WTF??? My cron job doesn't run?!$#%

Source: This SO Answer

Here's a checklist guide to debug not running cronjobs:

• Is the Cron daemon running?
  • Run ps ax | grep cron and look for cron.
  • Debian: service cron status, if not running, service cron start or service cron restart
• Is cron working?
  • * * * * * /bin/echo "cron works" >> /tmp/file
  • Syntax correct? See below.
  • You obviously need to have write access to the file you are redirecting the output to. A unique file name in /tmp which does not currently exist should always be writable.
• Is the command working standalone?
  • Check if the script has an error, by doing a dry run on the CLI
  • When testing your command, test as the user whose crontab you are editing, which might not be your login or root
• Can cron run your job?
  • Check /var/log/cron.log or /var/log/messages for errors.
  • Ubuntu: grep CRON /var/log/syslog
  • Redhat: /var/log/cron
• Check permissions
  • Set executable flag on the command: chmod +x /var/www/app/cron/do-stuff.php
  • If you redirect the output of your command to a file, verify you have permission to write to that file/directory
• Check paths
  • Check she-bangs / hashbangs line
  • Do not rely on environment variables like PATH, as their value will likely not be the same under cron as under an interactive session
• Don't suppress output while debugging
  • Commonly used is this suppression: 30 1 * * * command > /dev/null 2>&1
  • Re-enable the standard output or standard error message output by removing >/dev/null 2>&1 altogether; or perhaps redirect to a file in a location where you have write access: >>cron.out 2>&1 will append standard output and standard error to cron.out in the invoking user's home directory.

Still not working? Yikes!

• Raise the cron debug level
  • Debian
    • In /etc/default/cron
    • Set EXTRA_OPTS="-L 2"
    • service cron restart
    • tail -f /var/log/syslog to see the scripts executed
  • Ubuntu
    • In /etc/rsyslog.d/50-default.conf
    • Add or comment out line cron.crit /var/log/cron.log
    • Reload logger sudo /etc/init.d/rsyslog reload
    • Re-run cron
    • Open /var/log/cron.log and look for detailed error output
• Reminder: deactivate log level, when you are done with debugging
• Run cron and check log files again

Cronjob Syntax

# Minute  Hour  Day of Month      Month         Day of Week    User Command    
# (0-59) (0-23)   (1-31)    (1-12 or Jan-Dec) (0-6 or Sun-Sat)  

0       2       *             *                *          root /usr/bin/find

This syntax is only correct for the root user. Regular user crontab syntax doesn't have the User field (regular users aren't allowed to run code as any other user);

# Minute  Hour  Day of Month      Month         Day of Week    Command    
# (0-59) (0-23)   (1-31)    (1-12 or Jan-Dec) (0-6 or Sun-Sat)  

0       2       *             *                *          /usr/bin/find

Crontab Commands

• crontab -l
  • Lists all the user's cron tasks.
• crontab -e, for a specific user: crontab -e -u agentsmith
  • Starts edit session of your crontab file.
  • When you exit the editor, the modified crontab is installed automatically.
• crontab -r
  • Removes your crontab entry from the cron spooler, but not from crontab file.

Courtesy: Jens A. Koch