SRJahirTech

Linux Full Tutorial -- Part 8: Cron Jobs & Task Scheduling

By Suraj Ahir 2025-12-13 11 min read

Every production Linux server needs automated maintenance. Backups must run nightly. Log files must rotate weekly. Disk usage must be checked hourly. Expired sessions must be cleaned daily. Doing these manually is unreliable and exhausting. Cron is the built-in Linux job scheduler that handles all of this automatically.

Cron Syntax

# Format: minute hour day month weekday command
# *  *  *  *  *  command
# |  |  |  |  |
# |  |  |  |  +-- Day of week (0-7, 0 and 7 = Sunday)
# |  |  |  +----- Month (1-12)
# |  |  +-------- Day of month (1-31)
# |  +----------- Hour (0-23)
# +-------------- Minute (0-59)

# Examples:
# Every minute
* * * * * command

# Every hour at minute 0
0 * * * * command

# Every day at 2:30am
30 2 * * * command

# Every Monday at 9am
0 9 * * 1 command

# Every 5 minutes
*/5 * * * * command

# 1st of every month at midnight
0 0 1 * * command

Managing Crontabs

crontab -e          # Edit your crontab (uses EDITOR variable)
crontab -l          # List your crontab
crontab -r          # Remove your crontab (CAREFUL!)
crontab -u ubuntu   # View/edit another user's crontab (as root)

# System-wide cron directories
ls /etc/cron.d/     # Per-application cron files
ls /etc/cron.daily/ # Scripts run daily by system
ls /etc/cron.hourly/ ls /etc/cron.weekly/ /etc/cron.monthly/

Real Cron Job Examples

# Edit crontab with: crontab -e

# Daily database backup at 1am
0 1 * * * /home/ubuntu/scripts/backup-db.sh >> /var/log/backup.log 2>&1

# Check disk every 15 minutes, alert if over 85%
*/15 * * * * /usr/local/bin/check-disk.sh

# Rotate logs every Sunday at 3am
0 3 * * 0 /usr/sbin/logrotate /etc/logrotate.conf

# Restart application every night at 4am (memory leak workaround)
0 4 * * * systemctl restart myapp >> /var/log/restart.log 2>&1

# Clean temp files older than 7 days, run daily at 2am
0 2 * * * find /tmp -type f -mtime +7 -delete

# Sync S3 backup every 6 hours
0 */6 * * * aws s3 sync /data/backups s3://my-backups/daily/

Output and Logging

# By default, cron emails output to the user
# Silence output entirely:
* * * * * command > /dev/null 2>&1

# Log stdout and stderr to a file:
* * * * * command >> /var/log/mycron.log 2>&1

# Log with timestamps:
0 2 * * * echo "$(date) Starting backup" >> /var/log/backup.log && /scripts/backup.sh >> /var/log/backup.log 2>&1

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

systemd Timers -- Modern Alternative

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

[Service]
Type=oneshot
User=ubuntu
ExecStart=/home/ubuntu/scripts/backup-db.sh

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

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

[Install]
WantedBy=timers.target

# Enable and check:
systemctl daemon-reload
systemctl enable --now backup.timer
systemctl list-timers
journalctl -u backup.service -f

The at Command

echo "/scripts/maintenance.sh" | at 2:30am
echo "/scripts/deploy.sh" | at now + 10 minutes
echo "/scripts/task.sh" | at 14:00 tomorrow

atq          # List pending at jobs
atrm 3       # Remove job number 3

Frequently Asked Questions

In Part 9, we cover package management -- installing, updating, and managing software on Linux servers.

Written by

Suraj Ahir

Cloud & DevOps engineer running four live production services on my own AWS infrastructure. I write everything on this site myself — no ghostwriters, no AI filler.

More about me → GitHub LinkedIn

Advanced Cron Patterns for Production

#!/bin/bash
# Production-grade cron script with locking and alerting

LOCKFILE="/tmp/daily-cleanup.lock"
LOG="/var/log/daily-cleanup.log"
SLACK_WEBHOOK="${SLACK_WEBHOOK_URL}"

log() { echo "[$(date +%Y-%m-%dT%H:%M:%S)] $1" | tee -a $LOG; }

notify_slack() {
    [ -n "$SLACK_WEBHOOK" ] && curl -s -X POST         -H "Content-type: application/json"         -d "{"text": "$1"}"         "$SLACK_WEBHOOK" > /dev/null
}

# Prevent concurrent runs with flock
exec 9>"$LOCKFILE"
if ! flock -n 9; then
    log "Another instance is already running. Exiting."
    exit 0
fi

trap "log 'Script complete'; flock -u 9; rm -f $LOCKFILE" EXIT
trap "log 'Script failed!'; notify_slack ':red_circle: Daily cleanup FAILED'" ERR
set -e

log "Starting daily cleanup"

find /tmp -type f -mtime +1 -delete
find /var/log/myapp -name "*.log" -mtime +30 -exec gzip {} \;
aws s3 sync /var/backups s3://my-backups/daily/ --storage-class STANDARD_IA

notify_slack ":white_check_mark: Daily cleanup completed successfully"

Cron Security Best Practices

# Restrict who can use cron
echo "ubuntu" | sudo tee /etc/cron.allow  # Only ubuntu user
echo "ALL" | sudo tee /etc/cron.deny      # Deny all others

# Set correct ownership and permissions on crontab
ls -la /etc/cron.*
# drwxr-xr-x root root /etc/cron.d
# drwxr-xr-x root root /etc/cron.daily
# All cron directories: owned by root, 755 permissions

# Check for world-writable cron files (security risk)
find /etc/cron* -perm -o+w -type f 2>/dev/null

# Run cron jobs with minimal environment
# Add this at top of cron scripts:
export HOME=/home/ubuntu
export PATH=/usr/local/bin:/usr/bin:/bin
export LANG=en_US.UTF-8