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 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) (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.

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), dtrace_mib(4), loader.conf(5), sysctl.conf(5), security(7), jail(8), 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(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.