Sysctl

Sysctl is a kernel utility that allows you to query and set kernel variables. It is rare that you need to tinker with a running OS X kernel, but special applications may require it.

Viewing kernel variables

There are over 100 kernel state variables. To view a specific variable, use:
sysctl variable-name

To view all variables, use:
sysctl -a

Setting kernel variables

You must be root to set a kernel variable. To set a kernel variable, use the -w switch:
sysctl -w variable-name=value

If you are running OS X as a file server and have a lot of people connecting and sharing files, you might want to increase the maximum number of open files allowed by the kernel. This can be tuned using the kern.maxfiles variable. The default value is 12288. To increase it to 50000, use:
sysctl -w kern.maxfiles=50000