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25 .Dd September 29, 2021
30 .Nd kernel bootstrapping final stage
36 kernel bootstrapping process.
37 On IA32 (i386) architectures, it is a
40 It is linked statically to
42 and usually located in the directory
45 It provides a scripting language that can be used to
46 automate tasks, do pre-configuration or assist in recovery
48 This scripting language is roughly divided in
50 The smaller one is a set of commands
51 designed for direct use by the casual user, called "builtin
52 commands" for historical reasons.
53 The main drive behind these commands is user-friendliness.
55 During initialization,
57 will probe for a console and set the
59 variable, or set it to serial console
61 if the previous boot stage used that.
62 If multiple consoles are selected, they will be listed separated by spaces.
63 Then, devices are probed,
72 is processed if available.
73 These files are processed through the
75 command, which reads all of them into memory before processing them,
76 making disk changes possible.
80 has not been tried, and if
84 (not case sensitive), then an
87 If the system gets past this point,
91 will engage interactive mode.
92 Please note that historically even when
96 user will be able to interrupt autoboot process by pressing some key
97 on the console while kernel and modules are being loaded.
99 cases such behaviour may be undesirable, to prevent it set
105 will engage interactive mode only if
111 builtin commands take parameters from the command line.
113 the only way to call them from a script is by using
116 In the case of an error, an error message will be displayed and
117 the interpreter's state will be reset, emptying the stack and restoring
120 The builtin commands available are:
122 .Bl -tag -width Ds -compact
123 .It Ic autoboot Op Ar seconds Op Ar prompt
124 Proceeds to bootstrap the system after a number of seconds, if not
125 interrupted by the user.
126 Displays a countdown prompt
127 warning the user the system is about to be booted,
128 unless interrupted by a key press.
129 The kernel will be loaded first if necessary.
130 Defaults to 10 seconds.
133 Displays statistics about disk cache usage.
137 .It Ic boot Ar kernelname Op Cm ...
138 .It Ic boot Fl flag Cm ...
139 Immediately proceeds to bootstrap the system, loading the kernel
141 Any flags or arguments are passed to the kernel, but they
142 must precede the kernel name, if a kernel name is provided.
148 Displays text on the screen.
149 A new line will be printed unless
154 Displays memory usage statistics.
155 For debugging purposes only.
157 .It Ic help Op topic Op subtopic
158 Shows help messages read from
159 .Pa /boot/loader.help .
162 will list the topics available.
164 .It Ic include Ar file Op Ar
165 Process script files.
166 Each file, in turn, is completely read into memory,
167 and then each of its lines is passed to the command line interpreter.
168 If any error is returned by the interpreter, the include
169 command aborts immediately, without reading any other files, and
170 returns an error itself (see
177 Loads a kernel, kernel loadable module (kld), disk image,
178 or file of opaque contents tagged as being of the type
180 Kernel and modules can be either in a.out or ELF format.
181 Any arguments passed after the name of the file to be loaded
182 will be passed as arguments to that file.
185 type to make the kernel create a file-backed
188 This is useful for booting from a temporary rootfs.
189 Currently, argument passing does not work for the kernel.
197 encryption keyfile for the given provider name.
198 The key index can be specified via
200 or will default to zero.
206 Displays a listing of files in the directory
208 or the root directory if
213 is specified, file sizes will be shown too.
216 Lists all of the devices from which it may be possible to load modules,
217 as well as ZFS pools.
220 is specified, more details are printed, including ZFS pool information
221 in a format that resembles
226 Displays loaded modules.
229 is specified, more details are shown.
231 .It Ic lszfs Ar filesystem
232 A ZFS extended command that can be used to explore the ZFS filesystem
234 Lists the immediate children of the
236 The filesystem hierarchy is rooted at a filesystem with the same name
239 .It Ic more Ar file Op Ar
240 Display the files specified, with a pause at each
244 .It Ic pnpscan Op Fl v
245 Scans for Plug-and-Play devices.
246 This is not functional at present.
253 Reads a line of input from the terminal, storing it in
256 A timeout can be specified with
258 though it will be canceled at the first key pressed.
259 A prompt may also be displayed through the
264 Immediately reboots the system.
266 .It Ic set Ar variable
267 .It Ic set Ar variable Ns = Ns Ar value
268 Set loader's environment variables.
270 .It Ic show Op Va variable
271 Displays the specified variable's value, or all variables and their
277 Remove all modules from memory.
279 .It Ic unset Va variable
282 from the environment.
285 Lists available commands.
287 .Ss BUILTIN ENVIRONMENT VARIABLES
288 Environment variables can be set and unset through the
292 builtins, and can have their values interactively examined through the
296 Their values can also be accessed as described in
299 Notice that these environment variables are not inherited by any shell
300 after the system has been booted.
302 A few variables are set automatically by
304 Others can affect the behavior of either
306 or the kernel at boot.
307 Some options may require a value,
308 while others define behavior just by being set.
309 Both types of builtin variables are described below.
310 .Bl -tag -width bootfile
311 .It Va autoboot_delay
314 will wait before booting.
