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

:

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

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:

<code>$ sudo vim /etc/rsyslog.d/50-default.conf</code>

Uncomment the line:

<code>cron.*    /var/log/cron.log</code>

Then restart rsyslog:

<code>$ sudo service rsyslog restart</code>

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.

<code># minute hour day month weekday command</code>

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:

<code>5 10 1 5 * command</code>

Run at 03:00 and 06:00 daily:

<code>0 3,6 * * * command</code>

Run at 08:20, 09:20, 10:20, 11:20 daily:

<code>20 8-11 * * * command</code>

Run every 5 minutes:

<code>*/5 * * * * command</code>

Run every Monday at 10:00:

<code>* 10 * * 1 command</code>

Run every minute:

<code>* * * * * command</code>

Run hourly at minute 0:

<code>0 * * * * command</code>

Run daily at midnight:

<code>0 0 * * * command</code>

Run monthly on the 1st at midnight:

<code>0 0 1 * * command</code>

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

: