sfdisk
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.
sfdisk doesn’t understand GUID Partition Table (GPT) and it is not
designed for large partitions. In particular case use more advanced
GNU parted(8).
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 val-
ues, ask for a listing with sectors as unit.
Check partitions
The third type of invocation: sfdisk -V device will apply various con-
sistency 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 partitioning of device from its
standard input, and then to change the partition tables on that disk.
Thus, it is possible to use sfdisk from a shell script. When sfdisk
determines that its standard input is a terminal, it will be conversa-
tional; 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 partitions, 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 immediately.
-? 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 indicated
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_equipment/disks/2/5’ or so).
-Anumber
Make the indicated partition(s) active, and all others inac-
tive.
-c or --id number [Id]
If no Id argument given: print the partition Id of the indi-
cated partition. If an Id argument is present: change the type
(Id) of the indicated partition 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 output, and expect
descriptors for them on input.
-C cylinders
Specify the number of cylinders, possibly overriding what the
kernel thinks.
-H heads
Specify the number of heads, possibly overriding what the ker-
nel 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 contains 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.
-E or --DOS-extended
Take the starting sector numbers of "inner" extended partitions
to be relative to the starting cylinder boundary of the outer
one, (like some versions of DOS do) rather than to the starting
sector (like Linux does). (The fact that there is a difference
here means that one should always let extended partitions start
at cylinder boundaries if DOS and Linux should interpret the
partition table in the same way. Of course one can only know
where cylinder boundaries are when one knows what geometry DOS
will use for this disk.)
--IBM or --leave-last
Certain IBM diagnostic programs assume that they can use the
last cylinder on a disk for disk-testing purposes. If you think
you might ever run such programs, use this option to tell
sfdisk that it should not allocate the last cylinder. Some-
times 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 kernel re-read
the partition table). This can be useful for checking in
advance that the final BLKRRPART 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 con-
tinue 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 unfortunate 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 prob-
lems 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 or f
(‘extended partition’) the starting sector of the partition again con-
tains 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 partitions. Other operating systems have slightly different
conventions. Linux also accepts type 85 as equivalent to 5 and f -
this can be useful if one wants to have extended partitions under
Linux past the 1024 cylinder boundary, without 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 partition 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 com-
putes 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 sector/cylin-
der/...
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 partition, the
defaults are: Linux partition, Extended partition, 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.
The command
sfdisk /dev/hdb << EOF
,3,L
,60,L
,19,S
,,E
,130,L
,130,L
,130,L
,,L
EOF
will partition /dev/hdb into two Linux partitions of 3 and 60 cylin-
ders, a swap space of 19 cylinders, and an extended partition covering
the rest. Inside the extended partition there are four Linux logical
partitions, three of 130 cylinders and one covering the rest.
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 par-
titions 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 information in the partition table. DOS
FORMAT expects DOS FDISK to clear the first 512 bytes of the data area
of a partition 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 parti-
tion. 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 corrup-
tion turn out to be due to bad partitioning, with one filesystem over-
running the start of the next and corrupting its superblock. I have
even had this problem with the supposedly-reliable DRDOS. This was
quite possibly due to DRDOS-6.0’s FDISK command. Unless I created a
blank track or cylinder between the DRDOS partition and the immedi-
ately 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.efdisk: ‘Dr. DOS 5.0 and 6.0 has been
reported to have problems cooperating with Linux, and with this ver-
sion 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 hex-
adecimal 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’ com-
mand ‘t’ to change the system code of any Linux partitions to some
number less than hexadecimal 80; I suggest 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.
There is no support for non-DOS partition types.
AUTHOR
A. E. Brouwer (aeb@cwi.nl)
SEE ALSO
cfdisk(8), fdisk(8), mkfs(8), parted(8)
Linux 1 September 1995 SFDISK(8)