315 Configuration options are described in
318 Instructs the kernel to prompt the user for the name of the root device
319 when the kernel is booted.
321 Instructs the kernel to try to mount the root file system from CD-ROM.
323 Instructs the kernel to start in the DDB debugger, rather than
324 proceeding to initialize when booted.
326 Instructs the kernel to mount the statically compiled-in root file system.
328 Selects gdb-remote mode for the kernel debugger by default.
329 .It Va boot_multicons
330 Enables multiple console support in the kernel early on boot.
331 In a running system, console configuration can be manipulated
336 All kernel console output is suppressed when console is muted.
337 In a running system, the state of console muting can be manipulated by the
341 During the device probe, pause after each line is printed.
343 Force the use of a serial console even when an internal console
346 Prevents the kernel from initiating a multi-user startup; instead,
347 a single-user mode will be entered when the kernel has finished
350 Setting this variable causes extra debugging information to be printed
351 by the kernel during the boot phase.
353 List of semicolon-separated search path for bootable kernels.
356 .It Va comconsole_speed
357 Defines the speed of the serial console (i386 and amd64 only).
358 If the previous boot stage indicated that a serial console is in use
359 then this variable is initialized to the current speed of the console
361 Otherwise it is set to 115200 unless this was overridden using the
362 .Va BOOT_COMCONSOLE_SPEED
368 variable take effect immediately.
369 .It Va comconsole_port
370 Defines the base i/o port used to access console UART
371 (i386 and amd64 only).
372 If the variable is not set, its assumed value is 0x3F8, which
373 corresponds to PC port COM1, unless overridden by
374 .Va BOOT_COMCONSOLE_PORT
375 variable during the compilation of
379 variable automatically set
381 environment variable to provide a hint to kernel for location of the console.
382 Loader console is changed immediately after variable
385 .It Va comconsole_pcidev
386 Defines the location of a PCI device of the 'simple communication'
387 class to be used as the serial console UART (i386 and amd64 only).
388 The syntax of the variable is
389 .Li 'bus:device:function[:bar]' ,
390 where all members must be numeric, with possible
392 prefix to indicate a hexadecimal value.
395 member is optional and assumed to be 0x10 if omitted.
396 The bar must decode i/o space.
398 .Va comconsole_pcidev
399 automatically sets the variable
401 to the base of the selected bar, and hint
402 .Va hw.uart.console .
403 Loader console is changed immediately after variable
404 .Va comconsole_pcidev
407 Defines the current console or consoles.
408 Multiple consoles may be specified.
409 In that case, the first listed console will become the default console for
410 userland output (e.g.\& from
413 Selects the default device to loader the kernel from.
415 .Dl Ic loader_device:
420 .Dl Ic zfs:zroot/ROOT/default:
422 Sets the device for kernel dumps.
423 This can be used to ensure that a device is configured before the corresponding
427 has been processed, allowing kernel panics that happen during the early stages
428 of boot to be captured.
436 Sets the list of binaries which the kernel will try to run as the initial
438 The first matching binary is used.
440 .Dq Li /sbin/init:/sbin/oinit:/sbin/init.bak:\:/rescue/init .
450 if the Forth's current state is interpreting.
452 Define the number of lines on the screen, to be used by the pager.
454 Sets the list of directories which will be searched for modules
455 named in a load command or implicitly required by a dependency.
456 The default value for this variable is
457 .Dq Li /boot/kernel;/boot/modules .
459 Sets the number of IDE disks as a workaround for some problems in
460 finding the root disk at boot.
461 This has been deprecated in favor of
468 .Dq Li "${interpret}" .
471 is unset, the default prompt is
473 .It Va root_disk_unit
474 If the code which detects the disk unit number for the root disk is
475 confused, e.g.\& by a mix of SCSI and IDE disks, or IDE disks with
476 gaps in the sequence (e.g.\& no primary slave), the unit number can
477 be forced by setting this variable.
479 By default the value of
481 is used to set the root file system
482 when the kernel is booted.
483 This can be overridden by setting
488 Other variables are used to override kernel tunable parameters.
489 The following tunables are available:
491 .It Va efi.rt.disabled
492 Disable UEFI runtime services in the kernel, if applicable.
493 Runtime services are only available and used if the kernel is booted in a UEFI
496 Limit the amount of physical memory the system will use.
497 By default the size is in bytes, but the
498 .Cm k , K , m , M , g
502 are also accepted and indicate kilobytes, megabytes and gigabytes
504 An invalid suffix will result in the variable being ignored by the
506 .It Va hw.pci.host_start_mem , hw.acpi.host_start_mem
507 When not otherwise constrained, this limits the memory start
509 The default is 0x80000000 and should be set to at least size of the
510 memory and not conflict with other resources.
511 Typically, only systems without PCI bridges need to set this variable
512 since PCI bridges typically constrain the memory starting address
513 (and the variable is only used when bridges do not constrain this
515 .It Va hw.pci.enable_io_modes
516 Enable PCI resources which are left off by some BIOSes or are not
517 enabled correctly by the device driver.
518 Tunable value set to ON (1) by default, but this may cause problems
519 with some peripherals.
