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

sysctl — get or set kernel state

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

sysctl [-bdeFhlNnotqWx] [-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 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.
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.
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.
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.
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 the length of variables along with their values. This option cannot be combined with the -N option.
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`/'
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`
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.
Print the type of the variable.
Display only writable variables. 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).

The string and integer information is summarized below. For a detailed description of these variables 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
hw.activecpu integer no
hw.busfrequency integer no
hw.busfrequency_max integer no
hw.busfrequency_min integer no
hw.byteorder integer no
hw.cacheconfig struct no
hw.cachelinesize integer no
hw.cachesize struct no
hw.cpu64bit_capable integer no
hw.cpufamily integer no
hw.cpufrequency integer no
hw.cpufrequency_max integer no
hw.cpufrequency_min integer no
hw.cpusubtype integer no
hw.cputhreadtype integer no
hw.cputype integer no
hw.l1dcachesize integer no
hw.l1icachesize integer no
hw.l2cachesize integer no
hw.l3cachesize integer no
hw.logicalcpu integer no
hw.logicalcpu_max integer no
hw.memsize integer no
hw.ncpu integer no
hw.packages integer no
hw.pagesize integer no
hw.physicalcpu integer no
hw.physicalcpu_max integer no
hw.tbfrequency integer no
kern.argmax integer no
kern.bootargs string no
kern.boottime struct no
kern.clockrate struct no
kern.coredump integer yes
kern.corefile string yes
kern.flush_cache_on_write integer yes
kern.hostid integer yes
kern.hostname string yes
kern.job_control integer no
kern.maxfiles integer yes
kern.maxfilesperproc integer yes
kern.maxnbuf integer yes
kern.maxproc integer yes
kern.maxprocperuid integer yes
kern.maxvnodes integer yes
kern.msgbuf integer yes
kern.nbuf integer no
kern.netboot integer no
kern.ngroups integer no
kern.nisdomainname string yes
kern.num_files integer no
kern.num_tasks integer no
kern.num_taskthreads integer no
kern.num_threads integer no
kern.num_vnodes integer no
kern.nx integer yes
kern.osrelease string no
kern.osrevision integer no
kern.ostype string no
kern.osversion string yes
kern.posix1version integer no
kern.procname string yes
kern.safeboot integer no
kern.saved_ids integer no
kern.secure_kernel integer no
kern.securelevel integer yes
kern.singleuser integer no
kern.sleeptime struct no
kern.slide integer no
kern.stack_depth_max integer no
kern.stack_size integer no
kern.sugid_coredump integer yes
kern.sugid_scripts integer yes
kern.symfile string no
kern.usrstack integer no
kern.usrstack64 integer no
kern.uuid string no
kern.version string no
kern.waketime struct no
machdep.cpu.address_bits.physical integer no
machdep.cpu.address_bits.virtual integer no
machdep.cpu.brand integer no
machdep.cpu.brand_string string no
machdep.cpu.cache.L2_associativity integer no
machdep.cpu.cache.linesize integer no
machdep.cpu.cache.size integer no
machdep.cpu.core_count integer no
machdep.cpu.cores_per_package integer no
machdep.cpu.extfamily integer no
machdep.cpu.extfeature_bits integer no
machdep.cpu.extfeatures string no
machdep.cpu.extmodel integer no
machdep.cpu.family integer no
machdep.cpu.feature_bits integer no
machdep.cpu.features string no
machdep.cpu.leaf7_feature_bits integer no
machdep.cpu.leaf7_features string no
machdep.cpu.logical_per_package integer no
machdep.cpu.max_basic integer no
machdep.cpu.max_ext integer no
machdep.cpu.microcode_version integer no
machdep.cpu.model integer no
machdep.cpu.processor_flag integer no
machdep.cpu.signature integer no
machdep.cpu.stepping integer no
machdep.cpu.thread_count integer no
machdep.cpu.tlb.data.large integer no
machdep.cpu.tlb.data.large_level1 integer no
machdep.cpu.tlb.data.small integer no
machdep.cpu.tlb.data.small_level1 integer no
machdep.cpu.tlb.inst.large integer no
machdep.cpu.tlb.inst.small integer no
machdep.cpu.tlb.shared integer no
machdep.cpu.ucupdate integer yes
machdep.cpu.vendor string no
machdep.cpu.xsave.extended_state integer no
machdep.tsc.deep_idle_rebase integer yes
machdep.tsc.frequency integer no
machdep.tsc.nanotime.generation integer no
machdep.tsc.nanotime.shift integer no
net.inet.ip.forwarding integer yes
net.inet.ip.portrange.first integer yes
net.inet.ip.portrange.hifirst integer yes
net.inet.ip.portrange.hilast integer yes
net.inet.ip.portrange.last integer yes
net.inet.ip.portrange.lowfirst integer yes
net.inet.ip.portrange.lowlast integer yes
net.inet.ip.redirect integer yes
net.inet.ip.ttl integer yes
net.inet.udp.checksum integer yes
net.inet.udp.maxdgram integer yes
vm.loadavg struct no
vm.swapusage struct 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.cs_path string no
user.expr_nest_max integer no
user.line_max 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.posix2_version integer no
user.re_dup_max integer no
user.stream_max integer no
user.tzname_max integer no

Note that the hw.ncpu attribute is deprecated, and one of the more specific MIBs, hw.logicalcpu, hw.logicalcpu_max, hw.physicalcpu, or hw.physicalcpu_max, should be used instead.

<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), sysctl.conf(5)

A sysctl utility first appeared in 4.4BSD.

In FreeBSD 2.2, sysctl was significantly remodeled.

August 18, 2023 macOS 15.6