NAME

sysctl — get or set kernel state

SYNOPSIS

sysctl [-Aanq] [-f file] [name[=value] ...]

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, using 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:

$ sysctl machdep

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.
-f file: Process name[=value] arguments from file in sysctl.conf(5) format.
-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]: Retrieve the specified variable name, or attempt to set it to value. Multiple name[=value] arguments may be given. If given, -f file is processed first.

The information available from sysctl consists of integers, strings, and tables. For a detailed description of the variables, see sysctl(2). Tables can only be retrieved by special purpose programs such as ps(1), systat(1), and netstat(1).

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.mounts

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

/etc/sysctl.conf: sysctl variables to set at system startup

EXAMPLES

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

$ sysctl kern.maxproc

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

# sysctl
  kern.maxproc=1000

To retrieve information about the system clock rate:

$ sysctl kern.clockrate

To retrieve information about the load average history:

$ sysctl vm.loadavg

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

# sysctl
  fs.posix.setuid=0

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

# sysctl
  net.inet.tcp.baddynamic=749,750,751,760,761,871

# sysctl
  net.inet.udp.baddynamic=749,750,751,760,761,871,1024-2048

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 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

To place core dumps from issetugid(2) programs (in this example bgpd(8)) into a safe place for debugging purposes:

# mkdir -m 700 /var/crash/bgpd
# sysctl kern.nosuidcoredump=3

HISTORY

sysctl first appeared in 4.4BSD.