Cron & Scheduling

Cron & Scheduling

Schedule and manage recurring tasks. Covers cron syntax, crontab management, systemd timers, one-off scheduling, timezone handling, monitoring, and common failure patterns.

When to Use

• -Running scripts on a schedule (backups, reports, cleanup)
• -Setting up systemd timers (modern cron alternative)
• -Debugging why a scheduled job didn't run
• -Handling timezones in scheduled tasks
• -Monitoring and alerting on job failures
• -Running one-off delayed commands

Cron Syntax

The five fields

┌───────── 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)
│ │ │ │ │
* * * * * command

Common schedules

Every minute
* * * * * /path/to/script.sh

Every 5 minutes
*/5 * * * * /path/to/script.sh

Every hour at :00
0 * * * * /path/to/script.sh

Every day at 2:30 AM
30 2 * * * /path/to/script.sh

Every Monday at 9:00 AM
0 9 * * 1 /path/to/script.sh

Every weekday at 8:00 AM
0 8 * * 1-5 /path/to/script.sh

First day of every month at midnight
0 0 1 * * /path/to/script.sh

Every 15 minutes during business hours (Mon-Fri 9-17)
*/15 9-17 * * 1-5 /path/to/script.sh

Twice a day (9 AM and 5 PM)
0 9,17 * * * /path/to/script.sh

Every quarter (Jan, Apr, Jul, Oct) on the 1st at midnight
0 0 1 1,4,7,10 * /path/to/script.sh

Every Sunday at 3 AM
0 3 * * 0 /path/to/script.sh

Special strings (shorthand)

@reboot    /path/to/script.sh   # Run once at startup
@yearly    /path/to/script.sh   # 0 0 1 1 *
@monthly   /path/to/script.sh   # 0 0 1 * *
@weekly    /path/to/script.sh   # 0 0 * * 0
@daily     /path/to/script.sh   # 0 0 * * *
@hourly    /path/to/script.sh   # 0 * * * *

Crontab Management

Edit current user's crontab
crontab -e

List current crontab
crontab -l

Edit another user's crontab (root)
sudo crontab -u www-data -e

Remove all cron jobs (be careful!)
crontab -r

Install crontab from file
crontab mycrontab.txt

Backup crontab
crontab -l > crontab-backup-$(date +%Y%m%d).txt

Crontab best practices

Set PATH explicitly (cron has minimal PATH)
PATH=/usr/local/bin:/usr/bin:/bin

Set MAILTO for error notifications
MAILTO=admin@example.com

Set shell explicitly
SHELL=/bin/bash

Full crontab example
PATH=/usr/local/bin:/usr/bin:/bin
MAILTO=admin@example.com
SHELL=/bin/bash

Backups
0 2 * * * /opt/scripts/backup.sh >> /var/log/backup.log 2>&1

Cleanup old logs
0 3 * * 0 find /var/log/myapp -name "*.log" -mtime +30 -delete

Health check
*/5 * * * * /opt/scripts/healthcheck.sh || /opt/scripts/alert.sh "Health check failed"

Systemd Timers

Create a timer (modern cron replacement)

/etc/systemd/system/backup.service
[Unit]
Description=Daily backup

[Service]
Type=oneshot
ExecStart=/opt/scripts/backup.sh
User=backup
StandardOutput=journal
StandardError=journal

/etc/systemd/system/backup.timer
[Unit]
Description=Run backup daily at 2 AM

[Timer]
OnCalendar=*-*-* 02:00:00
Persistent=true
RandomizedDelaySec=300

[Install]
WantedBy=timers.target

Enable and start the timer
sudo systemctl daemon-reload
sudo systemctl enable --now backup.timer

Check timer status
systemctl list-timers
systemctl list-timers --all

Check last run
systemctl status backup.service
journalctl -u backup.service --since today

Run manually (for testing)
sudo systemctl start backup.service

Disable timer
sudo systemctl disable --now backup.timer

OnCalendar syntax

Systemd calendar expressions

Daily at midnight
OnCalendar=daily
or: OnCalendar=*-*-* 00:00:00

Every Monday at 9 AM
OnCalendar=Mon *-*-* 09:00:00

Every 15 minutes
OnCalendar=*:0/15

Weekdays at 8 AM
OnCalendar=Mon..Fri *-*-* 08:00:00

First of every month
OnCalendar=*-*-01 00:00:00

Every 6 hours
OnCalendar=0/6:00:00

Specific dates
OnCalendar=2026-02-03 12:00:00

Test calendar expressions
systemd-analyze calendar "Mon *-*-* 09:00:00"
systemd-analyze calendar "*:0/15"
systemd-analyze calendar --iterations=5 "Mon..Fri *-*-* 08:00:00"

Advantages over cron

Systemd timers vs cron:
+ Logs in journald (journalctl -u service-name)
+ Persistent: catches up on missed runs after reboot
+ RandomizedDelaySec: prevents thundering herd
+ Dependencies: can depend on network, mounts, etc.
+ Resource limits: CPUQuota, MemoryMax, etc.
+ No lost-email problem (MAILTO often misconfigured)
-More files to create (service + timer)
-More verbose configuration

One-Off Scheduling

at (run once at a specific time)

Schedule a command
echo "/opt/scripts/deploy.sh" | at 2:00 AM tomorrow
echo "reboot" | at now + 30 minutes
echo "/opt/scripts/report.sh" | at 5:00 PM Friday

Interactive (type commands, Ctrl+D to finish)
at 10:00 AM
> /opt/scripts/task.sh
> echo "Done" | mail -s "Task complete" admin@example.com
> <Ctrl+D>

List pending jobs
atq

View job details
at -c <job-number>

Remove a job
atrm <job-number>

sleep-based (simplest)

Run something after a delay
(sleep 3600 && /opt/scripts/task.sh) &

With nohup (survives logout)
nohup bash -c "sleep 7200 && /opt/scripts/task.sh" &

Timezone Handling

Cron runs in the system timezone by default
Check system timezone
timedatectl
date +%Z

Set timezone for a specific cron job
Method 1: TZ variable in crontab
TZ=America/New_York
0 9 * * * /opt/scripts/report.sh

Method 2: In the script itself
#!/bin/bash
export TZ=UTC
All date operations now use UTC

Method 3: Wrapper
TZ=Europe/London date '+%Y-%m-%d %H:%M:%S'

List available timezones
timedatectl list-timezones
timedatectl list-timezones | grep America

DST pitfalls

Problem: A job scheduled for 2:30 AM may run twice or not at all
during DST transitions.

