Process Management and AutomationLesson 5.3

Scheduling Bash scripts with cron and systemd timers

cron syntax, crontab fields, crontab -e, environment in cron, systemd timer units, OnCalendar syntax, cron vs systemd comparison, logging cron jobs, run-parts, anacron

Cron: Time-Based Job Scheduling

Cron runs commands on a schedule. The syntax is five fields followed by the command.

# Crontab format: min hour day month weekday command
# * means "every"

# Every day at 2:30 AM
30 2 * * * /opt/scripts/backup.sh >> /var/log/backup.log 2>&1

# Every 15 minutes
*/15 * * * * /opt/scripts/health-check.sh

# Weekdays at 8 AM
0 8 * * 1-5 /opt/scripts/send-report.sh

# First day of each month
0 0 1 * * /opt/scripts/monthly-cleanup.sh

Cron runs with a minimal environment — no PATH to your tools, no HOME-based configs. Always use absolute paths in cron scripts or set PATH at the top of the crontab.

# Top of crontab
PATH=/usr/local/bin:/usr/bin:/bin
SHELL=/bin/bash

systemd Timers (Modern Alternative)

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

[Timer]
OnCalendar=*-*-* 02:30:00
Persistent=true  # run if missed while system was off

[Install]
WantedBy=timers.target

systemctl enable --now backup.timer
systemctl list-timers  # view all timers and next run time

Prefer systemd timers on modern Linux: they survive missed runs (Persistent=true), integrate with journald for logging, and can depend on other services.