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

sysctl — get or set kernel state

sysctl [-n] name ...

sysctl [-n] -w name=value ...

sysctl [-n] -aA

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 tables. Those with string or integer values will be printed as with the -a flag; for the table values, the name of the utility to retrieve them is given.

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`

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 tables. The tabular information can only be retrieved by special purpose programs such as ps, systat, and netstat. The string and integer information is summaried 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.

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.maxfiles 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.link_max integer no
kern.max_canon integer no
kern.max_input integer no
kern.name_max integer no
kern.path_max integer no
kern.pipe_buf integer no
kern.chown_restricted integer no
kern.no_trunc integer no
kern.vdisable integer no
kern.boottime struct no
vm.loadavg struct no
machdep.console_device dev_t no
net.inet.ip.forwarding integer yes
net.inet.ip.redirect integer yes
net.inet.ip.ttl integer yes
net.inet.icmp.maskrepl integer yes
net.inet.udp.checksum integer yes
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
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

The sysctl program can get or set debugging variables that have been identified for its display. This information can be obtained by using the command:

sysctl debug
In addition, sysctl can extract information about the filesystems that have been compiled into the running system. This information can be obtained by using the command: 
sysctl vfs
By default, only filesystems that are actively being used are listed. Use of the -A flag lists all the filesystems compiled into the running kernel.

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

<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

sysctl(3)

sysctl first appeared in 4.4BSD.

4.4BSD-Lite2 May 9, 1995 SYSCTL(8)