The Linux kernel accepts certain 'command-line options' or 'boot
time parameters' at the moment it is started. In general, this
is used to supply the kernel with information about hardware
parameters that the kernel would not be able to determine on its
own, or to avoid/override the values that the kernel would
otherwise detect.
When the kernel is booted directly by the BIOS, you have no
opportunity to specify any parameters. So, in order to take
advantage of this possibility you have to use a boot loader that
is able to pass parameters, such as GRUB.
The argument list
The kernel command line is parsed into a list of strings (boot
arguments) separated by spaces. Most of the boot arguments have
the form:
name[=value_1][,value_2]...[,value_10]
where 'name' is a unique keyword that is used to identify what
part of the kernel the associated values (if any) are to be given
to. Note the limit of 10 is real, as the present code handles
only 10 comma separated parameters per keyword. (However, you
can reuse the same keyword with up to an additional 10 parameters
in unusually complicated situations, assuming the setup function
supports it.)
Most of the sorting is coded in the kernel source file
init/main.c. First, the kernel checks to see if the argument is
any of the special arguments 'root=', 'nfsroot=', 'nfsaddrs=',
'ro', 'rw', 'debug', or 'init'. The meaning of these special
arguments is described below.
Then it walks a list of setup functions to see if the specified
argument string (such as 'foo') has been associated with a setup
function ('foo_setup()') for a particular device or part of the
kernel. If you passed the kernel the line foo=3,4,5,6 then the
kernel would search the bootsetups array to see if 'foo' was
registered. If it was, then it would call the setup function
associated with 'foo' (foo_setup()) and hand it the arguments 3,
4, 5, and 6 as given on the kernel command line.
Anything of the form 'foo=bar' that is not accepted as a setup
function as described above is then interpreted as an environment
variable to be set. A (useless?) example would be to use
'TERM=vt100' as a boot argument.
Any remaining arguments that were not picked up by the kernel and
were not interpreted as environment variables are then passed
onto PID 1, which is usually the init(1) program. The most
common argument that is passed to the init process is the word
'single' which instructs it to boot the computer in single user
mode, and not launch all the usual daemons. Check the manual
page for the version of init(1) installed on your system to see
what arguments it accepts.
General non-device-specific boot arguments
'init=...'
This sets the initial command to be executed by the
kernel. If this is not set, or cannot be found, the
kernel will try /sbin/init, then /etc/init, then
/bin/init, then /bin/sh and panic if all of this fails.
'nfsaddrs=...'
This sets the NFS boot address to the given string. This
boot address is used in case of a net boot.
'nfsroot=...'
This sets the NFS root name to the given string. If this
string does not begin with '/' or ',' or a digit, then it
is prefixed by '/tftpboot/'. This root name is used in
case of a net boot.
'root=...'
This argument tells the kernel what device is to be used
as the root filesystem while booting. The default of this
setting is determined at compile time, and usually is the
value of the root device of the system that the kernel was
built on. To override this value, and select the second
floppy drive as the root device, one would use
'root=/dev/fd1'.
The root device can be specified symbolically or
numerically. A symbolic specification has the form
/dev/XXYN, where XX designates the device type (e.g., 'hd'
for ST-506 compatible hard disk, with Y in 'a'–'d'; 'sd'
for SCSI compatible disk, with Y in 'a'–'e'), Y the driver
letter or number, and N the number (in decimal) of the
partition on this device.
Note that this has nothing to do with the designation of
these devices on your filesystem. The '/dev/' part is
purely conventional.
The more awkward and less portable numeric specification
of the above possible root devices in major/minor format
is also accepted. (For example, /dev/sda3 is major 8,
minor 3, so you could use 'root=0x803' as an alternative.)
'rootdelay='
This parameter sets the delay (in seconds) to pause before
attempting to mount the root filesystem.
'rootflags=...'
This parameter sets the mount option string for the root
filesystem (see also fstab(5)).
'rootfstype=...'
The 'rootfstype' option tells the kernel to mount the root
filesystem as if it where of the type specified. This can
be useful (for example) to mount an ext3 filesystem as
ext2 and then remove the journal in the root filesystem,
in fact reverting its format from ext3 to ext2 without the
need to boot the box from alternate media.
'ro' and 'rw'
The 'ro' option tells the kernel to mount the root
filesystem as 'read-only' so that filesystem consistency
check programs (fsck) can do their work on a quiescent
filesystem. No processes can write to files on the
filesystem in question until it is 'remounted' as
read/write capable, for example, by 'mount -w -n -o
remount /'. (See also mount(8).)
