⏰ Cron Deep Dive

# Edit YOUR crontab (opens in $EDITOR)
crontab -e

# List your current crontab
crontab -l

# Remove your crontab entirely
crontab -r

# Edit another user's crontab (as root)
sudo crontab -e -u alice
sudo crontab -l -u alice
sudo crontab -r -u alice

# Install a crontab from a file
crontab mycrontab.txt

# Backup your crontab
crontab -l > ~/mycrontab.bak

# Check cron daemon status
sudo systemctl status crond    # RHEL/Rocky
sudo systemctl status cron     # Debian/Ubuntu

# Run every minute (useful for testing — remove after!)
* * * * * /usr/local/bin/myscript.sh

# Every 5 minutes
*/5 * * * * /usr/local/bin/check-service.sh

# Every day at 2:30 AM
30 2 * * * /usr/local/bin/nightly-backup.sh

# Every weekday (Mon-Fri) at 8 AM
0 8 * * 1-5 /usr/local/bin/send-report.sh

# Every Monday at 6 AM
0 6 * * 1 /usr/local/bin/weekly-cleanup.sh

# First day of every month at midnight
0 0 1 * * /usr/local/bin/monthly-report.sh

# Twice a day — 6 AM and 6 PM
0 6,18 * * * /usr/local/bin/sync-data.sh

# Every 15 minutes during business hours (8 AM - 6 PM)
*/15 8-18 * * 1-5 /usr/local/bin/poll-queue.sh

# Once a year — Jan 1 at midnight
@yearly /usr/local/bin/annual-cleanup.sh

# At every reboot
@reboot /usr/local/bin/startup-tasks.sh

Cron has multiple layers — knowing which one to use matters:

# USER CRONTABS — managed with crontab -e
# Stored in /var/spool/cron/crontabs/ (per user)
# Format: minute hour dom month dow command
# Run as the user who owns the crontab

# SYSTEM CRONTAB — /etc/crontab
# Has an extra USERNAME field:
# minute hour dom month dow USERNAME command
cat /etc/crontab

# /etc/cron.d/ — drop-in cron files (same format as /etc/crontab)
# Best place for package and system-level scheduled jobs
ls /etc/cron.d/

# /etc/cron.daily/ /etc/cron.weekly/ /etc/cron.monthly/
# Drop executable scripts here — run by run-parts at the time above
ls /etc/cron.daily/
ls /etc/cron.weekly/

The most common reason cron jobs fail when run manually they work fine — cron runs with a minimal environment:

# Cron's environment is minimal — very different from your shell
# Check what cron sees by logging it
* * * * * env > /tmp/cron-environment.txt

# Typical cron environment:
# HOME=/root
# LOGNAME=root
# PATH=/usr/bin:/bin   <-- much shorter than your interactive PATH
# SHELL=/bin/sh        <-- sh, not bash!

# SOLUTIONS:

# 1. Set PATH at top of crontab
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
30 2 * * * myscript.sh

# 2. Use full paths in cron commands (most reliable)
30 2 * * * /usr/local/bin/myscript.sh

# 3. Set SHELL explicitly
SHELL=/bin/bash
30 2 * * * myscript.sh

# 4. Source your profile inside the script
# #!/bin/bash
# source /etc/profile
# source ~/.bash_profile

# By default cron emails output to the user running the job
# Set MAILTO to redirect or suppress

# Send output to a specific address
MAILTO=sysadmin@example.com
30 2 * * * /usr/local/bin/backup.sh

# Suppress all email (discard stdout and stderr)
MAILTO=""
30 2 * * * /usr/local/bin/backup.sh

# Redirect output to a log file instead
30 2 * * * /usr/local/bin/backup.sh >> /var/log/backup.log 2>&1

# Log with timestamp
30 2 * * * echo "$(date): Starting backup" >> /var/log/backup.log; \
           /usr/local/bin/backup.sh >> /var/log/backup.log 2>&1

# Send errors to log but discard normal output
30 2 * * * /usr/local/bin/backup.sh > /dev/null 2>> /var/log/backup-errors.log

#!/bin/bash
# nightly-backup.sh — example production cron script

# Use full paths — cron has a minimal PATH
RSYNC=/usr/bin/rsync
LOGGER=/usr/bin/logger
DATE=$(date '+%Y-%m-%d %H:%M:%S')

# Log function — writes to both syslog and log file
log() {
    echo "[$DATE] $*" | tee -a /var/log/nightly-backup.log
    $LOGGER -t nightly-backup "$*"
}

# Lock file — prevent overlapping runs
LOCKFILE=/var/run/nightly-backup.lock
if [ -f "$LOCKFILE" ]; then
    log "ERROR: Already running (lockfile exists). Exiting."
    exit 1
fi
touch "$LOCKFILE"
trap 'rm -f "$LOCKFILE"' EXIT

log "Starting nightly backup"

# Do the work
$RSYNC -av --delete /data/ /backup/nightly/ >> /var/log/nightly-backup.log 2>&1
EXIT_CODE=$?

if [ $EXIT_CODE -eq 0 ]; then
    log "Backup completed successfully"
else
    log "ERROR: Backup failed with exit code $EXIT_CODE"
    exit $EXIT_CODE
fi

# /etc/cron.d/ files use the same format as /etc/crontab
# (includes the username field)

# Create a system cron job for log rotation check
sudo tee /etc/cron.d/logcheck << 'EOF'
SHELL=/bin/bash
PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin
MAILTO=sysadmin@example.com

# minute hour dom month dow user  command
0       6    *   *   *    root  /usr/sbin/logcheck
EOF

# Correct permissions — cron.d files must not be world-writable
sudo chmod 644 /etc/cron.d/logcheck
sudo chown root:root /etc/cron.d/logcheck

# Verify cron picks it up (no restart needed)
sudo systemctl status crond
sudo ls -la /etc/cron.d/

# Control which users can use crontab

# /etc/cron.allow — whitelist (if exists, ONLY these users can use cron)
sudo tee /etc/cron.allow << 'EOF'
root
alice
bob
EOF

# /etc/cron.deny — blacklist (these users cannot use cron)
sudo tee /etc/cron.deny << 'EOF'
tempuser
contractor1
EOF

# Logic:
# cron.allow exists → only listed users can use cron
# cron.allow absent, cron.deny exists → everyone except listed users
# Neither file exists → only root can use cron (RHEL default)
# Both empty → no one can use cron

# List all user crontabs on the system
sudo ls -la /var/spool/cron/
sudo ls -la /var/spool/cron/crontabs/   # Debian/Ubuntu path