SYSCTL(8) System Manager's Manual SYSCTL(8)
sysctl -- get or set kernel state
sysctl [-AdeMnq] [-r | -x] [name ...]
sysctl [-nq] [-r | -x] -w name[?]=value ...
sysctl [-en] [-r | -x] -a
sysctl [-nq] [-r | -x] -f file
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 `/' character may
also be used as a separator and a leading separator character is
accepted. If name specifies a non-leaf node in the MIB, all the nodes
underneath name will be printed.
The following options are available:
-A List all the known MIB names including tables, unless any MIB
arguments or -f file are given. Those with string or integer
values will be printed as with the -a flag; for table or
structure values that sysctl is not able to print, the name of
the utility to retrieve them is given. Errors in retrieving or
setting values will be directed to stdout instead of stderr.
-a List all the currently available string or integer values. The
use of a solitary separator character (either `.' or `/') by
itself has the same effect. Any given name arguments are ignored
if this option is specified.
-d Descriptions of each of the nodes selected will be printed
instead of their values.
-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 -n is specified or a
variable is being set.
-f Specifies the name of a file to read and process. Blank lines
and comments (beginning with `#') are ignored. Line
continuations with `\' are permitted. Remaining lines are
processed similarly to command line arguments of the form name or
name=value. The -w flag is implied by -f. Any name arguments
-M Makes sysctl print the MIB instead of any of the actual values
contained in the MIB. This causes the entire MIB to be printed
unless specific MIB arguments or -f file are also given.
-n 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`
-q Used to indicate that nothing should be printed for reads or
writes unless an error is detected. For reads, not finding a
variable does not print an error, but exits with an error code.
This is useful just for testing that a variable exists.
-r Raw output form. Values printed are in their raw binary forms as
retrieved directly from the kernel. Some additional nodes that
sysctl cannot print directly can be retrieved with this flag.
This option conflicts with the -x option.
-w Sets the MIB style name given to the value given. The MIB style
name and value must be separated by `=' with no whitespace. To
prevent an error if the MIB style name does not exist (as would
be the case with optional kernel components), one can separate
the MIB style name and the value with `?='. Only integral and
string values can be set via this method.
-x Makes sysctl print the requested value in a hexadecimal
representation instead of its regular form. If specified more
than once, the output for each value resembles that of hexdump(1)
when given the -C flag. This option conflicts with the -r
The `proc' top-level MIB has a special semantic: it represent per-process
values and as such may differ from one process to another. The second-
level name is the pid of the process (in decimal form), or the special
word `curproc'. For variables below `proc.<pid>.rlimit', the integer
value may be replaced with the string `unlimited' if it matches the magic
value used to disable a limit.
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. See sysctl(7) for description
of available MIBs.
CREATION AND DELETION
New nodes are allowed to be created by the superuser when the kernel is
running at security level 0. These new nodes may refer to existing
kernel data or to new data that is only instrumented by sysctl(3) itself.
The syntax for creating new nodes is ``//create=new.node.path'' followed
by one or more of the following attributes separated by commas. The use
of a double separator (both `/' and `.' can be used as separators) as the
prefix tells sysctl that the first series of tokens is not a MIB name,
but a command. It is recommended that the double separator preceding the
command not be the same as the separator used in naming the MIB entry so
as to avoid possible parse conflicts. The ``value'' assigned, if one is
given, must be last.
o type=<T> where T must be one of ``node'', ``int'', ``string'',
``quad'', or ``struct''. If the type is omitted, the ``node'' type
o size=<S> here, S asserts the size of the new node. Nodes of type
``node'' should not have a size set. The size may be omitted for
nodes of types ``int'' or ``quad''. If the size is omitted for a
node of type ``string'', the size will be determined by the length of
the given value, or by the kernel for kernel strings. Nodes of type
``struct'' must have their size explicitly set.
o addr=<A> or symbol=<A> The kernel address of the data being
instrumented. If ``symbol'' is used, the symbol must be globally
visible to the in-kernel ksyms(4) driver.
o n=<N> The MIB number to be assigned to the new node. If no number is
specified, the kernel will assign a value.