"Spring forward": 2:30 AM doesn't exist (clock jumps 2:00 → 3:00)
"Fall back": 2:30 AM happens twice

Mitigation:
1. Schedule critical jobs outside 1:00-3:00 AM
2. Use UTC for the schedule: TZ=UTC in crontab
3. Make jobs idempotent (safe to run twice)
4. Systemd timers handle DST correctly

Monitoring and Debugging

Why didn't my cron job run?

1. Check cron daemon is running
systemctl status cron    # Debian/Ubuntu
systemctl status crond   # CentOS/RHEL

2. Check cron logs
grep CRON /var/log/syslog           # Debian/Ubuntu
grep CRON /var/log/cron             # CentOS/RHEL
journalctl -u cron --since today    # systemd

3. Check crontab actually exists
crontab -l

4. Test the command manually (with cron's environment)
env -i HOME=$HOME SHELL=/bin/sh PATH=/usr/bin:/bin /opt/scripts/backup.sh
If it fails here but works normally → PATH or env issue

5. Check permissions
ls -la /opt/scripts/backup.sh   # Must be executable
ls -la /var/spool/cron/         # Crontab file permissions

6. Check for syntax errors in crontab
cron silently ignores lines with errors

7. Check if output is being discarded
By default, cron emails output. If no MTA, output is lost.
Always redirect: >> /var/log/myjob.log 2>&1

Job wrapper with logging and alerting

#!/bin/bash
cron-wrapper.sh — Run a command with logging, timing, and error alerting
Usage: cron-wrapper.sh <job-name> <command> [args...]

set -euo pipefail

JOB_NAME="${1:?Usage: cron-wrapper.sh <job-name> <command> [args...]}"
shift
COMMAND=("$@")

LOG_DIR="/var/log/cron-jobs"
mkdir -p "$LOG_DIR"
LOG_FILE="$LOG_DIR/$JOB_NAME.log"

log() { echo "[$(date -u '+%Y-%m-%dT%H:%M:%SZ')] $*" >> "$LOG_FILE"; }

log "START: ${COMMAND[*]}"
START_TIME=$(date +%s)

if "${COMMAND[@]}" >> "$LOG_FILE" 2>&1; then
    ELAPSED=$(( $(date +%s) - START_TIME ))
    log "SUCCESS (${ELAPSED}s)"
else
    EXIT_CODE=$?
    ELAPSED=$(( $(date +%s) - START_TIME ))
    log "FAILED with exit code $EXIT_CODE (${ELAPSED}s)"
    # Alert (customize as needed)
    echo "Cron job '$JOB_NAME' failed with exit $EXIT_CODE" | \
        mail -s "CRON FAIL: $JOB_NAME" admin@example.com 2>/dev/null || true
    exit $EXIT_CODE
fi

Use in crontab:
0 2 * * * /opt/scripts/cron-wrapper.sh daily-backup /opt/scripts/backup.sh
*/5 * * * * /opt/scripts/cron-wrapper.sh health-check /opt/scripts/healthcheck.sh

Lock to prevent overlap

Prevent concurrent runs (job takes longer than interval)
Method 1: flock
* * * * * flock -n /tmp/myjob.lock /opt/scripts/slow-job.sh

Method 2: In the script
LOCKFILE="/tmp/myjob.lock"
exec 200>"$LOCKFILE"
flock -n 200 || { echo "Already running"; exit 0; }
... do work ...

Idempotent Job Patterns

Idempotent backup (only creates if newer than last backup)
#!/bin/bash
BACKUP_DIR="/backups/$(date +%Y%m%d)"
[[ -d "$BACKUP_DIR" ]] && { echo "Backup already exists"; exit 0; }
mkdir -p "$BACKUP_DIR"
pg_dump mydb > "$BACKUP_DIR/mydb.sql"

Idempotent cleanup (safe to run multiple times)
find /tmp/uploads -mtime +7 -type f -delete 2>/dev/null || true

Idempotent sync (rsync only transfers changes)
rsync -az /data/ backup-server:/backups/data/

Tips

• -Always redirect output in cron jobs: >> /var/log/job.log 2>&1. Without this, output goes to mail (if configured) or is silently lost.
• -Test cron jobs by running them with env -i to simulate cron's minimal environment. Most failures are caused by missing PATH or environment variables.
• -Use flock to prevent overlapping runs when a job might take longer than its schedule interval.
• -Make all scheduled jobs idempotent. If a job runs twice (DST, manual trigger, crash recovery), it should produce the same result.
• -systemd-analyze calendar is invaluable for verifying timer schedules before deploying.
• -Never schedule critical jobs between 1:00 AM and 3:00 AM if DST applies. Use UTC schedules instead.
• -Log the start time, end time, and exit code of every cron job. Without this, debugging failures after the fact is guesswork.
• -Prefer systemd timers over cron for production services: you get journald logging, missed-run catchup (Persistent=true), and resource limits for free.