The 'rw' option tells the kernel to mount the root
filesystem read/write. This is the default.
'resume=...'
This tells the kernel the location of the suspend-to-disk
data that you want the machine to resume from after
hibernation. Usually, it is the same as your swap
partition or file. Example:
resume=/dev/hda2
'reserve=...'
This is used to protect I/O port regions from probes. The
form of the command is:
reserve=iobase,extent[,iobase,extent]...
In some machines it may be necessary to prevent device
drivers from checking for devices (auto-probing) in a
specific region. This may be because of hardware that
reacts badly to the probing, or hardware that would be
mistakenly identified, or merely hardware you don't want
the kernel to initialize.
The reserve boot-time argument specifies an I/O port
region that shouldn't be probed. A device driver will not
probe a reserved region, unless another boot argument
explicitly specifies that it do so.
For example, the boot line
reserve=0x300,32 blah=0x300
keeps all device drivers except the driver for 'blah' from
probing 0x300-0x31f.
'panic=N'
By default, the kernel will not reboot after a panic, but
this option will cause a kernel reboot after N seconds (if
N is greater than zero). This panic timeout can also be
set by
echo N > /proc/sys/kernel/panic
'reboot=[warm|cold][,[bios|hard]]'
Since Linux 2.0.22, a reboot is by default a cold reboot.
One asks for the old default with 'reboot=warm'. (A cold
reboot may be required to reset certain hardware, but
might destroy not yet written data in a disk cache. A
warm reboot may be faster.) By default, a reboot is hard,
by asking the keyboard controller to pulse the reset line
low, but there is at least one type of motherboard where
that doesn't work. The option 'reboot=bios' will instead
jump through the BIOS.
'nosmp' and 'maxcpus=N'
(Only when __SMP__ is defined.) A command-line option of
'nosmp' or 'maxcpus=0' will disable SMP activation
entirely; an option 'maxcpus=N' limits the maximum number
of CPUs activated in SMP mode to N.
Boot arguments for use by kernel developers
'debug'
Kernel messages are handed off to a daemon (e.g., klogd(8)
or similar) so that they may be logged to disk. Messages
with a priority above console_loglevel are also printed on
the console. (For a discussion of log levels, see
syslog(2).) By default, console_loglevel is set to log
messages at levels higher than KERN_DEBUG. This boot
argument will cause the kernel to also print messages
logged at level KERN_DEBUG. The console loglevel can also
be set on a booted system via the /proc/sys/kernel/printk
file (described in syslog(2)), the syslog(2)
SYSLOG_ACTION_CONSOLE_LEVEL operation, or dmesg(8).
'profile=N'
It is possible to enable a kernel profiling function, if
one wishes to find out where the kernel is spending its
CPU cycles. Profiling is enabled by setting the variable
prof_shift to a nonzero value. This is done either by
specifying CONFIG_PROFILE at compile time, or by giving
the 'profile=' option. Now the value that prof_shift gets
will be N, when given, or CONFIG_PROFILE_SHIFT, when that
is given, or 2, the default. The significance of this
variable is that it gives the granularity of the
profiling: each clock tick, if the system was executing
kernel code, a counter is incremented:
profile[address >> prof_shift]++;
The raw profiling information can be read from
/proc/profile. Probably you'll want to use a tool such as
readprofile.c to digest it. Writing to /proc/profile will
clear the counters.
Boot arguments for ramdisk use
(Only if the kernel was compiled with CONFIG_BLK_DEV_RAM.) In
general it is a bad idea to use a ramdisk under Linux—the system
will use available memory more efficiently itself. But while
booting, it is often useful to load the floppy contents into a
ramdisk. One might also have a system in which first some
modules (for filesystem or hardware) must be loaded before the
main disk can be accessed.
In Linux 1.3.48, ramdisk handling was changed drastically.
Earlier, the memory was allocated statically, and there
was a 'ramdisk=N' parameter to tell its size. (This could
also be set in the kernel image at compile time.) These
days ram disks use the buffer cache, and grow dynamically.
For a lot of information on the current ramdisk setup, see
the kernel source file Documentation/blockdev/ramdisk.txt
(Documentation/ramdisk.txt in older kernels).
There are four parameters, two boolean and two integral.
'load_ramdisk=N'
If N=1, do load a ramdisk. If N=0, do not load a ramdisk.
