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SYSCTL(8) System Manager's Manual SYSCTL(8)

sysctl — get or set kernel state

sysctl [-bdehiNnoRTtqx] [-B bufsize] [-f filename] name[=value[,value]] ...

sysctl [-bdehNnoRTtqx] [-B bufsize] -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:

Equivalent to -o -a (for compatibility).
List all the currently available non-opaque values. This option is ignored if one or more variable names are specified on the command line.
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.
bufsize
Set the buffer size to read from the sysctl to bufsize. This is necessary for a sysctl that has variable length, and the probe value of 0 is a valid length, such as kern.arandom.
Print the description of the variable instead of its value.
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.
filename
Specify a file which contains a pair of name and value in each line. sysctl reads and processes the specified file first and then processes the name and value pairs in the command line argument.
Format output for human, rather than machine, readability.
Ignore unknown OIDs. The purpose is to make use of sysctl for collecting data from a variety of machines (not all of which are necessarily running exactly the same software) easier.
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(1) (ports/shells/zsh), use the following code: 
listsysctls () { set -A reply $(sysctl -AN ${1%.*}) }
compctl -K listsysctls sysctl

To enable completion of variable names in tcsh(1), use:

complete sysctl 'n/*/`sysctl -Na`/'
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`
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.
Suppress some warnings generated by sysctl to standard error.
Display only variables that are settable via loader (CTLFLAG_TUN).
Print the type of the variable.
Display only writable variables that are not statistical. Useful for determining the set of runtime tunable sysctls.
Equivalent to -x -a (for compatibility).
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, 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.

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 and integer values can be set using sysctl.

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.logsigexit integer yes
security.bsd.suser_enabled integer yes
security.bsd.see_other_uids integer yes
security.bsd.unprivileged_proc_debug integer yes
security.bsd.unprivileged_read_msgbuf 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.realmem integer no
machdep.adjkerntz integer yes
machdep.disable_rtc_set integer yes
machdep.guessed_bootdev string no
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

<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

The sysctl utility exits 0 on success, and >0 if an error occurs.

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:

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.

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

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

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.

September 24, 2018 FreeBSD-12.0