crontab 不运行原因排查

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" >> /file
   • Syntax correct? See below.
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

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

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.

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