(This is the default.)
'prompt_ramdisk=N'
If N=1, do prompt for insertion of the floppy. (This is
the default.) If N=0, do not prompt. (Thus, this
parameter is never needed.)
'ramdisk_size=N' or (obsolete) 'ramdisk=N'
Set the maximal size of the ramdisk(s) to N kB. The
default is 4096 (4 MB).
'ramdisk_start=N'
Sets the starting block number (the offset on the floppy
where the ramdisk starts) to N. This is needed in case
the ramdisk follows a kernel image.
'noinitrd'
(Only if the kernel was compiled with CONFIG_BLK_DEV_RAM
and CONFIG_BLK_DEV_INITRD.) These days it is possible to
compile the kernel to use initrd. When this feature is
enabled, the boot process will load the kernel and an
initial ramdisk; then the kernel converts initrd into a
"normal" ramdisk, which is mounted read-write as root
device; then /linuxrc is executed; afterward the "real"
root filesystem is mounted, and the initrd filesystem is
moved over to /initrd; finally the usual boot sequence
(e.g., invocation of /sbin/init) is performed.
For a detailed description of the initrd feature, see the
kernel source file Documentation/admin-guide/initrd.rst
(or Documentation/initrd.txt before Linux 4.10).
The 'noinitrd' option tells the kernel that although it
was compiled for operation with initrd, it should not go
through the above steps, but leave the initrd data under
/dev/initrd. (This device can be used only once: the data
is freed as soon as the last process that used it has
closed /dev/initrd.)
Boot arguments for SCSI devices
General notation for this section:
iobase -- the first I/O port that the SCSI host occupies. These
are specified in hexadecimal notation, and usually lie in the
range from 0x200 to 0x3ff.
irq -- the hardware interrupt that the card is configured to use.
Valid values will be dependent on the card in question, but will
usually be 5, 7, 9, 10, 11, 12, and 15. The other values are
usually used for common peripherals like IDE hard disks,
floppies, serial ports, and so on.
scsi-id -- the ID that the host adapter uses to identify itself
on the SCSI bus. Only some host adapters allow you to change
this value, as most have it permanently specified internally.
The usual default value is 7, but the Seagate and Future Domain
TMC-950 boards use 6.
parity -- whether the SCSI host adapter expects the attached
devices to supply a parity value with all information exchanges.
Specifying a one indicates parity checking is enabled, and a zero
disables parity checking. Again, not all adapters will support
selection of parity behavior as a boot argument.
'max_scsi_luns=...'
A SCSI device can have a number of 'subdevices' contained
within itself. The most common example is one of the new
SCSI CD-ROMs that handle more than one disk at a time.
Each CD is addressed as a 'Logical Unit Number' (LUN) of
that particular device. But most devices, such as hard
disks, tape drives, and such are only one device, and will
be assigned to LUN zero.
Some poorly designed SCSI devices cannot handle being
probed for LUNs not equal to zero. Therefore, if the
compile-time flag CONFIG_SCSI_MULTI_LUN is not set, newer
kernels will by default probe only LUN zero.
To specify the number of probed LUNs at boot, one enters
'max_scsi_luns=n' as a boot arg, where n is a number
between one and eight. To avoid problems as described
above, one would use n=1 to avoid upsetting such broken
devices.
SCSI tape configuration
Some boot time configuration of the SCSI tape driver can
be achieved by using the following:
st=buf_size[,write_threshold[,max_bufs]]
The first two numbers are specified in units of kB. The
default buf_size is 32k B, and the maximum size that can
be specified is a ridiculous 16384 kB. The
write_threshold is the value at which the buffer is
committed to tape, with a default value of 30 kB. The
maximum number of buffers varies with the number of drives
detected, and has a default of two. An example usage
would be:
st=32,30,2
Full details can be found in the file
Documentation/scsi/st.txt (or drivers/scsi/README.st for
older kernels) in the Linux kernel source.
Hard disks
IDE Disk/CD-ROM Driver Parameters
The IDE driver accepts a number of parameters, which range
from disk geometry specifications, to support for broken
controller chips. Drive-specific options are specified by
using 'hdX=' with X in 'a'–'h'.
Non-drive-specific options are specified with the prefix
'hd='. Note that using a drive-specific prefix for a non-
drive-specific option will still work, and the option will
just be applied as expected.