o flags=<F> A concatenated string of single letters that govern the
behavior of the node. Flags currently available are:
a Allow anyone to write to the node, if it is writable.
h ``Hidden''. sysctl must be invoked with -A or the hidden node
must be specifically requested in order to see it
i ``Immediate''. Makes the node store data in itself, rather than
allocating new space for it. This is the default for nodes of
type ``int'' and ``quad''. This is the opposite of owning data.
o ``Own''. When the node is created, separate space will be
allocated to store the data to be instrumented. This is the
default for nodes of type ``string'' and ``struct'' where it is
not possible to guarantee sufficient space to store the data in
the node itself.
p ``Private''. Nodes that are marked private, and children of
nodes so marked, are only viewable by the superuser. Be aware
that the immediate data that some nodes may store is not
necessarily protected by this.
x ``Hexadecimal''. Make sysctl default to hexadecimal display of
the retrieved value
r ``Read-only''. The data instrumented by the given node is read-
only. Note that other mechanisms may still exist for changing
the data. This is the default for nodes that instrument data.
w ``Writable''. The data instrumented by the given node is
writable at any time. This is the default for nodes that can
o value=<V> An initial starting value for a new node that does not
reference existing kernel data. Initial values can only be assigned
for nodes of the ``int'', ``quad'', and ``string'' types.
New nodes must fit the following set of criteria:
o If the new node is to address an existing kernel object, only one of
the ``symbol'' or ``addr'' arguments may be given.
o The size for a ``struct'' type node must be specified; no initial
value is expected or permitted.
o Either the size or the initial value for a ``string'' node must be
o The node which will be the parent of the new node must be writable.
If any of the given parameters describes an invalid configuration, sysctl
will emit a diagnostic message to the standard error and exit.
Descriptions can be added by the super-user to any node that does not
have one, provided that the node is not marked with the ``PERMANENT''
flag. The syntax is similar to the syntax for creating new nodes with
the exception of the keyword that follows the double separator at the
start of the command: ``//describe=new.node.path=new node description''.
Once a description has been added, it cannot be changed or removed.
When destroying nodes, only the path to the node is necessary, i.e.,
``//destroy=old.node.path''. No other parameters are expected or
permitted. Nodes being destroyed must have no children, and their parent
must be writable. Nodes that are marked with the ``PERMANENT'' flag (as
assigned by the kernel) may not be deleted.
In all cases, the initial `=' that follows the command (eg, ``create'',
``destroy'', or ``describe'') may be replaced with another instance of
the separator character, provided that the same separator character is
used for the length of the name specification.
/etc/sysctl.conf sysctl variables set at boot time
For example, to retrieve the maximum number of processes allowed in the
system, one would use the following request:
To set the maximum number of processes allowed in the system to 1000, one
would use the following request:
sysctl -w kern.maxproc=1000
Information about the system clock rate may be obtained with:
Information about the load average history may be obtained with:
To view the values of the per-process variables of the current shell, the
can be used if the shell interpreter replaces $$ with its pid (this is
true for most shells).
To redirect core dumps to the /var/tmp/<username> directory,
sysctl -w proc.$$.corename=/var/tmp/%u/%n.core
should be used.
sysctl -w proc.curproc.corename=/var/tmp/%u/%n.core
changes the value for the sysctl process itself, and will not have the
To create the root of a new sub-tree called ``local'' add some children
to the new node, and some descriptions:
sysctl -w //create=local
sysctl -w //describe=local=my local sysctl tree
sysctl -w //create=local.esm_debug,type=int,symbol=esm_debug,flags=w
sysctl -w //describe=local.esm_debug=esm driver debug knob
sysctl -w //create=local.audiodebug,type=int,symbol=audiodebug,flags=w
sysctl -w //describe=local.audiodebug=generic audio debug knob
Note that the children are made writable so that the two debug settings
in question can be tuned arbitrarily.
To destroy that same subtree:
sysctl -w //destroy=local.esm_debug
sysctl -w //destroy=local.audiodebug
sysctl -w //destroy=local
sysctl(3), ksyms(4), sysctl(7)
sysctl first appeared in 4.4BSD.
NetBSD 6.1.5 August 2, 2011 NetBSD 6.1.5