Computer Science
SFDISK(8) Linux Programmer's Manual SFDISK(8)
NAME
sfdisk - Partition table manipulator for Linux
SYNOPSIS
sfdisk [options] device
sfdisk -s [partition]
DESCRIPTION
sfdisk has four (main) uses: list the size of a partition,
list the partitions on a device, check the partitions on a
device, and - very dangerous - repartition a device.
List Sizes
sfdisk -s partition gives the size of partition in blocks.
This may be useful in connection with programs like mkswap
or so. Here partition is usually something like /dev/hda1
or /dev/sdb12, but may also be an entire disk, like
/dev/xda.
% sfdisk -s /dev/hda9
81599
%
If the partition argument is omitted, sfdisk will list the
sizes of all disks, and the total:
% sfdisk -s
/dev/hda: 208896
/dev/hdb: 1025136
/dev/hdc: 1031063
/dev/sda: 8877895
/dev/sdb: 1758927
total: 12901917 blocks
%
List Partitions
The second type of invocation: sfdisk -l [options] device
will list the partitions on this device. If the device
argument is omitted, the partitions on all hard disks are
listed.
% sfdisk -l /dev/hdc
Disk /dev/hdc: 16 heads, 63 sectors, 2045 cylinders
Units = cylinders of 516096 bytes, blocks of 1024 bytes, counting from 0
Device Boot Start End #cyls #blocks Id System
/dev/hdc1 0+ 406 407- 205096+ 83 Linux native
/dev/hdc2 407 813 407 205128 83 Linux native
/dev/hdc3 814 2044 1231 620424 83 Linux native
/dev/hdc4 0 - 0 0 0 Empty
%
The trailing - and + signs indicate that rounding has
taken place, and that the actual value is slightly less
(more). To see the exact values, ask for a listing with
sectors as unit.
Check partitions
The third type of invocation: sfdisk -V device will apply
various consistency checks to the partition tables on
device. It prints `OK' or complains. The -V option can be
used together with -l. In a shell script one might use
sfdisk -V -q device which only returns a status.
Create partitions
The fourth type of invocation: sfdisk device will cause
sfdisk to read the specification for the desired parti-
tioning of device from its standard input, and then to
change the partition tables on that disk. Thus, it is pos-
sible to use sfdisk from a shell script. When sfdisk
determines that its standard input is a terminal, it will
be conversational; otherwise it will abort on any error.
BE EXTREMELY CAREFUL - ONE TYPING MISTAKE AND ALL YOUR
DATA IS LOST
As a precaution, one can save the sectors changed by
sfdisk:
% sfdisk /dev/hdd -O hdd-partition-sectors.save
...
%
Then, if you discover that you did something stupid before
anything else has been written to disk, it may be possible
to recover the old situation with
% sfdisk /dev/hdd -I hdd-partition-sectors.save
%
(This is not the same as saving the old partition table: a
readable version of the old partition table can be saved
using the -d option. However, if you create logical parti-
tions, the sectors describing them are located somewhere
on disk, possibly on sectors that were not part of the
partition table before. Thus, the information the -O
option saves is not a binary version of the output of -d.)
There are many options.
OPTIONS
-v or --version
Print version number of sfdisk and exit immedi-
ately.
-? or --help
Print a usage message and exit immediately.
-T or --list-types
Print the recognized types (system Id's).
-s or --show-size
List the size of a partition.
-g or --show-geometry
List the kernel's idea of the geometry of the indi-
cated disk(s).
-l or --list
List the partitions of a device.
-d Dump the partitions of a device in a format useful
as input to sfdisk. For example,
% sfdisk -d /dev/hda > hda.out
% sfdisk /dev/hda < hda.out
will correct the bad last extended partition that
the OS/2 fdisk creates.
-V or --verify
Test whether partitions seem correct. (See above.)
-i or --increment
Number cylinders etc. starting from 1 instead of 0.
-N number
Change only the single partition indicated. For
example:
% sfdisk /dev/hdb -N5
,,,*
%
will make the fifth partition on /dev/hdb bootable
(`active') and change nothing else. (Probably this
fifth partition is called /dev/hdb5, but you are
free to call it something else, like `/my_equip-
ment/disks/2/5' or so).
-Anumber
Make the indicated partition(s) active, and all
others inactive.
-c or --id number [Id]
If no Id argument given: print the partition Id of
the indicated partition. If an Id argument is pre-
sent: change the type (Id) of the indicated parti-
tion to the given value. This option has the two
very long forms --print-id and --change-id. For
example:
% sfdisk --print-id /dev/hdb 5
6
% sfdisk --change-id /dev/hdb 5 83
OK
first reports that /dev/hdb5 has Id 6, and then
changes that into 83.
