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NAME

sysctl — get or set kernel state

SYNOPSIS

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.

When retrieving a variable, a subset of the MIB name may be specified to retrieve a list of variables in that subset. For example, to list all the machdep variables:

When setting a variable, the MIB name should be followed by an equal sign and the new value.

The options are as follows:

-A

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.

-a

List all the currently available string or integer values. This is the default, if no parameters are given to sysctl.

-n

Suppress printing of the field name, only output the field value. Useful for setting shell variables. For example, to set the psize shell variable to the pagesize of the hardware:

# set psize=`sysctl -n hw.pagesize`

-q

Suppress all output when setting a variable. This option overrides the behaviour of -n.

name=value

Attempt to set the specified variable name to value.

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(1), systat(1), and netstat(1). The string and integer information is summarized below. For a detailed description of these variables, see sysctl(3). The changeable column indicates whether a process with appropriate privilege can change the value.

Note: not all of the variables are relevant to all architectures, and a few require a kernel compiled with non-standard options(4).

The sysctl program can extract information about the filesystems that have been compiled into the running system. This information can be obtained by using the command:

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.

FILES

⟨sys/sysctl.h⟩

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

⟨dev/rndvar.h⟩

definitions for random(4) device's statistics structure

⟨sys/socket.h⟩

definitions for second level network identifiers

⟨sys/gmon.h⟩

definitions for third level profiling identifiers

⟨uvm/uvm_param.h⟩

definitions for second level virtual memory identifiers

⟨uvm/uvm_swap_encrypt.h⟩

definitions for third level virtual memory identifiers

⟨netinet/in.h⟩

definitions for third level IPv4/v6 identifiers and fourth level IPv4/v6 identifiers

⟨netinet/ip_divert.h⟩

definitions for fourth level divert identifiers

⟨netinet/icmp_var.h⟩

definitions for fourth level ICMP identifiers

⟨netinet6/icmp6.h⟩

definitions for fourth level ICMPv6 identifiers

⟨netinet/tcp_var.h⟩

definitions for fourth level TCP identifiers

⟨netinet/udp_var.h⟩

definitions for fourth level UDP identifiers

⟨ddb/db_var.h⟩

definitions for second level ddb identifiers

⟨sys/mount.h⟩

definitions for second level vfs identifiers

⟨nfs/nfs.h⟩

definitions for third level NFS identifiers

⟨ufs/ffs/ffs_extern.h⟩

definitions for third level FFS identifiers

⟨machine/cpu.h⟩

definitions for second level CPU identifiers

EXAMPLES

To retrieve the maximum number of processes allowed in the system:

To set the maximum number of processes allowed in the system to 1000:

To retrieve information about the system clock rate:

To retrieve information about the load average history:

To make the chown(2) system call use traditional BSD semantics (don't clear setuid/setgid bits):

To set the list of reserved TCP ports that should not be allocated by the kernel dynamically:

This can be used to keep daemons from stealing a specific port that another program needs to function. List elements may be separated by commas and/or whitespace; a hyphen may be used to specify a range of ports.

It is also possible to add or remove ports from the current list:

# sysctl net.inet.tcp.baddynamic=+748,6000-6999
# sysctl net.inet.tcp.baddynamic=-871

To adjust the number of kernel nfsio threads used to service asynchronous I/O requests on an NFS client machine:

The default is 4; 20 is the maximum. See nfssvc(2) and nfsd(8) for further discussion.

To set the amount of shared memory available in the system and the maximum number of shared memory segments:

# sysctl kern.shminfo.shmmax=33554432
# sysctl kern.shminfo.shmseg=32

SEE ALSO

sysctl(3), options(4), sysctl.conf(5)

HISTORY

sysctl first appeared in 4.4BSD.