Master Linux Startup and Scheduling: chkconfig, rc.local, at & crontab Explained

Why Automate?

Automation saves manpower, runs tasks during low‑traffic periods, reduces errors, and eliminates the need to manually repeat commands.

Boot‑time Startup

Many services need to start automatically when the system boots. The chkconfig command manages which services start at which runlevels.

Linux Runlevels

0 – Halt

1 – Single‑user mode

2 – Multi‑user, no NFS

3 – Multi‑user with NFS

4 – Unused

5 – Multi‑user with graphical interface

6 – Reboot

Common chkconfig commands:

chkconfig --list               # List current startup settings</code>
<code>chkconfig --add xxxd          # Add service xxxd to startup list</code>
<code>chkconfig [--level 1-6] xxxd on|off   # Enable/disable at specific levels</code>
<code>chkconfig --del xxxd         # Remove service from startup list

Editing rc.d Files

Directly edit files under /etc/rc.d/ (or /etc/rc.local) to customize boot actions, e.g., adding /usr/local/apache/bin/apachectl start to launch Apache on boot.

One‑Time Scheduling with at

The at utility runs a single command at a specified time.

# at 14:30               # Schedule a job at 14:30 today</code>
<code>at> /path/to/script.sh   # Enter the command to run</code>
<code>at> Ctrl+D               # Finish input

Common time specifications: at 09:00 tomorrow – 9 AM the next day at now + 30 minutes – 30 minutes from now at midnight – at midnight at 18:00 pm – 6 PM today

Note: the atd daemon may be disabled by default and must be started manually.

Recurring Scheduling with crontab

The cron daemon runs scheduled jobs defined in crontab files.

Configuration Files

/var/spool/cron/

– per‑user crontabs /etc/crontab – system‑wide jobs /etc/cron.d/ – additional crontab files

Directories /etc/cron.hourly, /etc/cron.daily, /etc/cron.weekly, /etc/cron.monthly for scripts executed at those intervals.

Basic crontab Commands

crontab -e          # Edit current user's crontab</code>
<code>crontab -l          # List current crontab</code>
<code>crontab -r          # Remove current crontab

A crontab line consists of five time fields followed by the command. Supported symbols: * – every possible value / – step values (e.g., */5 every 5 units) - – range (e.g., 4-6 hours 4 to 6) , – list of values (e.g., 2,5 on Tuesdays and Fridays)

Example entries:

# 0 0 25 12 *      # Run at midnight on December 25</code>
<code>*/5 * * * *      # Every 5 minutes</code>
<code>* 4-6 * * *      # Every minute during 4 AM‑6 AM daily</code>
<code>* * * * 2,5      # Every minute on Tuesdays and Fridays

Using Shell Scripts with Cron

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

#!/bin/sh

a="hello world"
echo $a

Then add to crontab, e.g., */5 * * * * /usr/sh/test.sh or run a PHP script: /phppath/php /filepath/test.php This runs the script every five minutes.