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
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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
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use the equivalent sysctl command as
follows:
sysctl -w kernel.sysrq="1"
kernel.sysrq = 1
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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.
