Cron Jobs Done Right: Scheduling That Doesn't Break

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

# Every hour at minute 0
0 * * * * /path/to/script.sh

# Every day at midnight
0 0 * * * /path/to/script.sh

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

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

# Every 15 minutes
*/15 * * * * /path/to/script.sh

# Every weekday at 6 PM
0 18 * * 1-5 /path/to/script.sh

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

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@reboot     # Run once at startup
@yearly     # 0 0 1 1 *
@annually   # Same as @yearly
@monthly    # 0 0 1 * *
@weekly     # 0 0 * * 0
@daily      # 0 0 * * *
@midnight   # Same as @daily
@hourly     # 0 * * * *

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# Edit current user's crontab
crontab -e

# List current user's crontab
crontab -l

# Edit another user's crontab (as root)
crontab -u username -e

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

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# System-wide crontab (includes user field)
/etc/crontab

# Drop-in directory (no user field needed)
/etc/cron.d/

# Periodic directories (scripts run by run-parts)
/etc/cron.hourly/
/etc/cron.daily/
/etc/cron.weekly/
/etc/cron.monthly/

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0 * * * * root /path/to/script.sh

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# Bad - relies on user PATH
* * * * * backup.sh

# Good - explicit path
* * * * * /usr/local/bin/backup.sh

# Or set PATH in crontab
PATH=/usr/local/bin:/usr/bin:/bin
* * * * * backup.sh

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#!/bin/bash
export PATH=/usr/local/bin:/usr/bin:/bin
export HOME=/home/myuser

# Now your script has the expected environment

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# Load user's environment
* * * * * . ~/.profile; /path/to/script.sh

# Or bash login shell
* * * * * bash -l -c '/path/to/script.sh'

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# Redirect stdout and stderr to file
* * * * * /path/to/script.sh >> /var/log/myjob.log 2>&1

# With timestamp
* * * * * /path/to/script.sh >> /var/log/myjob.log 2>&1; echo "---$(date)---" >> /var/log/myjob.log

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# Send to specific email
MAILTO=alerts@example.com
* * * * * /path/to/script.sh

# Disable email
MAILTO=""
* * * * * /path/to/script.sh

# Only email on error
* * * * * /path/to/script.sh > /dev/null

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# Run exactly as cron would
env -i /bin/sh -c '/path/to/script.sh'

# Check cron logs
grep CRON /var/log/syslog
journalctl -u cron

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* * * * * flock -n /tmp/myjob.lock /path/to/script.sh

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#!/bin/bash
LOCKFILE=/tmp/myjob.lock

if ! mkdir "$LOCKFILE" 2>/dev/null; then
    echo "Already running"
    exit 1
fi

trap "rm -rf $LOCKFILE" EXIT

# Your job here

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#!/bin/bash
set -e  # Exit on any error

# Commands here

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#!/bin/bash
set -e

cleanup() {
    if [ $? -ne 0 ]; then
        curl -X POST https://hooks.slack.com/... \
            -d '{"text":"Cron job failed!"}'
    fi
}
trap cleanup EXIT

# Your job here

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#!/bin/bash
# /usr/local/bin/cron-wrapper

JOB_NAME="$1"
shift

if ! "$@"; then
    echo "[$JOB_NAME] Failed at $(date)" | mail -s "Cron Failure: $JOB_NAME" alerts@example.com
    exit 1
fi

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* * * * * /usr/local/bin/cron-wrapper backup /path/to/backup.sh

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# Set timezone in crontab
TZ=America/New_York
0 9 * * * /path/to/script.sh  # 9 AM Eastern

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# Run at 14:00 UTC (9 AM Eastern during EST)
0 14 * * * /path/to/script.sh

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# Random delay up to 5 minutes
* * * * * sleep $((RANDOM \% 300)) && /path/to/script.sh

# Or use systemd timer's RandomizedDelaySec

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# Bad
* * * * * cd mydir && ./script.sh

# Good
* * * * * cd /home/user/mydir && ./script.sh

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#!/bin/bash
exec >> /var/log/myjob.log 2>&1
echo "=== Starting at $(date) ==="

# Your job here

echo "=== Finished at $(date) ==="

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# Kill job if it runs longer than 1 hour
* * * * * timeout 3600 /path/to/script.sh

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# Create service account
sudo useradd -r -s /usr/sbin/nologin cronjobs

# Set up crontab for that user
sudo crontab -u cronjobs -e

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#!/bin/bash
START=$(date +%s)

# Your job here
result=$?

END=$(date +%s)
DURATION=$((END - START))

# Send metrics
curl -X POST "https://metrics.example.com/cron" \
    -d "job=backup&duration=$DURATION&status=$result"

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# /etc/systemd/system/backup.timer
[Unit]
Description=Daily Backup Timer

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

[Install]
WantedBy=timers.target

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# /etc/systemd/system/backup.service
[Unit]
Description=Backup Job

[Service]
Type=oneshot
ExecStart=/path/to/backup.sh
User=backup
Nice=10
IOSchedulingClass=idle

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sudo systemctl enable backup.timer
sudo systemctl start backup.timer
systemctl list-timers