ~50–80 ms on modern hardware. Invoking python my_helper.py once per second = ~5–8% of one core doing nothing but importing stdlib.

If a status-bar value needs Python logic:

• Inline it in bash when possible (the rules above almost always suffice).
• Run a persistent Python daemon that writes to a FIFO / Unix socket / tmpfile; the bash hot-path reads from it with read / $(<file).
• Use a compiled helper (Go/Rust/C) if Python startup is the only issue — a static binary startup is sub-millisecond.

R7. Cap risk with a systemd slice

Even a correct script can regress. Put status-bar / monitoring work in a resource-capped user slice so the blast radius is bounded:

# ~/.config/systemd/user/monitors.slice
[Slice]
CPUQuota=50%
MemoryMax=512M
MemorySwapMax=0   # REQUIRED on zram systems — see oom-prevention skill
TasksMax=256

Launch i3blocks (or individual persist scripts) under that slice, e.g. via a user service with Slice=monitors.slice, so every child inherits the cap.

R8. Measure before and after

For any "fast" shell script, time 10k invocations:

time for _ in {1..10000}; do ./script.sh >/dev/null; done

Target: a 1-Hz script should take < 2 ms per invocation on a modern desktop. A 5-second-interval script can afford ~20 ms. If you're over budget, count the execve with strace -c and remove forks.

R9. Never flush+rebuild firewall rules unconditionally in a poll loop

iptables -F chain followed by dozens of iptables -A calls every tick is one of the most expensive fork patterns possible — each rule insert is its own fork, plus it contends the xtables lock with the kernel's own netfilter subsystem (netd on Android, firewalld/nftables on Linux). This is a specific enough, and costly enough, instance of R6-style unconditional recomputation to call out on its own: a real incident (phone_focus_mode/dns_enforcer.sh, see feedback-iptables-hash-gate-before-rebuild in this user's memory) rebuilt ~100 rules every 20s with no state check at all, and measured out to pegging netd at a sustained ~50% CPU, contributing to a phone overheating.

Fix: check whether the current state is already correct before touching iptables at all —

# Cheap check (2-3 forks): is the chain already right?
chain_intact() {
    "$1" -C OUTPUT -j "$2" >/dev/null 2>&1 || return 1
    [ "$("$1" -S "$2" 2>/dev/null | grep -c '^-A')" = "$3" ]
}

# Only pay for the expensive flush+rebuild (50-100+ forks) when it's wrong.
chain_intact iptables MY_CHAIN "$expected_count" || rebuild_my_chain

This preserves self-healing (a flush by the kernel or another process is still detected and repaired on the next tick) while skipping the ~95% of ticks where nothing changed. If diagnosing whether this pattern is already live in a running system, kill -STOP/kill -CONT the suspect script's PID and compare /proc/<pid>/stat utime+stime jiffy deltas of the netfilter daemon before/after — don't trust dumpsys cpuinfo (Android) or similar cached snapshot tools, some builds don't refresh them between calls.

Python-specific rules (for daemons, not hot-loop callees)

• Use pathlib.Path.read_text() / read_bytes() — one syscall, no subprocess.
• Open /sys / /proc files with the builtin open(); they're tiny reads.
• For event loops, use asyncio / selectors to block on fds (same idea as read in bash) instead of time.sleep() in a polling loop.
• Don't shell out with subprocess.run("sensors") when /sys/class/hwmon exists.
• Cache psutil objects across ticks — psutil.cpu_percent(interval=None) uses deltas and is O(1) after the first call.

Common red flags (search for these in review)

• while true / while : with a sleep and no event source
• $(…|…|…) chains with three or more pipes in a status-bar script
• | awk, | grep, | tr, | cut, | sed, | head, | tail where bash builtins would do
• $(cat foo) anywhere — always replaceable with $(<foo)
• echo … | bc — replaceable with bash integer math
• sensors, acpi, free, lspci, iwgetid in a per-second script
• python … / node … invoked per tick
• No set -u (silent typo bugs compound over thousands of ticks)
• iptables -F / ip6tables -F followed by unconditional -A rebuilds with no preceding "is this already correct?" check (see R9)

Verification checklist before shipping

1. shellcheck script.sh — clean.
2. strace -c -f script.sh 2>&1 | grep -E 'execve|clone' — fork count matches expectation.
3. time for _ in {1..10000}; do script.sh >/dev/null; done — under budget.
4. For persist scripts: run for 60 s under perf stat -p $PID — CPU time near zero when idle.
5. Running under the monitors.slice unit — verify with systemctl --user status monitors.slice.

Reference implementations in this repo

• linux_configuration/i3-configuration/i3blocks/volume.sh — persist mode with pactl subscribe.
• linux_configuration/i3-configuration/i3blocks/gpu_monitor.sh — persist mode with nvidia-smi --loop.
• linux_configuration/i3-configuration/i3blocks/battery_status.sh — zero-fork via /sys/class/power_supply.
• linux_configuration/i3-configuration/i3blocks/cpu_monitor.sh — zero-fork via /proc/loadavg + /sys/class/hwmon.
• linux_configuration/i3-configuration/i3blocks/motherboard_temp.sh — zero-fork via /sys/class/hwmon.
• linux_configuration/scripts/system-maintenance/systemd/monitors.slice — resource-cap slice.
• phone_focus_mode/dns_enforcer.sh::chain_intact()/expected_rule_count() — R9's hash/count-gate pattern, fixed 2026-07-03 after it was found pegging Android's netd at ~50% CPU.
• phone_focus_mode/hosts_enforcer.sh::sync_magisk_module — the original hash-before-write pattern R9 mirrors.