Master Linux Automation: Startup Scripts, chkconfig, at & crontab Explained

Why Automate?

Automation reduces manual effort, avoids peak‑hour load, minimizes human error, and ensures repeatable execution of routine tasks.

Boot‑time Service Startup

Linux uses runlevels to define system states. The traditional SysV runlevels are:

0 – Halt

1 – Single‑user mode

2 – Multi‑user without NFS

3 – Multi‑user with NFS

4 – Unused

5 – Multi‑user with graphical interface

6 – Reboot

The chkconfig utility manages which services start in which runlevels.

chkconfig --list               # List services and their runlevel settings</code>
<code>chkconfig --add <em>service</em>          # Add a new service to the management list</code>
<code>chkconfig --del <em>service</em>          # Remove a service from the list</code>
<code>chkconfig [--level 1-6] <em>service</em> on|off  # Enable or disable for specific levels (default 2‑5)

Typical output shows the enabled levels, e.g. xxxd 0:off 1:off 2:on 3:on 4:on 5:on 6:off.

For simple cases you can edit /etc/rc.d/rc.local (or /etc/rc.local on newer distributions) with a text editor and add the exact command you want to run at boot, for example:

vim /etc/rc.d/rc.local</code>
<code>/usr/local/apache/bin/apachectl start

After saving, ensure the script is executable:

chmod +x /etc/rc.d/rc.local

One‑time Scheduling with at

The at daemon runs a single command at a specified time. The daemon atd is often disabled by default; start it with:

systemctl start atd</code>
<code>systemctl enable atd   # optional, to start at boot

Schedule a job:

# at 14:30</code>
<code>at> /path/to/command</code>
<code>at> ^D   # Ctrl+D to finish

Common time specifications:

at 09:15 tomorrow          # next day at 09:15</code>
<code>at now + 10 minutes       # 10 minutes from now</code>
<code>at midnight               # today at 00:00</code>
<code>at 18:00 pm               # today at 18:00

Pending jobs are stored under /var/spool/at and can be listed with atq and removed with atrm.

Recurring Scheduling with crontab

The cron daemon executes tasks repeatedly according to a schedule defined in crontab files.

Configuration locations

/var/spool/cron/

– per‑user crontabs (named after the user) /etc/crontab – system‑wide crontab (includes a user field) /etc/cron.d/ – additional crontab fragments /etc/cron.hourly, /etc/cron.daily, /etc/cron.weekly, /etc/cron.monthly – directories for scripts run at the indicated interval

Basic crontab commands

crontab -e          # Edit current user’s crontab (opens $EDITOR)</code>
<code>crontab -l          # List current crontab entries</code>
<code>crontab -r          # Remove the current crontab

Crontab line syntax

A line consists of five time fields followed by the command: * * * * * command Field order: minute (0‑59), hour (0‑23), day of month (1‑31), month (1‑12), day of week (0‑7, where both 0 and 7 represent Sunday).

Special symbols: * – every possible value / – step values (e.g., */5 every 5 units) - – range (e.g., 4-6 for hours 4 through 6) , – list of discrete values (e.g., 2,5 for Tuesday and Friday)

Example entries

0 0 25 12 *          # Run at 00:00 on December 25</code>
<code>*/5 * * * *          # Every 5 minutes</code>
<code>* 4-6 * * *          # Every minute during 04:00‑06:59 each day</code>
<code>* * * * 2,5          # Every minute on Tuesdays and Fridays

Running scripts from cron

For complex logic place the commands in an executable script and invoke the script from crontab. Example shell script:

#!/bin/sh</code>
<code>echo "Hello world"

Make it executable: chmod +x /usr/local/bin/test.sh Then add to crontab: */5 * * * * /usr/local/bin/test.sh PHP scripts can be run similarly, specifying the interpreter:

*/5 * * * * /usr/bin/php /path/to/script.php

Reference

Source URL: https://reurl.cc/v5o0xA