Background Jobs, wait and Signals

Doing several things at once

Ending a command with & runs it in the background; the script continues immediately and $! holds the background process ID. wait pauses until background jobs finish: wait $pid waits for one and returns its exit status, plain wait waits for all, and wait -n (Bash 4.3+) returns when any one finishes, which allows simple concurrency limits. Always check statuses: a background job's failure does not stop the script under set -e unless you wait for it. Signals are how processes are told to stop or reload: SIGTERM (15, polite stop, the default for kill), SIGINT (2, Ctrl+C), SIGHUP (1, often "reload configuration"), and SIGKILL (9, cannot be caught, use as a last resort). Long-running scripts should handle TERM and INT with traps so they stop child processes cleanly. Use timeout 30s cmd to bound how long a command may run.

Fork, run in parallel, wait

Background jobs run alongside the script, which later waits and collects their exit statuses.

Parallel health checks with a concurrency limit

At most 4 checks run at once, and every failure is counted.

#!/usr/bin/env bash
set -uo pipefail

check() { timeout 10s curl -fsS "https://$1/health" > /dev/null; }

max_jobs=4 failures=0
mapfile -t hosts < hosts.txt

for host in "${hosts[@]}"; do
    check "$host" &
    while (( $(jobs -rp | wc -l) >= max_jobs )); do
        wait -n || (( failures++ )) || true
    done
done
while (( $(jobs -rp | wc -l) > 0 )); do
    wait -n || (( failures++ )) || true
done

echo "$failures of ${#hosts[@]} hosts failed" >&2
(( failures == 0 ))

Background failures are silent unless you wait

A command started with & can fail without anyone noticing. Collect PIDs, wait for each and check the status, or the script will report success while half the work failed.

त्वरित जाँच: Which signal cannot be caught or ignored by a process?

• SIGKILL
• SIGTERM
• SIGINT
• SIGHUP