521 Set the size of a number of statically allocated system tables; see
523 for a description of how to select an appropriate value for this
525 When set, this tunable replaces the value declared in the kernel
526 compile-time configuration file.
527 .It Va kern.ipc.nmbclusters
528 Set the number of mbuf clusters to be allocated.
529 The value cannot be set below the default
530 determined when the kernel was compiled.
531 .It Va kern.ipc.nsfbufs
534 buffers to be allocated.
537 Not all architectures use such buffers; see
540 .It Va kern.maxswzone
541 Limits the amount of KVM to be used to hold swap
542 metadata, which directly governs the
543 maximum amount of swap the system can support,
544 at the rate of approximately 200 MB of swap space
545 per 1 MB of metadata.
546 This value is specified in bytes of KVA space.
547 If no value is provided, the system allocates
548 enough memory to handle an amount of swap
549 that corresponds to eight times the amount of
550 physical memory present in the system.
552 Note that swap metadata can be fragmented,
553 which means that the system can run out of
554 space before it reaches the theoretical limit.
555 Therefore, care should be taken to not configure
556 more swap than approximately half of the
559 Running out of space for swap metadata can leave
560 the system in an unrecoverable state.
561 Therefore, you should only change
562 this parameter if you need to greatly extend the
563 KVM reservation for other resources such as the
565 .Va kern.ipc.nmbclusters .
566 Modifies kernel option
567 .Dv VM_SWZONE_SIZE_MAX .
568 .It Va kern.maxbcache
569 Limits the amount of KVM reserved for use by the
570 buffer cache, specified in bytes.
571 The default maximum is 200MB on i386,
573 This parameter is used to
574 prevent the buffer cache from eating too much
575 KVM in large-memory machine configurations.
576 Only mess around with this parameter if you need to
577 greatly extend the KVM reservation for other resources
578 such as the swap zone or
579 .Va kern.ipc.nmbclusters .
581 the NBUF parameter will override this limit.
583 .Dv VM_BCACHE_SIZE_MAX .
584 .It Va kern.msgbufsize
585 Sets the size of the kernel message buffer.
586 The default limit of 96KB is usually sufficient unless
587 large amounts of trace data need to be collected
588 between opportunities to examine the buffer or
590 Overrides kernel option
592 .It Va machdep.disable_mtrrs
593 Disable the use of i686 MTRRs (x86 only).
594 .It Va net.inet.tcp.tcbhashsize
595 Overrides the compile-time set value of
597 or the preset default of 512.
598 Must be a power of 2.
599 .It Va twiddle_divisor
600 Throttles the output of the
602 I/O progress indicator displayed while loading the kernel and modules.
603 This is useful on slow serial consoles where the time spent waiting for
604 these characters to be written can add up to many seconds.
605 The default is 16; a value of 32 spins half as fast,
606 while a value of 8 spins twice as fast.
608 Sets the size of kernel memory (bytes).
609 This overrides the value determined when the kernel was compiled.
612 .It Va vm.kmem_size_min
613 .It Va vm.kmem_size_max
614 Sets the minimum and maximum (respectively) amount of kernel memory
615 that will be automatically allocated by the kernel.
616 These override the values determined when the kernel was compiled.
620 .Dv VM_KMEM_SIZE_MAX .
624 supports the following format for specifying ZFS filesystems which
627 refers to a device specification:
629 .Ar zfs:pool/filesystem:
633 is a ZFS filesystem name as described in
638 does not have an entry for the root filesystem and
639 .Va vfs.root.mountfrom
642 refers to a ZFS filesystem, then
644 will instruct kernel to use that filesystem as the root filesystem.
648 command line provides several ways of compromising system security,
649 including, but not limited to:
653 Booting from removable storage.
655 One can prevent unauthorized access
658 command line by booting unconditionally in
660 In order for this to be effective, one should also configure the firmware
661 (BIOS or UEFI) to prevent booting from unauthorized devices.
663 .Bl -tag -width /boot/loader_simp -compact
664 .It Pa /boot/loader_simp
667 .It Pa /boot/loader.rc
672 Boot in single user mode:
676 Load the kernel, a splash screen, and then autoboot in five seconds.
677 Notice that a kernel must be loaded before any other
679 command is attempted.
680 .Bd -literal -offset indent
683 load -t splash_image_data /boot/chuckrulez.bmp
687 Set the disk unit of the root device to 2, and then boot.
688 This would be needed in a system with two IDE disks,
689 with the second IDE disk hardwired to ada2 instead of ada1.
690 .Bd -literal -offset indent
692 boot /boot/kernel/kernel
695 Set the default device used for loading a kernel from a ZFS filesystem:
696 .Bd -literal -offset indent
697 set currdev=zfs:tank/ROOT/knowngood:
701 The following values are thrown by
703 .Bl -tag -width XXXXX -offset indent
705 Any type of error in the processing of a builtin.
716 Out of interpreting text.
718 Need more text to succeed -- will finish on next run.
741 .An Michael Smith Aq msmith@FreeBSD.org .
projects / FreeBSD / FreeBSD.git / blob