cron & at Jobs

# crontab format
# ┌───────── minute (0-59)
# │  ┌────── hour (0-23)
# │  │  ┌─── day of month (1-31)
# │  │  │  ┌── month (1-12)
# │  │  │  │  ┌── day of week (0-7, 0=7=Sun)
# │  │  │  │  │
# *  *  *  *  *  command to execute

# Examples
30 2 * * * /usr/bin/backup.sh           # Daily at 2:30 AM
0 */4 * * * /usr/bin/cleanup.sh         # Every 4 hours
0 9-17 * * 1-5 /usr/bin/work-hours.sh   # Weekdays 9-5
0 0 1 * * /usr/bin/monthly-report.sh    # First of month
0 0 * * 0 /usr/bin/weekly-cleanup.sh    # Every Sunday

# Edit crontab
crontab -e

# List crontab entries
crontab -l

# Remove all crontab entries
crontab -r

# Specific user's crontab
sudo crontab -u username -l

# Cron has a minimal environment
# PATH=/usr/bin:/bin
# SHELL=/bin/sh
# No user environment variables

# Common pitfalls:
# - Commands not found (limited PATH)
# - No email output (no terminal)
# - Different working directory

# Fix: set environment at top of crontab
PATH=/usr/local/bin:/usr/bin:/bin
HOME=/root
MAILTO=admin@example.com

# Or use full paths
0 3 * * * /usr/bin/python3 /opt/scripts/backup.py

# Source environment in script
#!/usr/bin/env bash
source /etc/profile.d/app.sh
# ... rest of script

# Redirect output
0 3 * * * /usr/bin/backup.sh >> /var/log/backup.log 2>&1

# Problem: job takes longer than interval, jobs overlap

# Solution 1: flock (recommended)
0 * * * * flock -n /var/run/backup.lock /usr/bin/backup.sh
# -n: non-blocking, fail immediately if lock is held

# Solution 2: flock with timeout
0 * * * * flock -w 300 /var/run/backup.lock /usr/bin/backup.sh
# -w: wait up to 300 seconds for lock

# Solution 3: Lock in script
#!/usr/bin/env bash
exec 200>/var/run/backup.lock
flock -n 200 || { echo "Already running"; exit 1; }
# ... rest of script

# Solution 4: PID file
#!/usr/bin/env bash
PIDFILE=/var/run/backup.pid
if [ -f "$PIDFILE" ] && kill -0 $(cat "$PIDFILE") 2>/dev/null; then
    echo "Already running"
    exit 1
fi
echo $$ > "$PIDFILE"
trap 'rm -f "$PIDFILE"' EXIT
# ... rest of script

# System cron directories
/etc/cron.d/           # Package-provided cron jobs
/etc/cron.daily/       # Scripts run once daily
/etc/cron.hourly/      # Scripts run once hourly
/etc/cron.weekly/      # Scripts run once weekly
/etc/cron.monthly/     # Scripts run once monthly

# /etc/crontab (system-wide, has USER field)
# minute hour day month weekday user command
0 3 * * * root /usr/bin/backup.sh

# Anacron (for systems not running 24/7)
cat /etc/anacrontab
# 1   5   cron.daily    nice run-parts /etc/cron.daily
# 7   25  cron.weekly   nice run-parts /etc/cron.weekly
# @monthly 45 cron.monthly nice run-parts /etc/cron.monthly
# delay   period   identifier   command

# Schedule a job for later
echo "/usr/bin/backup.sh" | at 2:00 AM
echo "reboot" | at now + 30 minutes

# Interactive at prompt
at now + 1 hour
# at> /usr/bin/backup.sh
# at> Ctrl+D

# List pending jobs
atq
# 1234  2024-01-15 02:00 a root

# View job details
at -c 1234

# Delete a job
atrm 1234

# at.allow and at.deny
# /etc/at.allow — users allowed to use at
# /etc/at.deny — users denied from using at

# Batch mode (runs when system load is low)
echo "/usr/bin/heavy-task.sh" | batch