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

Boot‑time startup

Use chkconfig to manage services across run‑levels (0–6). Run‑levels: 0 shutdown, 1 single‑user, 2 multi‑user without NFS, 3 multi‑user with NFS, 4 unused, 5 graphical, 6 reboot.

chkconfig --list                # list services and their run‑level status
chkconfig --add myservice       # add a new service
chkconfig --level 2,3,4,5 myservice on   # enable at levels 2‑5
chkconfig --del myservice       # remove service

Alternatively edit scripts under /etc/rc.d/ or add commands to /etc/rc.local, e.g. /usr/local/apache/bin/apachectl start to launch Apache on boot.

One‑time tasks with at

The at utility schedules a single execution. After invoking at with a time specification, enter commands, finish with EOF (Ctrl‑D).

# at 14:30 tomorrow
at> /usr/bin/backup.sh
at> ^D

Common time formats:

at 14:30 tomorrow

at now + 10 minutes

at midnight

at 9:00 pm

Ensure the atd daemon is running; start it if disabled.

Recurring tasks with cron

The cron daemon reads crontab files. User crontabs are in /var/spool/cron/; system crontab is /etc/crontab and /etc/cron.d/. Directories /etc/cron.hourly, /etc/cron.daily, /etc/cron.weekly, /etc/cron.monthly run scripts at fixed intervals.

Cron syntax

A crontab line consists of five time fields (minute hour day‑of‑month month day‑of‑week) followed by the command. Special characters: * – every possible value / – step values (e.g., */5 every 5 units) - – range (e.g., 1-5) , – list of specific values (e.g., 1,3,5)

# Example entries
0 0 25 12 *          # Dec 25 at 00:00
*/5 * * * *          # every 5 minutes
* 4-6 * * *          # at 04:00, 05:00, 06:00 daily
* * * * 2,5          # every Tuesday and Friday

Using shell scripts with cron

Place complex logic in a script and invoke it from crontab. Example script /usr/sh/test.sh:

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

Add to crontab: */5 * * * * /usr/sh/test.sh PHP scripts can be scheduled similarly, e.g. /usr/bin/php /path/to/script.php.