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Crontab

Crontab is the program to manage cron jobs for individual users. Edit, list, and remove scheduled tasks.

Basic Crontab Commands

CommandDescription
crontab -eEdit current user’s crontab
crontab -lList current user’s cron jobs
crontab -rRemove current user’s crontab (deletes all jobs)
crontab -iInteractive remove (asks for confirmation)
crontab -vShow last edit date and time
crontab -u [user]Edit/list crontab for specific user (requires sudo)
crontab [file]Install crontab from file

Editing Crontab

# Edit crontab with default editor
crontab -e

# Use specific editor
EDITOR=nano crontab -e
EDITOR=vi crontab -e

# Change default editor
export EDITOR=nano
crontab -e

# View what editor is used
echo $EDITOR

Crontab File Format

# Format: minute hour day month weekday command
# Example:
0 9 * * 1-5 /home/user/backup.sh

# Comments start with #
# Blank lines are ignored
# One job per line

Crontab Examples

Time-Based Jobs

# Minute field (0-59)
0 9 * * * command        # At 9:00
15 9 * * * command       # At 9:15
*/5 9 * * * command      # Every 5 minutes during 9 AM hour

# Hour field (0-23)
0 9 * * * command        # At 9:00
0 21 * * * command       # At 9:00 PM (21:00)
0 */4 * * * command      # Every 4 hours

# Day field (1-31)
0 0 1 * * command        # 1st of month
0 0 15 * * command       # 15th of month
0 0 1,15 * * command     # 1st and 15th

# Month field (1-12)
0 0 1 1 * command        # January 1st
0 0 1 6 * command        # June 1st
0 0 * 12 * command       # Daily in December

# Weekday field (0-7, both 0 and 7 are Sunday)
0 0 * * 0 command        # Sunday
0 0 * * 1 command        # Monday
0 0 * * 1-5 command      # Monday-Friday
0 0 * * 0,6 command      # Saturday-Sunday (weekend)

Common Scheduling Patterns

# Every minute
* * * * * command

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

# Every hour at 15 minutes past
15 * * * * command

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

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

# Every 1st and 15th at midnight
0 0 1,15 * * command

# Every weekday (Monday-Friday) at 5 PM
0 17 * * 1-5 command

# Every 6 hours
0 */6 * * * command

# Every 2 hours during business hours (9 AM - 5 PM)
0 9-17/2 * * 1-5 command

# Run twice daily (9 AM and 5 PM)
0 9,17 * * * command

# First day of month at 1 AM
0 1 1 * * command

# Last day of month (approximation)
0 0 28-31 * * command

# Every Sunday and Friday at 10 PM
0 22 * * 0,5 command

Managing Multiple Users’ Crontabs

# View another user's crontab (requires sudo)
sudo crontab -u username -l

# Edit another user's crontab (requires sudo)
sudo crontab -u username -e

# Remove another user's crontab (requires sudo)
sudo crontab -u username -r

# Remove with confirmation (requires sudo)
sudo crontab -u username -i

# Install crontab for another user (requires sudo)
sudo crontab -u username -i crontab_file

Working with Crontab Files

Export Crontab

# Save current crontab to file
crontab -l > my_crontab.txt

# Save another user's crontab
sudo crontab -u username -l > username_crontab.txt

Import Crontab

# Install crontab from file (replaces existing)
crontab my_crontab.txt

# Install for another user
sudo crontab -u username username_crontab.txt

# Append to existing crontab
crontab -l > temp.txt
cat additional_jobs.txt >> temp.txt
crontab temp.txt
rm temp.txt

Add Single Job to Crontab

# Method 1: Using echo
(crontab -l 2>/dev/null; echo "0 9 * * * /backup.sh") | crontab -

# Method 2: Edit file then import
crontab -l > cron_backup.txt
echo "0 9 * * * /backup.sh" >> cron_backup.txt
crontab cron_backup.txt
rm cron_backup.txt

# Method 3: Using heredoc
(crontab -l 2>/dev/null || true; cat <<EOF
0 9 * * * /home/user/backup.sh
0 0 * * 0 /home/user/weekly-task.sh
EOF
) | crontab -