-uS or -uB or -uC or -uM
Accept or report in units of sectors (blocks,
cylinders, megabytes, respectively). The default is
cylinders, at least when the geometry is known.
-x or --show-extended
Also list non-primary extended partitions on out-
put, and expect descriptors for them on input.
-C cylinders
Specify the number of cylinders, possibly overrid-
ing what the kernel thinks.
-H heads
Specify the number of heads, possibly overriding
what the kernel thinks.
-S sectors
Specify the number of sectors, possibly overriding
what the kernel thinks.
-f or --force
Do what I say, even if it is stupid.
-q or --quiet
Suppress warning messages.
-L or --Linux
Do not complain about things irrelevant for Linux.
-D or --DOS
For DOS-compatibility: waste a little space. (More
precisely: if a partition cannot contain sector 0,
e.g. because that is the MBR of the device, or con-
tains the partition table of an extended partition,
then sfdisk would make it start the next sector.
However, when this option is given it skips to the
start of the next track, wasting for example 33
sectors (in case of 34 sectors/track), just like
certain versions of DOS do.) Certain Disk Managers
and boot loaders (such as OSBS, but not LILO or the
OS/2 Boot Manager) also live in this empty space,
so maybe you want this option if you use one.
--IBM or --leave-last
Certain IBM diagnostic programs assume that they
can use the last cylinder on a disk for disk-test-
ing purposes. If you think you might ever run such
programs, use this option to tell sfdisk that it
should not allocate the last cylinder. Sometimes
the last cylinder contains a bad sector table.
-n Go through all the motions, but do not actually
write to disk.
-R Only execute the BLKRRPART ioctl (to make the ker-
nel re-read the partition table). This can be use-
ful for checking in advance that the final BLKR-
RPART will be successful, and also when you changed
the partition table `by hand' (e.g., using dd from
a backup). If the kernel complains (`device busy
for revalidation (usage = 2)') then something still
uses the device, and you still have to unmount some
file system, or say swapoff to some swap partition.
--no-reread
When starting a repartitioning of a disk, sfdisk
checks that this disk is not mounted, or in use as
a swap device, and refuses to continue if it is.
This option suppresses the test. (On the other
hand, the -f option would force sfdisk to continue
even when this test fails.)
-O file
Just before writing the new partition, output the
sectors that are going to be overwritten to file
(where hopefully file resides on another disk, or
on a floppy).
-I file
After destroying your filesystems with an unfortu-
nate sfdisk command, you would have been able to
restore the old situation if only you had preserved
it using the -O flag.
THEORY
Block 0 of a disk (the Master Boot Record) contains among
other things four partition descriptors. The partitions
described here are called primary partitions.
A partition descriptor has 6 fields:
struct partition {
unsigned char bootable; /* 0 or 0x80 */
hsc begin_hsc;
unsigned char id;
hsc end_hsc;
unsigned int starting_sector;
unsigned int nr_of_sectors;
}
The two hsc fields indicate head, sector and cylinder of
the begin and the end of the partition. Since each hsc
field only takes 3 bytes, only 24 bits are available,
which does not suffice for big disks (say > 8GB). In fact,
due to the wasteful representation (that uses a byte for
the number of heads, which is typically 16), problems
already start with 0.5GB. However Linux does not use
these fields, and problems can arise only at boot time,
before Linux has been started. For more details, see the
lilo documentation.
Each partition has a type, its `Id', and if this type is 5
(`extended partition') the starting sector of the parti-
tion again contains 4 partition descriptors. MSDOS only
uses the first two of these: the first one an actual data
partition, and the second one again an extended partition
(or empty). In this way one gets a chain of extended par-
titions. Other operating systems have slightly different
conventions. Linux also accepts type 85 as equivalent to
5 - this can be useful if one wants to have extended par-
titions under Linux past the 1024 cylinder boundary, with-
out DOS FDISK hanging. (If there is no good reason, you
should just use 5, which is understood by other systems.)
Partitions that are not primary or extended are called
logical. Often, one cannot boot from logical partitions
(because the process of finding them is more involved than
just looking at the MBR). Note that of an extended parti-
tion only the Id and the start are used. There are various
conventions about what to write in the other fields. One
should not try to use extended partitions for data storage
or swap.
INPUT FORMAT
sfdisk reads lines of the form
<start> <size> <id> <bootable> <c,h,s> <c,h,s>
where each line fills one partition descriptor.
Fields are separated by whitespace, or comma or semicolon
possibly followed by whitespace; initial and trailing
whitespace is ignored. Numbers can be octal, decimal or
hexadecimal, decimal is default. When a field is absent
or empty, a default value is used.
The <c,h,s> parts can (and probably should) be omitted -
sfdisk computes them from <start> and <size> and the disk
geometry as given by the kernel or specified using the -H,
-S, -C flags.
