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Monitor bash scripts: trap EXIT to ping on success or failure

Define a _finish function that checks $? and either curls the success URL or POSTs to /fail, then add trap '_finish' EXIT. Works with set -e: any failing command exits the script, the trap fires, and the exit-code check sends the right ping. Absence detection catches SIGKILL and scripts that never start. Free, no monitor cap.

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On this page
  1. The snippets
  2. What this catches
  3. Questions people ask

The snippets

Create a monitor in the console, set the schedule plus a grace window, copy the ping URL, and paste the trap header at the top of your script.

Minimal trap: exit code only

Works with any bash script. set -euo pipefail makes every non-zero command exit immediately so the trap always fires on the first error: 

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

PING="${LASTPING_URL:-https://ping.lastping.dev/<your-monitor-id>}"

_finish() {
  local ec=$?
  if [ "$ec" -eq 0 ]; then
    curl -fsS --retry 3 --max-time 10 "$PING" >/dev/null
  else
    curl -fsS --retry 3 --max-time 10 \
      -X POST \
      --data "script exited with code $ec" \
      "$PING/fail" >/dev/null || true
  fi
}
trap '_finish' EXIT

# ── your job starts here ──────────────────────────────────────
echo "Starting backup..."
rsync -a /data/ /backup/
echo "Done."

Capture stderr for the fail body

Without set -e, redirect stderr to a temp file. If the script exits non-zero, POST the contents of that file to /fail so the alert body contains the actual error output, not just an exit code: 

#!/usr/bin/env bash
set -uo pipefail   # -e omitted so we control the exit

PING="${LASTPING_URL:-https://ping.lastping.dev/<your-monitor-id>}"
ERR_LOG=$(mktemp)

_finish() {
  local ec=$?
  if [ "$ec" -eq 0 ]; then
    curl -fsS --retry 3 --max-time 10 "$PING" >/dev/null
  else
    # POST stderr (up to 10,000 bytes) to /fail
    curl -fsS --retry 3 --max-time 10 \
      -X POST \
      --data-binary "@${ERR_LOG}" \
      "$PING/fail" >/dev/null || true
  fi
  rm -f "$ERR_LOG"
}
trap '_finish' EXIT

# capture stderr for the fail body
exec 2>"$ERR_LOG"

# ── your job ──────────────────────────────────────────────────
some_command_that_might_fail || exit 1

Optional: /start ping for overrun detection

A /start ping arms the hung-job alarm: if no success ping arrives inside the grace window an incident opens even if the script just hangs rather than crashing: 

PING="${LASTPING_URL:-https://ping.lastping.dev/<your-monitor-id>}"
RID="$$-$(date +%s)"   # PID + unix timestamp

curl -fsS --retry 3 --max-time 10 "$PING/start?rid=$RID" >/dev/null

_finish() {
  local ec=$?
  if [ "$ec" -eq 0 ]; then
    curl -fsS --retry 3 --max-time 10 "$PING?rid=$RID" >/dev/null
  else
    curl -fsS --retry 3 --max-time 10 \
      -X POST --data "exited $ec" \
      "$PING/fail" >/dev/null || true
  fi
}
trap '_finish' EXIT

Why SIGKILL can't be trapped

trap catches EXIT, ERR, TERM, and INT, but not SIGKILL. An OOM-killed process sends no ping. That is exactly what absence-detection handles: the missing ping itself opens the incident after the grace window. See the cron monitoring guide for the scheduler side and the Python guide for a language-level comparison. Also available: LastPing MCP server for AI-agent provisioning.

What this catches

The trap reports what it can; the silence reports the rest.

  • A failing command (set -e): bash exits immediately on any non-zero command, the trap fires, and the exit code goes to /fail.
  • A non-zero explicit exit: exit 1 triggers the EXIT trap; the body can include the captured stderr output.
  • SIGTERM: the EXIT trap fires on graceful shutdown signals, giving you a chance to report before the process ends.
  • SIGKILL and OOM kills: untrapable; handled by absence detection: no ping reaches LastPing within the schedule+grace window → incident.
  • The script never ran: missing cron entry, missing executable, dead host. Absence → incident.

Questions people ask

Front-loaded answers: the most important fact first.

  • How do I monitor a bash cron script?

    Set PING to your LastPing URL, define a _finish function that checks $? and either curls the success URL or POSTs to /fail, then add trap '_finish' EXIT before your job code. set -e causes bash to exit on any non-zero command, so the trap fires on the first failure.

  • Does trap EXIT work with set -e?

    Yes. With set -e any failing command exits the script immediately; bash still runs the EXIT trap when the script exits, regardless of the cause. The $? inside the trap reflects the exit code that triggered the exit, and the trap handler checks it to determine whether to send the success or failure ping.

  • Can I send the actual error message, not just the exit code?

    Yes. Without set -e (drop -e from the shebang line so you control exits manually), redirect stderr to a temp file with exec 2>$ERR_LOG before your job code. In the _finish trap, if the exit code is non-zero, POST the contents of that file to the /fail endpoint using curl --data-binary @$ERR_LOG. Up to 10,000 bytes are stored with the ping event.

  • What if the script is killed with SIGKILL?

    SIGKILL cannot be caught or trapped: bash does not run EXIT handlers when killed with SIGKILL. Absence-detection monitoring handles this: the missing ping itself opens an incident after the grace window, without any trap code running.

  • Is bash cron monitoring free?

    Yes. LastPing is free for individuals with no monitor cap. See how it compares to Healthchecks.io and Cronitor.

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SIGKILL can't be trapped: absence detection is the safety net.

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