Master Linux Automation: Startup Scripts, at, and crontab Explained

When a Linux‑based website is in production, routine maintenance such as resource monitoring, log rotation, and data cleanup often needs to run automatically; this article introduces common Linux automation tasks.

Save manpower – a single script can replace manual work.

Run at night to avoid peak traffic.

Increase accuracy when properly configured.

Reduce hassle by eliminating repetitive command entry.

Boot Startup

Automatically executing commands at boot is a frequent requirement for starting services and processes.

chkconfig command

chkconfig allows you to enable or disable services at specific runlevels.

Runlevel 0: shutdown

Runlevel 1: single‑user mode

Runlevel 2: multi‑user mode without NFS

Runlevel 3: multi‑user mode with NFS

Runlevel 4: unavailable

Runlevel 5: multi‑user mode with graphical interface

Runlevel 6: reboot

Typical chkconfig usage:

chkconfig --list            // list current startup services</code><code>xxxd 0:off 1:off 2:on ... 6:off   // example output</code><code>chkconfig --add xxxd      // add a service</code><code>chkconfig --level 2,3,4 xxxd on   // enable at levels 2‑4</code><code>chkconfig --del xxxd      // remove a service

Editing rc.d files

You can directly edit files under /etc/rc.d/ (e.g., rc.local) to create custom boot‑time actions, such as starting Apache with /usr/local/apache/bin/apachectl start.

Using at for One‑Time Tasks

at

schedules a single execution of a command.

#at time               // specify execution time</code><code>at> operation        // enter the command</code><code>at> Ctrl+D           // finish editing

Common time formats:

at 14:30 tomorrow          // next day at 14:30</code><code>at now + 10 minutes        // 10 minutes from now</code><code>at midnight                // at midnight</code><code>at 9:00 pm                // today at 21:00

Check pending jobs in /var/spool/at. Note that the atd daemon is disabled by default and must be started manually.

Using crontab for Recurring Tasks

The built‑in cron daemon runs scheduled jobs defined in crontab files.

Cron Introduction

Cron consists of a daemon and configuration files. User‑specific crontabs reside in /var/spool/cron/, while system‑wide schedules are in /etc/crontab and /etc/cron.d/. Additional directories such as /etc/cron.hourly, /etc/cron.daily, /etc/cron.weekly, and /etc/cron.monthly allow scripts to run at those intervals.

Using crontab

Common commands:

crontab -e          // edit current user's crontab</code><code>crontab -l          // list crontab entries</code><code>crontab -r          // remove crontab

A crontab line consists of five time fields followed by the command. Operators: * – every possible value / – step values (e.g., every 5 minutes) - – range (e.g., 1‑5) , – list of specific values

Examples:

0 0 25 12 *          // December 25 at 00:00</code><code>*/5 * * * *          // every 5 minutes</code><code>* 4-6 * * *          // at 4,5,6 AM daily</code><code>* * * * 2,5          // every Tuesday and Friday

Combining with Simple Shell Scripts

For complex logic, place commands in a shell script and invoke it from crontab.

#!/bin/sh</code><code>a="hello world"</code><code>echo $a

Add the script to crontab, e.g.: */5 * * * * /usr/sh/test.sh – runs every five minutes.

Alternatively, execute a PHP script with:

/phppath/php /filepath/test.php