linux-cron-manager

name	linux-cron-manager
description	Use when user wants to schedule a task with cron, write a crontab entry, create a /etc/cron.d/ file, set up a cron job with locking and logging, convert a cron job to a systemd timer, debug a cron job that isn't running, or understand cron syntax and environment behavior.
version	1.5.0
author	Lehnert

Linux Cron Manager

Overview

Generates production-ready cron entries and /etc/cron.d/ files with proper environment setup, flock-based locking to prevent overlapping runs, structured logging, and failure alerting. Includes a full cron syntax reference and a conversion path to systemd timers for complex scheduling needs.

Language: Respond in the user's language. Cron files and scripts in English.

---

When to Use

• User wants to schedule any recurring task
• User wants a cron job that logs and alerts on failure
• User wants to prevent a cron job from running twice at once
• User wants to convert a cron job to a systemd timer
• User asks "my cron job isn't running" — debugging
• User wants to understand cron syntax or cron environment behavior

When NOT to Use

• User needs complex dependencies between jobs, socket activation, or resource limits → linux-systemd-manager
• User wants to write the script that cron will run → linux-shell-scriptor

---

Step 1 — Understand the Request

User says	Output
"run script X every Y"	/etc/cron.d/ entry + wrapper with locking + logging
"crontab entry for..."	Direct crontab line with explanation
"my cron job isn't running"	Debugging checklist
"convert cron to systemd timer"	Equivalent .service + .timer pair
"cron syntax for..."	Explanation + validated expression

---

Cron Syntax Reference

⚠️ Timezone: Cron uses the system timezone (from /etc/timezone). To override per-job, add TZ=America/New_York at the top of the /etc/cron.d/ file — this affects all entries in that file only.

┌──────────── minute       (0–59)
│ ┌────────── hour         (0–23)
│ │ ┌──────── day of month (1–31)
│ │ │ ┌────── month        (1–12 or JAN–DEC)
│ │ │ │ ┌──── day of week  (0–7, 0 and 7 = Sunday, or SUN–SAT)
│ │ │ │ │
* * * * *  user  command

Common expressions:

Schedule	Expression
Every minute	* * * * *
Every 5 minutes	*/5 * * * *
Every hour at :00	0 * * * *
Every hour at :30	30 * * * *
Daily at 2 AM	0 2 * * *
Daily at 2:30 AM	30 2 * * *
Every Monday at 3 AM	0 3 * * 1
Weekdays at 9 AM	0 9 * * 1-5
1st of every month	0 0 1 * *
Every 15 minutes	*/15 * * * *
Twice daily (6 AM + 6 PM)	0 6,18 * * *
Every Sunday at midnight	0 0 * * 0

Validate any expression at: https://crontab.guru

---

Production Cron Entry Template

Never write a bare cron entry. Always wrap with locking, logging, and error handling.

/etc/cron.d/ file format (preferred — survives user crontab edits)

/etc/cron.d/myapp-task:

# myapp-task — describe what this does
# Runs: daily at 2 AM
# Logs: /var/log/cron/myapp-task.log
# Owner: appuser

SHELL=/bin/bash
PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin
MAILTO=""
HOME=/home/appuser   # cron doesn't source profile — set HOME explicitly to avoid "~" surprises

0 2 * * *  appuser  /usr/bin/flock -n /var/lock/myapp-task.lock /opt/myapp/tasks/run-task.sh

Wrapper script (/opt/myapp/tasks/run-task.sh):

Always generate the wrapper script alongside the cron entry:

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

# ── Config ──────────────────────────────────────────────────
SCRIPT_NAME="myapp-task"
LOG_DIR="/var/log/cron"
LOG_FILE="${LOG_DIR}/${SCRIPT_NAME}.log"
LOCK_FILE="/var/lock/${SCRIPT_NAME}.lock"
MAX_LOG_SIZE_MB=50

# ── Logging ──────────────────────────────────────────────────
mkdir -p "$LOG_DIR"
log()   { echo "[$(date '+%Y-%m-%d %H:%M:%S')] [INFO]  $*" | tee -a "$LOG_FILE"; }
warn()  { echo "[$(date '+%Y-%m-%d %H:%M:%S')] [WARN]  $*" | tee -a "$LOG_FILE" >&2; }
error() { echo "[$(date '+%Y-%m-%d %H:%M:%S')] [ERROR] $*" | tee -a "$LOG_FILE" >&2; }

# ── Log rotation ─────────────────────────────────────────────
if [[ -f "$LOG_FILE" ]]; then
  LOG_SIZE_MB=$(du -m "$LOG_FILE" | cut -f1)
  if [[ "$LOG_SIZE_MB" -ge "$MAX_LOG_SIZE_MB" ]]; then
    ARCHIVED_LOG="${LOG_FILE}.$(date +%Y%m%d_%H%M%S)"
    mv "$LOG_FILE" "$ARCHIVED_LOG"
    gzip "$ARCHIVED_LOG"
  fi
fi

# ── Lock — prevent concurrent runs ───────────────────────────
exec 9>"$LOCK_FILE"
if ! flock -n 9; then
  warn "Another instance is already running. Exiting."
  exit 0
fi

# ── Trap — cleanup on exit ───────────────────────────────────
cleanup() {
  local exit_code=$?
  if [[ $exit_code -ne 0 ]]; then
    error "Task failed with exit code $exit_code"
    # Optional: send alert
    # echo "Cron job $SCRIPT_NAME failed" | mail -s "CRON FAILURE: $SCRIPT_NAME" [e-mail]
  fi
  flock -u 9
}
trap cleanup EXIT

# ── Main ─────────────────────────────────────────────────────
log "=== Starting $SCRIPT_NAME ==="

# --- YOUR TASK CODE HERE ---
# Example:
# log "Running backup..."
# /opt/backup/backup.sh
# log "Backup complete"
# ---------------------------

log "=== $SCRIPT_NAME complete ==="

---

Email / Alert on Failure

Add to the cron file to capture output and email on non-zero exit:

# Option 1: Send all output to email (requires working MTA)
MAILTO="[e-mail]"
0 2 * * *  appuser  /opt/myapp/tasks/run-task.sh

# Option 2: Only email on failure using a wrapper
0 2 * * *  appuser  /opt/myapp/tasks/run-task.sh || echo "myapp-task failed" | mail -s "CRON FAILURE" [e-mail]

# Option 3: Log to file and send to a webhook (Slack/Discord) on failure
0 2 * * *  appuser  /opt/myapp/tasks/run-task.sh 2>&1 | tee -a /var/log/cron/myapp.log | \
  grep -i "error\|fail" | while read line; do \
  curl -s -X POST -H 'Content-type: application/json' \
  --data "{\"text\":\"Cron failure: $line\"}" "${SLACK_WEBHOOK_URL}"; done

---

Environment Rules

Cron runs in a minimal environment — this is the #1 cause of cron jobs that "work manually but not in cron."

Always include in /etc/cron.d/ files:

SHELL=/bin/bash
PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin

Common environment pitfalls:

Problem	Cause	Fix
Command not found	PATH is minimal in cron	Use full path: /usr/bin/python3
Script works manually, not in cron	Missing env vars	Add source /etc/environment or export each var at top of wrapper script
App-specific env vars missing	.env file not loaded	Add set -a; source /opt/myapp/.env; set +a at top of wrapper script
Python/Node virtualenv not active	No shell init	Source virtualenv explicitly: source /opt/venv/bin/activate
HOME not set	Cron doesn't source profile	Add HOME=/home/appuser to cron file
No display/dbus	X11/dbus not available	Not suitable for GUI apps; use headless mode

---

Debugging Checklist

When a cron job isn't running or is failing silently:

# 1. Verify cron daemon is running
systemctl status cron    # Debian/Ubuntu
systemctl status crond   # RHEL/Rocky

# 2. Check cron logs
grep CRON /var/log/syslog | tail -30          # Debian/Ubuntu
journalctl -u cron --since "1 hour ago"       # systemd systems
grep cron /var/log/messages | tail -30        # RHEL/Rocky

# 3. Check the cron entry is valid
crontab -l -u appuser                         # list user crontab
ls -la /etc/cron.d/                           # list system cron jobs

# 4. Test the command manually as the cron user
sudo -u appuser /path/to/script.sh

# 5. Test with same minimal environment as cron
sudo -u appuser env -i HOME=/home/appuser \
  PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin \
  /path/to/script.sh

# 6. Verify file permissions
ls -la /path/to/script.sh        # must be executable
stat /etc/cron.d/myfile          # must be owned by root, mode 644

# 7. Check for syntax errors in /etc/cron.d/ files
# File must NOT have a .sh extension — cron ignores them
# File must NOT be world-writable

The #1 reason cron jobs "silently do nothing": output (stdout + stderr) is discarded unless captured. Always write logs inside the wrapper script using the log() function above, or add >> /var/log/cron/myapp.log 2>&1 to the cron entry.

Common reasons cron jobs silently do nothing:

Issue	Check
Script not executable	chmod +x script.sh
/etc/cron.d/ file has .sh extension	Rename to no extension
/etc/cron.d/ file is world-writable	chmod 644
Wrong user in cron entry	Verify user exists: id appuser
Script path wrong	Use absolute paths only in cron
Cron daemon not reloaded	systemctl reload cron after adding new file
Lock file stuck from previous crash	Delete /var/lock/scriptname.lock

---

Cron → Systemd Timer Conversion

When to switch to systemd timers instead of cron:

Use cron when	Use systemd timer when	Use Anacron when
Simple schedule, no dependencies	Need After= dependencies	System isn't always on (laptop, desktop)
Script already works standalone	Need resource limits (MemoryMax, CPUQuota)	Daily/weekly tasks that must run even after downtime
Quick setup needed	Need detailed logging via journalctl	No systemd available and Persistent=true is needed
No systemd available (Alpine, containers)	Need Persistent=true for missed runs	—

Anacron (/etc/anacrontab) — runs jobs that were missed while the system was off:

# period  delay  job-id         command
1         5      daily-backup   /opt/backup/backup.sh
7         10     weekly-report  /opt/reports/weekly.sh

Install: apt install anacron / dnf install cronie-anacron

Conversion example:

Cron entry:

0 2 * * *  appuser  /opt/myapp/backup.sh

Equivalent systemd timer — see /linux-systemd-manager for full output.

---

Output Format

Write files to ./cron/ in the current working directory:

cron/
  myapp-task          ← drop into /etc/cron.d/ (no extension!)
  run-task.sh         ← wrapper script with locking + logging

Then print ONLY:

✅ Cron job written to ./cron/

▶ Install:
  # ⚠️  /etc/cron.d files MUST be owned by root with mode 644.
  #     Cron silently ignores files with wrong ownership or permissions.
  sudo cp cron/myapp-task /etc/cron.d/myapp-task
  sudo chmod 644 /etc/cron.d/myapp-task
  sudo chown root:root /etc/cron.d/myapp-task
  sudo cp cron/run-task.sh /opt/myapp/tasks/run-task.sh
  sudo chmod 755 /opt/myapp/tasks/run-task.sh
  sudo chown appuser:appuser /opt/myapp/tasks/run-task.sh
  sudo systemctl reload cron          # tell cron to pick up the new /etc/cron.d/ file

▶ Verify cron picked it up:
  grep myapp-task /var/log/syslog
  # or: journalctl -u cron | grep myapp-task

▶ Test manually as cron user:
  sudo -u appuser /opt/myapp/tasks/run-task.sh

💡 For complex scheduling with dependencies or resource limits → /linux-systemd-manager