Cron Job Wrapper Shell Script

Long-running backup or import jobs started twice can corrupt data or exhaust disk. This cron wrapper uses flock so only one instance runs, while standardizing logs across every job in your crontab.

What this script does

Takes a job name and command as arguments
Uses non-blocking flock — second run exits cleanly if busy
Writes START/DONE lines with exit code to a per-job log
Works with any underlying script or pipeline
Keeps lock files under /var/lock

Prerequisites

util-linux provides flock
Cron entry calls the wrapper, not the raw script
Consistent job names (alphanumeric)

Step 1: Save the script

sudo nano /usr/local/bin/cron-wrapper.sh
sudo chmod +x /usr/local/bin/cron-wrapper.sh

Step 2: Full script (scroll to read)

cron-wrapper.sh

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

JOB="${1:?usage: cron-wrapper.sh JOB_NAME command [args...]}"
shift
CMD=("$@")
LOCK="/var/lock/${JOB}.lock"
LOG="/var/log/cron-${JOB}.log"

log(){ echo "[$(date '+%F %T')] $*" | tee -a "$LOG"; }

exec 9>"$LOCK"
if ! flock -n 9; then
  log "SKIP ${JOB} already running"
  exit 0
fi

log "LOCK acquired: $LOCK"
log "START ${JOB}: ${CMD[*]}"
set +e
"${CMD[@]}"
ec=$?
set -e
log "DONE ${JOB} exit=$ec"
log "LOCK released"
exit "$ec"

Scroll inside the box to read the full script.

Step 3: Configure settings

Example: cron-wrapper.sh nightly-backup /usr/local/bin/mysql-backup.sh
If flock fails, exit 0 so cron does not spam mail — tune if you want alerts on overlap
Complements crontab basics

Cron job wrapper shell script acquiring flock lock

Step 4: Test manually

sudo cron-wrapper.sh test-job sleep 30 &

sudo cron-wrapper.sh test-job sleep 5; echo second_exit=$?

Schedule with cron

sudo crontab -e

Add:

0 2 * * * /usr/local/bin/cron-wrapper.sh nightly-backup /usr/local/bin/mysql-backup.sh

Cron Job Wrapper Shell Script: flock, Logging and Duplicate-Run Prevention