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NOTE: CentOS Enterprise Linux is built from the Red Hat Enterprise Linux source code. Other than logo and name changes CentOS Enterprise Linux is compatible with the equivalent Red Hat version. This document applies equally to both Red Hat and CentOS Enterprise Linux.

5.4. Using the sysctl Command

The /sbin/sysctl command is used to view, set, and automate kernel settings in the /proc/sys/ directory.

For a quick overview of all settings configurable in the /proc/sys/ directory, type the /sbin/sysctl -a command as root. This creates a large, comprehensive list, a small portion of which looks something like the following:

net.ipv4.route.min_delay = 2
kernel.sysrq = 0
kernel.sem = 250     32000     32     128

This is the same information seen if each of the files were viewed individually. The only difference is the file location. For example, the /proc/sys/net/ipv4/route/min_delay file is listed as net.ipv4.route.min_delay, with the directory slashes replaced by dots and the proc.sys portion assumed.

The sysctl command can be used in place of echo to assign values to writable files in the /proc/sys/ directory. For example, instead of using the command

echo 1 > /proc/sys/kernel/sysrq

use the equivalent sysctl command as follows:

sysctl -w kernel.sysrq="1"
kernel.sysrq = 1

While quickly setting single values like this in /proc/sys/ is helpful during testing, this method does not work as well on a production system as special settings within /proc/sys/ are lost when the machine is rebooted. To preserve custom settings, add them to the /etc/sysctl.conf file.

Each time the system boots, the init program runs the /etc/rc.d/rc.sysinit script. This script contains a command to execute sysctl using /etc/sysctl.conf to determine the values passed to the kernel. Any values added to /etc/sysctl.conf therefore take effect each time the system boots.

 
 
  Published under the terms of the GNU General Public License Design by Interspire