Crontab

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

Basic Crontab Commands

Command	Description
`crontab -e`	Edit current user’s crontab
`crontab -l`	List current user’s cron jobs
`crontab -r`	Remove current user’s crontab (deletes all jobs)
`crontab -i`	Interactive remove (asks for confirmation)
`crontab -v`	Show 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

man crontab - Crontab manual
man 5 crontab - Crontab file format
Crontab Generator
Cron Syntax Checker

Last updated: 2026-03-30