SYSCTL(8)

NAME
sysctl - get or set kernel state

SYNOPSIS
sysctl [-bn] name ...
sysctl [-bn] -w name=value ...
sysctl [-bn] -aAX

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 -a flag can be used to list all the currently available string or integer values.

The -A flag will list all the known MIB names including opaques. Those with string or integer values will be printed as with the -a flag; for the opaque values, information about the format and the length is printed in addition the first few bytes is dumped in hex.

The -X flag is the same as -A except the entire value of opaque variables is hexdumped.

The -n flag specifies that the printing of the field name should be suppressed and that only its value should be output. This flag is useful for setting shell variables. For example, to save the pagesize in variable psize, use:
    set psize=`sysctl -n hw.pagesize`

The -b flag forces 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.

If just a MIB style name is given, the corresponding value is retrieved. If a value is to be set, the -w flag must be specified and the MIB name followed by an equal sign and the new value to be used.

The information available from sysctl consists of integers, strings, and opaques. sysctl 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, systat, and netstat.

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.
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 yes
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.update integer yes
kern.osreldate string no
kern.bootfile string 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
machdep.console_device dev_t no
machdep.adjkerntz integer yes
machdep.disable_rtc_set integer yes
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

EXAMPLES

For example, to retrieve the maximum number of processes allowed in the system, one would use the follow request:
    sysctl kern.maxproc

To set the maximum number of processes allowed in the system to 1000, one would use the follow request:
    sysctl -w kern.maxproc=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.

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

SEE ALSO
sysctl(3)

BUGS
sysctl 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.

HISTORY
sysctl first appeared in 4.4BSD.

In FreeBSD 2.2 sysctl was significantly remodeled.