Also note that 'hd=' can be used to refer to the next
unspecified drive in the (a, ..., h) sequence. For the
following discussions, the 'hd=' option will be cited for
brevity. See the file Documentation/ide/ide.txt (or
Documentation/ide.txt in older kernels, or
drivers/block/README.ide in ancient kernels) in the Linux
kernel source for more details.
The 'hd=cyls,heads,sects[,wpcom[,irq]]' options
These options are used to specify the physical geometry of
the disk. Only the first three values are required. The
cylinder/head/sectors values will be those used by fdisk.
The write precompensation value is ignored for IDE disks.
The IRQ value specified will be the IRQ used for the
interface that the drive resides on, and is not really a
drive-specific parameter.
The 'hd=serialize' option
The dual IDE interface CMD-640 chip is broken as designed
such that when drives on the secondary interface are used
at the same time as drives on the primary interface, it
will corrupt your data. Using this option tells the
driver to make sure that both interfaces are never used at
the same time.
The 'hd=noprobe' option
Do not probe for this drive. For example,
hdb=noprobe hdb=1166,7,17
would disable the probe, but still specify the drive
geometry so that it would be registered as a valid block
device, and hence usable.
The 'hd=nowerr' option
Some drives apparently have the WRERR_STAT bit stuck on
permanently. This enables a work-around for these broken
devices.
The 'hd=cdrom' option
This tells the IDE driver that there is an ATAPI
compatible CD-ROM attached in place of a normal IDE hard
disk. In most cases the CD-ROM is identified
automatically, but if it isn't then this may help.
Standard ST-506 Disk Driver Options ('hd=')
The standard disk driver can accept geometry arguments for
the disks similar to the IDE driver. Note however that it
expects only three values (C/H/S); any more or any less
and it will silently ignore you. Also, it accepts only
'hd=' as an argument, that is, 'hda=' and so on are not
valid here. The format is as follows:
hd=cyls,heads,sects
If there are two disks installed, the above is repeated
with the geometry parameters of the second disk.
Ethernet devices
Different drivers make use of different parameters, but they all
at least share having an IRQ, an I/O port base value, and a name.
In its most generic form, it looks something like this:
ether=irq,iobase[,param_1[,...param_8]],name
The first nonnumeric argument is taken as the name. The param_n
values (if applicable) usually have different meanings for each
different card/driver. Typical param_n values are used to
specify things like shared memory address, interface selection,
DMA channel and the like.
The most common use of this parameter is to force probing for a
second ethercard, as the default is to probe only for one. This
can be accomplished with a simple:
ether=0,0,eth1
Note that the values of zero for the IRQ and I/O base in the
above example tell the driver(s) to autoprobe.
The Ethernet-HowTo has extensive documentation on using multiple
cards and on the card/driver-specific implementation of the
param_n values where used. Interested readers should refer to
the section in that document on their particular card.
The floppy disk driver
There are many floppy driver options, and they are all listed in
Documentation/blockdev/floppy.txt (or Documentation/floppy.txt in
older kernels, or drivers/block/README.fd for ancient kernels) in
the Linux kernel source. See that file for the details.
The sound driver
The sound driver can also accept boot arguments to override the
compiled-in values. This is not recommended, as it is rather
complex. It is described in the Linux kernel source file
Documentation/sound/oss/README.OSS (drivers/sound/Readme.linux in
older kernel versions). It accepts a boot argument of the form:
sound=device1[,device2[,device3...[,device10]]]
where each deviceN value is of the following format 0xTaaaId and
the bytes are used as follows:
T - device type: 1=FM, 2=SB, 3=PAS, 4=GUS, 5=MPU401, 6=SB16,
7=SB16-MPU401
aaa - I/O address in hex.
I - interrupt line in hex (i.e., 10=a, 11=b, ...)
d - DMA channel.
As you can see, it gets pretty messy, and you are better off to
compile in your own personal values as recommended. Using a boot
argument of 'sound=0' will disable the sound driver entirely.
The line printer driver
'lp='
Syntax:
lp=0
lp=auto
lp=reset
lp=port[,port...]
You can tell the printer driver what ports to use and what
ports not to use. The latter comes in handy if you don't
want the printer driver to claim all available parallel
ports, so that other drivers (e.g., PLIP, PPA) can use
them instead.
The format of the argument is multiple port names. For
example, lp=none,parport0 would use the first parallel
port for lp1, and disable lp0. To disable the printer
driver entirely, one can use lp=0.
