Linux crontab 不执行的原因和如何检查错误大全

Linux crontab 不执行的原因和如何检查错误大全

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

  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

    • 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

    • in /etc/default/cron

    • set EXTRA_OPTS="-L 2"

    • service cron restart

    • tail -f /var/log/syslog to see the scripts executed

    • Debian

    • Ubuntu

    • 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/findCrontab 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.