SYSCTL(8) FreeBSD System Manager's Manual SYSCTL(8)
NAME
sysctl - get or set kernel state
SYNOPSIS
sysctl [-j jail] [-bdeFhiJlNnoqTtVWx] [-B bufsize] [-f filename]
name[=value[,value]] ...
sysctl [-j jail] [-bdeFhJlNnoqTtVWx] [-B bufsize] -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 values except for those
which are opaque or excluded from listing via the
CTLFLAG_SKIP flag. This option is ignored if one or more
variable names are specified on the command line.
-B 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.
-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.
-F Print the format of the variable. This is additional
information to describe the type of the variable and most
useful with struct types such as clockinfo, timeval, and
loadavg.
-f 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. Note that when the -j jail option is
specified, the file will be opened before attaching to the
jail and then be processed inside the jail.
-h Format output for human, rather than machine, readability.
-i 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.
-J Display only jail prision sysctl variables (CTLFLAG_PRISON).
-j jail Perform the actions inside the jail (by jail id or jail
name).
-l Show the length of variables along with their values. This
option cannot be combined with the -N option.
-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(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`/'
-n Do not show variable 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.
-T Display only variables that are settable via loader
(CTLFLAG_TUN).
-t Print the type of the variable.
-V Display only VNET sysctl variables (CTLFLAG_VNET).
-W Display only writable variables that are not statistical.
Useful for determining the set of runtime tunable sysctls.
-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, 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 variables see sysctl(3) and security(7).
The changeable column indicates whether a process with appropriate
privilege can change the value. String and integer values can be set
using sysctl.
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.logsigexit integer yes
security.bsd.suser_enabled integer yes
security.bsd.see_other_uids integer yes
security.bsd.see_other_gids integer yes
security.bsd.see_jail_proc 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
user.localbase string 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
EXIT STATUS
The sysctl utility exits 0 on success, and >0 if an error occurs.
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
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), dtrace_mib(4), loader.conf(5), sysctl.conf(5), security(7),
loader(8), jail(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(9) facility to traverse the sysctl tree and to retrieve
format and name information. This correct interface is being thought
about for the time being.
FreeBSD 15.1-STABLE-HBSD May 7, 2026 SYSCTL(8)