sysctl



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


NAME

     sysctl - get or set kernel state


SYNOPSIS

     sysctl [-Aanq] [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.

     -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.

     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


SEE ALSO

     sysctl(2), options(4), sysctl.conf(5)


HISTORY

     sysctl first appeared in 4.4BSD.

OpenBSD 6.4                    February 16, 2018                   OpenBSD 6.4

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