Bootable is specified as [*|-], with as default not-
bootable. (The value of this field is irrelevant for
Linux - when Linux runs it has been booted already - but
might play a role for certain boot loaders and for other
operating systems. For example, when there are several
primary DOS partitions, DOS assigns C: to the first among
these that is bootable.)
Id is given in hex, without the 0x prefix, or is
[E|S|L|X], where L (LINUX_NATIVE (83)) is the default, S
is LINUX_SWAP (82), E is EXTENDED_PARTITION (5), and X is
LINUX_EXTENDED (85).
The default value of start is the first nonassigned sec-
tor/cylinder/...
The default value of size is as much as possible (until
next partition or end-of-disk).
However, for the four partitions inside an extended parti-
tion, the defaults are: Linux partition, Extended parti-
tion, Empty, Empty.
But when the -N option (change a single partition only) is
given, the default for each field is its previous value.
EXAMPLE
The command
sfdisk /dev/hdc << EOF
0,407
,407
;
;
EOF
will partition /dev/hdc just as indicated above.
With the -x option, the number of input lines must be a
multiple of 4: you have to list the two empty partitions
that you never want using two blank lines. Without the -x
option, you give one line for the partitions inside a
extended partition, instead of four, and terminate with
end-of-file (^D). (And sfdisk will assume that your input
line represents the first of four, that the second one is
extended, and the 3rd and 4th are empty.)
DOS 6.x WARNING
The DOS 6.x FORMAT command looks for some information in
the first sector of the data area of the partition, and
treats this information as more reliable than the informa-
tion in the partition table. DOS FORMAT expects DOS FDISK
to clear the first 512 bytes of the data area of a parti-
tion whenever a size change occurs. DOS FORMAT will look
at this extra information even if the /U flag is given --
we consider this a bug in DOS FORMAT and DOS FDISK.
The bottom line is that if you use sfdisk to change the
size of a DOS partition table entry, then you must also
use dd to zero the first 512 bytes of that partition
before using DOS FORMAT to format the partition. For
example, if you were using sfdisk to make a DOS partition
table entry for /dev/hda1, then (after exiting sfdisk and
rebooting Linux so that the partition table information is
valid) you would use the command "dd if=/dev/zero
of=/dev/hda1 bs=512 count=1" to zero the first 512 bytes
of the partition. BE EXTREMELY CAREFUL if you use the dd
command, since a small typo can make all of the data on
your disk useless.
For best results, you should always use an OS-specific
partition table program. For example, you should make DOS
partitions with the DOS FDISK program and Linux partitions
with the Linux sfdisk program.
DRDOS WARNINGS
Stephen Tweedie reported (930515): `Most reports of
superblock corruption turn out to be due to bad partition-
ing, with one filesystem overrunning the start of the next
and corrupting its superblock. I have even had this prob-
lem with the supposedly-reliable DRDOS. This was quite
possibly due to DRDOS-6.0's FDISK command. Unless I cre-
ated a blank track or cylinder between the DRDOS partition
and the immediately following one, DRDOS would happily
stamp all over the start of the next partition. Mind you,
as long as I keep a little free disk space after any DRDOS
partition, I don't have any other problems with the two
coexisting on the one drive.'
A. V. Le Blanc writes in README.esfdisk: `Dr. DOS 5.0 and
6.0 has been reported to have problems cooperating with
Linux, and with this version of efdisk in particular.
This efdisk sets the system type to hexadecimal 81. Dr.
DOS seems to confuse this with hexadecimal 1, a DOS code.
If you use Dr. DOS, use the efdisk command 't' to change
the system code of any Linux partitions to some number
less than hexadecimal 80; I suggest 41 and 42 for the
moment.'
A. V. Le Blanc writes in his README.fdisk: `DR-DOS 5.0 and
6.0 are reported to have difficulties with partition ID
codes of 80 or more. The Linux `fdisk' used to set the
system type of new partitions to hexadecimal 81. DR-DOS
seems to confuse this with hexadecimal 1, a DOS code. The
values 82 for swap and 83 for file systems should not
cause problems with DR-DOS. If they do, you may use the
`fdisk' command `t' to change the system code of any Linux
partitions to some number less than hexadecimal 80; I sug-
gest 42 and 43 for the moment.'
In fact, it seems that only 4 bits are significant for the
DRDOS FDISK, so that for example 11 and 21 are listed as
DOS 2.0. However, DRDOS itself seems to use the full byte.
I have not been able to reproduce any corruption with
DRDOS or its fdisk.
BUGS
A corresponding interactive cfdisk (with curses interface)
is still lacking.
There are too many options.
AUTHOR
A. E. Brouwer (aeb@cwi.nl)
Linux 1.3.23 1 September 1995 1
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