---
name: "sysctl"
section: "8"
platform: "freebsd"
package: "base"
version: "16.0-CURRENT-20261027-61c70dc16a79"
release: "FreeBSD 16.0-CURRENT (2026-10-27)"
platforms: ["linux", "freebsd", "openbsd", "macos"]
url: "https://new.carta.tech/sysctl"
---

# sysctl(8)

get or set kernel state

## 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 prison 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)](/zsh/1.md) (*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)](/tcsh/1.md), 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)](/ps/1.md), [systat(1)](/systat/1.md), and [netstat(1)](/netstat/1.md).

Some of the variables which cannot be modified during normal system operation can be initialized via [loader(8)](/loader/8.md) tunables. This can for example be done by setting them in [loader.conf(5)](/loader.conf/5.md). Please refer to [loader.conf(5)](/loader.conf/5.md) 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)](/sysctl/3.md) and [security(7)](/security/7.md).

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

- In sys/sysctl.h

  definitions for top level identifiers, second level kernel and hardware identifiers, and user level identifiers

- In sys/socket.h

  definitions for second level network identifiers

- In sys/gmon.h

  definitions for third level profiling identifiers

- In vm/vm\_param.h

  definitions for second level virtual memory identifiers

- In netinet/in.h

  definitions for third level Internet identifiers and fourth level IP identifiers

- In netinet/icmp\_var.h

  definitions for fourth level ICMP identifiers

- In 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)](/sysctl/3.md), [dtrace\_mib(4)](/dtrace_mib/4.md), [loader.conf(5)](/loader.conf/5.md), [sysctl.conf(5)](/sysctl.conf/5.md), [security(7)](/security/7.md), [jail(8)](/jail/8.md), [loader(8)](/loader/8.md)

## 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)](/sysctl/9.md) facility to traverse the sysctl tree and to retrieve format and name information. This correct interface is being thought about for the time being.