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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 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 specifi-
cation 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 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 logi-
cal 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 inactive.
-c or --id number [Id]
If no Id argument given: print the partition Id of the indicated partition.
If an Id argument is present: 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, respec-
tively). 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 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 parti-
tion 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 Man-
agers 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 rela-
tive 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. 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 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 parti-
tion.
--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 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 parti-
tion 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 rep-
resentation (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 or f (‘extended parti-
tion’) the starting sector of the partition 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 partitions. Other operating systems have slightly different con-
ventions. 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 bound-
ary, 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, dec-
imal 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 sector/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 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 cylinders, 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 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
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 con-
sider 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 ta-
ble 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 partitioning, with one filesystem overrunning the start of the
next and corrupting its superblock. I have even had this problem with the suppos-
edly-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
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.efdisk: ‘Dr. DOS 5.0 and 6.0 has been reported to
have problems cooperating with Linux, and with this version of efdisk in particu-
lar. 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’ command
‘t’ to change the system code of any Linux partitions to some number less than hex-
adecimal 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.
SEE ALSO
cfdisk(8), fdisk(8), mkfs(8), parted(8)
Linux 1 September 1995 SFDISK(8)
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