Master Linux Automation: Startup Scripts, at, and Cron Scheduling

When operating a Linux‑based web site, routine maintenance such as monitoring resources, rotating logs, and performing data cleanup often needs to run automatically. This article introduces practical methods for automating typical Linux tasks.

Why Automate?

Save manpower – a single script can replace manual commands.

Run at night to avoid peak traffic and keep daytime performance unaffected.

Increase accuracy when the script is correctly configured.

Reduce the hassle of repeatedly typing commands.

Boot‑Time Startup

Automatically launching services or processes at system boot eliminates the need to manually start them each time.

Using chkconfig

Linux runlevels define the system state:

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

Typical chkconfig commands:

chkconfig --list               // List current startup services
xxxd 0:off 1:off 2:on ... 6:off // Example output showing service "xxxd" starts at levels 2‑5
chkconfig --add xxxd          // Add a new service
chkconfig [--level 1-6] xxxd on|off // Enable or disable at specific levels (default 2‑5)
chkconfig --del xxxd          // Remove the service

Editing rc.d files

Directly modify files under /etc/rc.d/. The rc.local script is a convenient place to add custom commands, e.g., /usr/local/apache/bin/apachectl start to launch Apache on boot.

One‑Time Scheduling with at

The at utility schedules a single execution of a command.

# at time          // Start at command with a time argument
at> operation      // Enter the command to run
at> Ctrl+D         // End input

Common time specifications:

at H:M tomorrow               // Next day at hour H, minute M
at now + n minutes/hours/days/weeks // After n units
at midnight                    // At midnight
at H:M pm/am                  // Today at H:M in the specified period

Scheduled jobs are stored in /var/spool/at. Note that the atd daemon is often disabled by default and must be started manually.

Recurring Scheduling with cron

The built‑in cron daemon handles periodic tasks. Cron jobs are defined in crontab files.

Cron Locations

/var/spool/cron/

– per‑user crontabs (named after the user). /etc/crontab – system‑wide schedule. /etc/cron.d/ – additional crontab files. /etc/cron.hourly, /etc/cron.daily, /etc/cron.weekly, /etc/cron.monthly – directories for scripts run at the corresponding intervals.

Using crontab

Common commands:

crontab [-u username]    // Operate on a specific user's crontab
  -e   // Edit the crontab (opens in vim)
  -l   // List current entries
  -r   // Remove the crontab

A crontab line consists of a time specification followed by the command. Time fields are minute, hour, day of month, month, and day of week. Special characters: * – every possible value / – step values (e.g., */5 every 5 units) - – range (e.g., 4-6 for hours 4 through 6) , – list of values (e.g., 2,5 for Tuesday and Friday)

Example entries:

# Minute Hour Day Month Weekday   Command
0 0 25 12 *   // Run at midnight on December 25th
*/5 * * * *   // Every 5 minutes
* 4-6 * * *   // Every minute during 4‑6 AM each day
* * * * 2,5   // Every minute on Tuesdays and Fridays

Combining Cron with Shell Scripts

For complex logic, place the commands in a shell script and invoke the script from cron.

#!/bin/sh
# Simple example script
a="hello world"
echo $a

Add the script to crontab, e.g.: */5 * * * * /usr/sh/test.sh Or run a PHP script via cron: /phppath/php /filepath/test.php This article provides the essential commands and concepts needed to set up automated tasks on a Linux server, improving reliability and reducing manual effort.