Remove Specific Job from Crontab

# Remove jobs matching pattern
crontab -l | grep -v "backup.sh" | crontab -

# Remove and keep a backup
crontab -l > cron_backup.txt
crontab -l | grep -v "backup.sh" | crontab -

# List jobs to remove first
crontab -l | grep "backup"

Crontab Environment Variables

# Set environment variables at top of crontab
SHELL=/bin/bash
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin
MAILTO=admin@example.com
HOME=/home/username

# Then define jobs
0 9 * * * /backup.sh
0 0 * * 0 /maintenance.sh

# To view existing environment
crontab -l | head -10

Output Redirection

# Send output to file
0 9 * * * /backup.sh > /var/log/backup.log 2>&1

# Append to log file
0 9 * * * /backup.sh >> /var/log/backup.log 2>&1

# Discard output
0 9 * * * /backup.sh > /dev/null 2>&1

# Email output (requires MAILTO set)
0 9 * * * /backup.sh

# Mail output only if error
0 9 * * * /backup.sh || (echo "Backup failed on $(hostname)" | mail -s "Backup Alert" admin@example.com)

# Log with timestamp
0 9 * * * echo "$(date): Backup started" >> /var/log/backup.log && /backup.sh >> /var/log/backup.log 2>&1

Crontab Restrictions

# /etc/cron.deny - Users who cannot use crontab
# /etc/cron.allow - Only these users can use crontab
# /etc/crontab - System-wide cron jobs

# System crontab format (includes username):
0 9 * * * root /usr/local/bin/backup.sh
0 * * * * www-data /var/www/cron.php

Viewing Crontab

List Crontabs

# Current user
crontab -l

# Specific user (with sudo)
sudo crontab -u username -l

# All users' crontabs (from /var/spool/cron/crontabs)
sudo ls -la /var/spool/cron/crontabs/

# View specific user's crontab directly
sudo cat /var/spool/cron/crontabs/username

Find All Crontabs

# Search all user crontabs
for user in $(cat /etc/passwd | cut -f1 -d:); do
  echo "=== $user ==="
  sudo crontab -u $user -l 2>/dev/null
done

# List all active cron jobs
sudo cat /var/spool/cron/crontabs/*

Troubleshooting

Crontab Not Working

# Check if cron service is running
sudo systemctl status cron
sudo service cron status

# Start cron service
sudo systemctl start cron
sudo service cron start

# Enable on boot
sudo systemctl enable cron

# Check user is allowed to use crontab
sudo test -f /etc/cron.allow && cat /etc/cron.allow
sudo test -f /etc/cron.deny && cat /etc/cron.deny

View Cron Logs

# Ubuntu/Debian
sudo tail -f /var/log/syslog | grep CRON

# CentOS/RHEL/Fedora
sudo tail -f /var/log/cron

# macOS
log stream --level debug --predicate 'eventMessage contains[cd] "cron"'

# Using journalctl (newer systems)
journalctl -u cron -f
journalctl -u cron -n 50  # Last 50 entries

Debug Crontab Issues

# Check crontab syntax
crontab -l

# Test script manually
/home/user/script.sh

# Run with bash explicitly
/bin/bash -x /home/user/script.sh

# Add logging to script
#!/bin/bash
{
  echo "Job started at $(date)"
  # ... rest of script
  echo "Job ended at $(date)"
} >> /var/log/myjob.log 2>&1

# Test environment
env > /tmp/cron_env.txt

Best Practices

  • Always use absolute paths for commands and files
  • Redirect output to log files
  • Add comments explaining each job
  • Test scripts manually before adding to crontab
  • Use MAILTO for error notifications
  • Backup crontab file: crontab -l > crontab_backup.txt
  • Set proper permissions on scripts
  • Use meaningful filenames in /tmp
  • Log output for debugging
  • Avoid using crontab -r without backup
  • Include error handling in scripts

Common Issues & Solutions

# Issue: "No such file or directory"
# Solution: Use absolute path
0 9 * * * /home/user/backup.sh  # Correct
0 9 * * * backup.sh              # Wrong

# Issue: Script not executable
# Solution: Make executable
chmod +x /home/user/backup.sh

# Issue: Environment variables not found
# Solution: Set in crontab or script
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin
0 9 * * * /backup.sh

# Issue: "Permission denied"
# Solution: Check file permissions
ls -la /home/user/backup.sh
chmod 755 /home/user/backup.sh

Resources

Last updated: 2026-03-30