[linux] CronJob not running

WTF?! My cronjob doesn't run?!

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

1. Is the Cron daemon running?

• Run ps ax | grep cron and look for cron.
• Debian: service cron start or service cron restart

2. 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.
• Probably also add 2>&1 to include standard error as well as standard output, or separately output standard error to another file with 2>>/tmp/errors

3. 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

4. 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

5. 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

6. 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

7. 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.
• If you are trying to figure out why something failed, the error messages will be visible in this file. Read it and understand it.

Still not working? Yikes!

1. 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

2. 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

1. crontab -l
   • Lists all the user's cron tasks.
2. 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.
3. crontab -r
   • Removes your crontab entry from the cron spooler, but not from crontab file.