The sysctl Interface

The sysctl interface lets you configure kernel parameters at runtime. sysctl lists kernel parameters, modifies the parameters temporarily through the command-line interface, loads the settings from a file, or modifies the kernel parameters permanently by adding in a configuration file for loading at boot time.

The parameters are organized in a hierarchical, or tree, structure. For example, all the parameters related to devices start with dev. The parameters for a specific device, such as cdrom, are under dev.cdrom.

By default, kernel support for sysctl is enabled. To enable sysctl support when building a new kernel, use menuconfig, choose General Setup from the main menu, and select sysctl support. Alternatively, you can edit the configuration file to set the CONFIG_ SYSCTL option.

Using sysctl

The sysctl command usually resides in the /sbin directory. You can view the complete list of tunable parameters and their settings by executing the sysctl a command. To set a specific parameter, use the w option. For example, the following command sets the message queue identifiers to 32:

 $ sysctl w kernel.shmmax=32 

Note that this change is temporary and does not persist across system reboots.

To make a change that will persist across reboots, use the p option to load a set of values from a configuration file. In the following example, the values from the configuration file /etc/sysctl.conf are loaded:

 $ sysctl p /etc/sysctl.conf 

For more information on different usage options, refer to the sysctl(8) man page.