Master Linux Cron: Create, Manage, and Debug Scheduled Tasks

Creating a Simple Scheduled Task

Run a command every minute that appends the current date to /home/xxx/time.log:

$ crontab -e
* * * * * echo `date` >> /home/xxx/time.log

The crontab executable resides at /usr/bin/crontab. User crontabs are stored in /var/spool/cron/crontabs/ and require root permission to access. Each file named after a user contains that user's scheduled jobs. Do not edit these files directly with vi.

Cron logs are written to /var/log/cron, but Ubuntu disables this by default. Enable logging by editing the rsyslog configuration: $ sudo vim /etc/rsyslog.d/50-default.conf Uncomment the line: cron.* /var/log/cron.log Then restart rsyslog: $ sudo service rsyslog restart Afterward, /var/log/cron will contain cron execution logs, which are essential for troubleshooting.

Controlling User Access to Cron

To deny a user from running cron jobs, add the username (one per line) to /etc/cron.deny. To explicitly allow users, use /etc/cron.allow, which takes precedence over /etc/cron.deny. It is recommended to use only one of these files to avoid confusion.

Cron Command Options

-u

: Only root can use this to manage other users' crontabs. -e: Edit the current crontab. -l: List the crontab entries. -r: Remove all crontab entries (use -e to edit specific lines).

Time Format

The cron schedule consists of five fields: minute, hour, day of month, month, day of week, followed by the command. # minute hour day month weekday command Special characters: * – any value. , – value list separator. - – range of values. /n – step values (every n units).

Common Scheduling Examples

Run on May 1st at 10:05 each year: 5 10 1 5 * command Run at 03:00 and 06:00 daily: 0 3,6 * * * command Run at 08:20, 09:20, 10:20, 11:20 daily: 20 8-11 * * * command Run every 5 minutes: */5 * * * * command Run every Monday at 10:00: * 10 * * 1 command Run every minute: * * * * * command Run hourly at minute 0: 0 * * * * command Run daily at midnight: 0 0 * * * command Run monthly on the 1st at midnight:

0 0 1 * * command

Configuring System‑Level Scheduled Tasks

While crontab -e edits user‑level jobs, system‑wide jobs are defined in /etc/crontab. Edit this file with root privileges, for example using vim /etc/crontab: