SYSCTL(8)

NAME
sysctl - get or set kernel state

SYNOPSIS
sysctl [-AbdeiNnoqwxX] name[=value] ...
sysctl [-bdeiNnoqwxX] -a

DESCRIPTION
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.

-i  Exit cleanly (exit code 0) with no output if the sysctl does not exist.

-N  Show only variable names, not their values. This is particularly useful with shells that offer programmable completion. To enable completion of variable names in zsh, use the following code:

        listsysctls () { set -A reply $(sysctl -AN ${1%.*}) }
        compctl -K listsysctls sysctl

-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  Exit (exit code N) with no output if the sysctl does not exist. Each failed mib on the command line increments N.

-w  Ignored (for compatibility).

-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, systat and netstat.

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.

The string and integer information is summarized below. For a detailed description of these variable see sysctl(3).

The changeable column indicates whether a process with appropriate privilege can change the value. String, integer, and devices values can be set using sysctl. For device values, value can be specified as a character device special file name. Special values off and none denote “no device”.
Name | Type | Changeable
kern.ostype | string | no
kern.osrelease | string | no
kern.osrevision | integer | no
kern.version | string | no
kern.maxvnodes | integer | yes
kern.maxproc | integer | no
kern.maxprocperuid | integer | yes
kern.maxfiles | integer | yes
kern.maxfilesperproc | integer | yes
kern.argmax | integer | no
kern.securelevel | integer | raise only
kern.hostname | string | yes
kern.hostid | integer | yes
kern.clockrate | struct | no
kern.posix1version | integer | no
kern.ngroups | integer | no
kern.job_control | integer | no
kern.saved_ids | integer | no
kern.boottime | struct | no
kern.domainname | string | yes
kern.filedelay | integer | yes
kern.dirdelay | integer | yes
kern.metadelay | integer | yes
kern.osreldate | integer | no
kern.bootfile | string | yes
kern.corefile | string | yes
kern.dumpdev | dev_t | yes
kern.logsigexit | integer | yes
vm.loadavg | struct | no
hw.machine | string | no
hw.model | string | no
hw.ncpu | integer | no
hw.byteorder | integer | no
hw.physmem | integer | no
hw.usermem | integer | no
hw.pagesize | integer | no
hw.floatingpoint | integer | no
hw.machine_arch | string | no
hw.sensors.<xname>.<type><numt> | struct | no
machdep.console_device | dev_t | no
machdep.adjkerntz | integer | yes
machdep.disable_rtc_set | integer | yes
user.cs_path | string | no
user.bc_base_max | integer | no
user.bc_dim_max | integer | no
user.bc_scale_max | integer | no
user.bc_string_max | integer | no
user.coll_weights_max | integer | no
user.expr_nest_max | integer | no
user.line_max | integer | no
user.re_dup_max | integer | no
user.posix2_version | integer | no
user.posix2_c_bind | integer | no
user.posix2_c_dev | integer | no
user.posix2_char_term | integer | no
user.posix2_fort_dev | integer | no
user.posix2_fort_run | integer | no
user.posix2_localedef | integer | no
user.posix2_sw_dev | integer | no
user.posix2_upe | integer | no
user.stream_max | integer | no
user.tzname_max | integer | no

FILES
<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
<sys/gmon.h>
    definitions for third level profiling identifiers
<vm/vm_param.h>
    definitions for second level virtual memory 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

EXAMPLES
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

The device used for crash dumps can be specified using:

    sysctl kern.dumpdev=/dev/somedev

which is equivalent to

    dumpon /dev/somedev

Information about the system clock rate may be obtained with:

    sysctl kern.clockrate

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.

COMPATIBILITY
The -w option has been deprecated and is silently ignored.

SEE ALSO
sysctl(3), loader.conf(5), sysctl.conf(5), loader(8)

HISTORY
A sysctl utility first appeared in 4.4BSD.

In FreeBSD 2.2, sysctl was significantly remodeled.

BUGS
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.