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Chapter 1 Executing Commands 27
Important: As the root user, you have sufficient privileges to do things that can cause
your server to stop working properly. Don’t execute commands as the root user unless
you know what you’re doing. Logging in as an administrator user and using
sudo
selectively might prevent you from making unintended changes.
Terminating Commands
To terminate the currently running command, enter Control-C. This keyboard shortcut
sends an abort signal to the command. In most cases this causes the command to
terminate, although commands may install signal handlers to trap this signal and
respond differently.
Scheduling Tasks
You can create scheduled tasks using the cron tool. cron is a daemon that executes
scheduled commands from a crontab file. The cron tool searches the /var/cron/tabs
folder for crontab files that are named after accounts in /etc/passwd, and loads the files
into memory. cron also searches for crontab files in the /etc/crontab folder, which are in
a different format. cron then cycles every minute, examining all stored crontab files and
checking each command to see if it should be run in the current minute.
When commands execute, any output is mailed to the owner of the crontab file or to
the user named in the MAILTO environment variable in the crontab file, if such exists.
When a crontab file has been modified, cron needs to be restarted. crontab is the
program used to install, deinstall, or list the tables used to drive the cron daemon.
Each user can have their own crontab file.
To configure your crontab file, use the crontab -e command. This displays an empty
crontab file.
An example of a configured crontab file:
SHELL=/bin/sh
PATH=/bin:/sbin:/usr/bin:/usr/sbin
HOME=/var/log
#min hour mday month wday command
30 18 * * 1-5 /usr/local/vscanx folder-name
50 23 * * 0 /usr/local/vscanx --summary folder-name
15 10 * * 6 /usr/local/vscanx --load /usr/local/conf1 /uz
458** 1 /usr/local/vscanx --f /usr/local/biglist
Listed below is an explanation of the crontab structure shown above.
The following crontab entry schedules a scan operation to run and produce a summary
at 18:30 every day, Monday through Friday:
30 18 * * 1-5 /usr/local/vscanx folder-name