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SYSCTL(8) FreeBSD System Manager's Manual SYSCTL(8)
NAME sysctl - get or set kernel state
SYNOPSIS sysctl [-bdeFhilNnoTtqWx] [-B bufsize] [-f filename] name[=value[,value]] ... sysctl [-bdeFhlNnoTtqWx] [-B bufsize] -a
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.
The following options are available:
-A Equivalent to -o -a (for compatibility).
-a List all the currently available values except for those which are opaque or excluded from listing via the CTLFLAG_SKIP flag. This option is ignored if one or more variable names are specified on the command line.
-b Force the value of the variable(s) to be output in raw, binary format. No names are printed and no terminating newlines are output. This is mostly useful with a single variable.
-B bufsize Set the buffer size to read from the sysctl to bufsize. This is necessary for a sysctl that has variable length, and the probe value of 0 is a valid length, such as kern.arandom.
-d Print the description of the variable instead of its value.
-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 either -N or -n is specified, or a variable is being set.
-f filename Specify a file which contains a pair of name and value in each line. sysctl reads and processes the specified file first and then processes the name and value pairs in the command line argument.
-F Print the format of the variable. This is additional information to describe the type of the variable and most useful with struct types such as clockinfo, timeval, and loadavg.
-h Format output for human, rather than machine, readability.
-i Ignore unknown OIDs. The purpose is to make use of sysctl for collecting data from a variety of machines (not all of which are necessarily running exactly the same software) easier.
-l Show the length of variables along with their values. This option cannot be combined with the -N option. compctl -K listsysctls sysctl
To enable completion of variable names in tcsh(1), use:
complete sysctl 'n/*/`sysctl -Na`/'
-n Do not show variable names. This option is useful for setting shell variables. For instance, to save the pagesize in variable psize, use:
set psize=`sysctl -n hw.pagesize`
-o Show opaque variables (which are normally suppressed). The format and length are printed, as well as a hex dump of the first sixteen bytes of the value.
-q Suppress some warnings generated by sysctl to standard error.
-T Display only variables that are settable via loader (CTLFLAG_TUN).
-t Print the type of the variable.
-W Display only writable variables that are not statistical. Useful for determining the set of runtime tunable sysctls.
-X Equivalent to -x -a (for compatibility).
-x As -o, but prints a hex dump of the entire value instead of just the first few bytes.
The information available from sysctl consists of integers, strings, and opaque types. The sysctl utility only knows about a couple of opaque types, and will resort to hexdumps for the rest. The opaque information is much more useful if retrieved by special purpose programs such as ps(1), systat(1), and netstat(1).
Some of the variables which cannot be modified during normal system operation can be initialized via loader(8) tunables. This can for example be done by setting them in loader.conf(5). Please refer to loader.conf(5) for more information on which tunables are available and how to set them.
The string and integer information is summarized below. For a detailed description of these variables see sysctl(3) and security(7).
The changeable column indicates whether a process with appropriate privilege can change the value. String and integer values can be set using sysctl.
Name 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 no kern.maxprocperuid integer yes kern.maxfiles integer yes kern.ngroups integer no kern.job_control integer no kern.saved_ids integer no kern.boottime struct no kern.domainname string yes kern.filedelay integer yes kern.dirdelay integer yes kern.metadelay integer yes kern.osreldate integer no kern.bootfile string yes kern.corefile string yes kern.logsigexit integer yes security.bsd.suser_enabled integer yes security.bsd.see_other_uids integer yes security.bsd.see_other_gids integer yes security.bsd.see_jail_proc integer yes security.bsd.unprivileged_proc_debug integer yes security.bsd.unprivileged_read_msgbuf integer yes vm.loadavg struct no 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 hw.floatingpoint integer no hw.machine_arch string no hw.realmem integer no machdep.adjkerntz integer yes machdep.disable_rtc_set integer yes machdep.guessed_bootdev string 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 user.stream_max integer no user.tzname_max integer no user.localbase string no
FILES <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 <netinet/udp_var.h> definitions for fourth level UDP identifiers
EXIT STATUS The sysctl utility exits 0 on success, and >0 if an error occurs.
EXAMPLES For example, to retrieve the maximum number of processes allowed in the system, one would use the following request:
sysctl kern.maxproc
To set the maximum number of processes allowed per uid to 1000, one would use the following request:
sysctl kern.maxprocperuid=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
More variables than these exist, and the best and likely only place to search for their deeper meaning is undoubtedly the source where they are defined.
COMPATIBILITY The -w option has been deprecated and is silently ignored.
SEE ALSO sysctl(3), loader.conf(5), sysctl.conf(5), security(7,) loader(8)
HISTORY A sysctl utility first appeared in 4.4BSD.
In FreeBSD 2.2, sysctl was significantly remodeled.
BUGS The sysctl utility presently exploits an undocumented interface to the kernel sysctl(9) facility to traverse the sysctl tree and to retrieve format and name information. This correct interface is being thought about for the time being.
FreeBSD 14.0-RELEASE-p11 August 18, 2023 FreeBSD 14.0-RELEASE-p11