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Get or set kernel state.

sysctl-bdehNnoqx name[=value] ...
sysctl [-bdehNnoqx] -a


The sysctl utility retrieves kernel state and allows processes with appropriate privilege to set kernel state. The state to be retrieved or set is described using a 'Management Information Base' ('MIB') style name, described as a dotted set of components.

The following options are available:

-A Equivalent to -o -a (for compatibility).
-a List all the currently available non-opaque values. This option is ignored if one or more variable names are specified on the command line.
-b Force the value of the variable(s) to be output in raw, binary format. No names are printed and no terminating newlines are output. This is mostly useful with a single variable.
-d Print the description of the variable instead of its value.
-e Separate the name and the value of the variable(s) with '='. This is useful for producing output which can be fed back to the sysctl utility. This option is ignored if either -N or -n is specified, or a variable is being set.
-h Format output for human, rather than machine, readability.
-N Show only variable names, not their values.
-n Show only variable values, not their names. This option is useful for setting shell variables. For instance, to save the pagesize in variable psize, use: set psize=`sysctl -n hw.pagesize`.
-o Show opaque variables (which are normally suppressed). The format and length are printed, as well as a hex dump of the first sixteen bytes of the value.
-q Suppress some warnings generated by sysctl to standard error.
-X Equivalent to -x -a (for compatibility).
-x As -o, but prints a hex dump of the entire value instead of just the first few bytes.

The information available from sysctl consists of integers, strings, devices (dev_t), and opaque types. The sysctl utility only knows about a couple of opaque types, and will resort to hexdumps for the rest. The opaque information is much more useful if retrieved by special purpose programs such as ps(1), systat(1), and netstat(1).

Some of the variables which cannot be modified during normal system operation can be initialized via loader(8) tunables. This can for example be done by setting them in loader.conf(5). Please refer to loader.conf(5) for more information on which tunables are available and how to set them.


<sys/sysctl.h> definitions for top level identifiers, second level kernel and hardware identifiers, and user level identifiers
<sys/socket.h> definitions for second level network identifiers
<netinet/in.h> definitions for third level Internet identifiers and fourth level IP identifiers
<netinet/icmp_var.h> definitions for fourth level ICMP identifiers
<netinet/udp_var.h> definitions for fourth level UDP identifiers


For example, to retrieve the maximum number of processes allowed in the system, one would use the following request:

sysctl kern.maxproc

To set the maximum number of processes allowed per uid to 1000, one would use the following request:

sysctl kern.maxprocperuid=1000

Information about the system clock rate may be obtained with:


Information about the load average history may be obtained with:

sysctl vm.loadavg

More variables than these exist, and the best and likely only place to search for their deeper meaning is undoubtedly the source where they are defined.


A sysctl utility first appeared in 4.4BSD.

In FreeBSD 2.2, sysctl was significantly remodeled.


The sysctl utility presently exploits an undocumented interface to the kernel sysctl facility to traverse the sysctl tree and to retrieve format and name information. This correct interface is being thought about for the time being.


INtime 4.0
See Also