tests: add nologin to build tests

[thirdparty/util-linux.git] / fdisks / fdisk.8

.\" Copyright 1992, 1993 Rickard E. Faith (faith@cs.unc.edu)
.\" Copyright 1998 Andries E. Brouwer (aeb@cwi.nl)
.\" Copyright 2012 Davidlohr Bueso <dave@gnu.org>
.\" Copyright (C) 2013 Karel Zak <kzak@redhat.com>
.\" May be distributed under the GNU General Public License
.TH FDISK 8 "September 2013" "util-linux" "System Administration"

.SH NAME
fdisk \- manipulate disk partition table

.SH SYNOPSIS
.B fdisk
.RB [ options ]
.I device
.sp
.B fdisk \-l
.RI [ device ...]

.SH DESCRIPTION
.B fdisk
is a dialog-driven program for creation and manipulation of partition tables.
It understands GPT, MBR, Sun, SGI and BSD partition tables.

Block devices can be divided into one or more logical disks called
.IR partitions .
This division is recorded in the
.IR "partition table" ,
usually found in sector 0 of the disk.
(In the BSD world one talks about `disk slices' and a `disklabel'.)

All partitioning is driven by device I/O limits (the topology) by default.
.B fdisk
is able to optimize the disk layout for a 4K-sector size and use an alignment offset on
modern devices for MBR and GPT.  It is always a good idea to follow \fBfdisk\fR's defaults
as the default values (e.g. first and last partition sectors) and partition
sizes specified by the +<size>{M,G,...} notation are always aligned according
to the device properties.

Note that
.BR partx (8)
provides a rich interface for scripts to print disk layouts,
.B fdisk
is mostly designed for humans.  Backward compatibility in the output of
.B fdisk
is not guaranteed.  The input (the commands) should always be backward compatible.

.SH OPTIONS
.TP
.BI "\-b " sectorsize
Specify the sector size of the disk.  Valid values are 512, 1024, 2048, and 4096.
(Recent kernels know the sector size.  Use this option only on old kernels or
to override the kernel's ideas.)  Since util-linux-2.17, \fBfdisk\fR differentiates
between logical and physical sector size.  This option changes both sector sizes to
.IB sectorsize .
.TP
.BR "\-c"[=\fImode\fR]
Specify the compatibility mode, 'dos' or 'nondos'.  The default is non-DOS
mode.  For backward compatibility, it is possible to use the option without
the \fImode\fR argument -- then the default is used.  Note that the optional
\fImode\fR argument cannot be separated from the \fB-c\fR option by a space,
the correct form is for example '-c=dos'.  This option is DEPRECATED.
.TP
.BI "\-C " cylinders
Specify the number of cylinders of the disk.
I have no idea why anybody would want to do so.  This option is DEPRECATED.
.TP
.BI "\-H " heads
Specify the number of heads of the disk.  (Not the physical number,
of course, but the number used for partition tables.)
Reasonable values are 255 and 16.  This option is DEPRECATED.
.TP
.BI "\-S " sectors
Specify the number of sectors per track of the disk.
(Not the physical number, of course, but the number used for
partition tables.)
A reasonable value is 63.  This option is DEPRECATED.
.TP
.BI \-h
Display a help text and exit.
.TP
.BR "\-L"[=\fIwhen\fR]
Colorize the output in interactive mode.  The optional argument \fIwhen\fP can
be \fBauto\fR, \fBnever\fR or \fBalways\fR.  The default is \fBauto\fR.
.TP
.B \-l
List the partition tables for the specified devices and then exit.
If no devices are given, those mentioned in
.I /proc/partitions
(if that file exists) are used.
.TP
.BI "\-s " partition...
Print the size (in blocks) of each given partition.  This option is DEPRECATED
in favour of
.BR blockdev (1).
.TP
.BI "\-t " type
Enable support only for disklabels of the specified \fItype\fP, and disable
support for all other types.
This is necessary for example to access a protective or hybrid MBR on devices
with GPT.
.TP
.BR "\-u"[=\fIunit\fR]
When listing partition tables, show sizes in 'sectors' or in 'cylinders'.  The
default is to show sizes in sectors.  For backward compatibility, it is possible
to use the option without the \fIunit\fR argument -- then the default is used.
Note that the optional \fIunit\fR argument cannot be separated from the \fB-u\fR
option by a space, the correct form is for example '-u=cylinders'.
.TP
.B \-v
Display version information and exit.

.SH DEVICES
The
.I device
is usually /dev/sda, /dev/sdb or so.  A device name refers to the entire disk.
Old systems without libata (a library used inside the Linux kernel to support
ATA host controllers and devices) make a difference between IDE and SCSI disks.
In such cases the device name will be /dev/hd* (IDE) or /dev/sd* (SCSI).

The
.I partition
is a device name followed by a partition number.  For example, /dev/sda1 is the
first partition on the first hard disk in the system.  See also Linux kernel
documentation (the Documentation/devices.txt file).

.SH DISK LABELS
.B GPT (GUID Partition Table)
.RS
GPT is modern standard for the layout of the partition table.  GPT uses 64-bit
logical block addresses, checksums, UUIDs and names for partitions and an
unlimited number of partitions (although the number of partitions is
usually restricted to 128 in many partitioning tools).

Note that the first sector is still reserved for a
.B protective MBR
in the GPT specification.  It prevents MBR-only partitioning tools
from mis-recognizing and overwriting GPT disks.

GPT is always a better choice than MBR, especially on modern hardware with a UEFI
boot loader.
.RE

.B DOS-type (MBR)
.RS
A DOS-type partition table can describe an unlimited number of partitions.  In sector 0
there is room for the description of 4 partitions (called `primary').  One of
these may be an extended partition; this is a box holding logical partitions,
with descriptors found in a linked list of sectors, each preceding the
corresponding logical partitions.  The four primary partitions, present or not,
get numbers 1-4.  Logical partitions are numbered starting from 5.

In a DOS-type partition table the starting offset and the size of each
partition is stored in two ways: as an absolute number of sectors (given in 32
bits), and as a
.B Cylinders/Heads/Sectors
triple (given in 10+8+6 bits).  The former is OK -- with 512-byte sectors this
will work up to 2 TB.  The latter has two problems.  First, these C/H/S fields
can be filled only when the number of heads and the number of sectors per track
are known.  And second, even if we know what these numbers should be, the 24
bits that are available do not suffice.  DOS uses C/H/S only, Windows uses
both, Linux never uses C/H/S.  The
.B C/H/S addressing is deprecated
and may be unsupported in some later fdisk version.

.B Please, read the DOS-mode section if you want DOS-compatible partitions.
.B fdisk
does not care about cylinder boundaries by default.
.RE

.B BSD/Sun-type
.RS
A BSD/Sun disklabel can describe 8 partitions, the third of which should be a `whole
disk' partition.  Do not start a partition that actually uses its first sector
(like a swap partition) at cylinder 0, since that will destroy the disklabel.
Note that a
.B BSD label
is usually nested within a DOS partition.
.RE

.B IRIX/SGI-type
.RS
An IRIX/SGI disklabel can describe 16 partitions, the eleventh of which should be an entire
`volume' partition, while the ninth should be labeled `volume header'.  The
volume header will also cover the partition table, i.e., it starts at block
zero and extends by default over five cylinders.  The remaining space in the
volume header may be used by header directory entries.  No partitions may
overlap with the volume header.  Also do not change its type or make some
filesystem on it, since you will lose the partition table.  Use this type of
label only when working with Linux on IRIX/SGI machines or IRIX/SGI disks under
Linux.
.RE

A sync() and an ioctl(BLKRRPART) (rereading the partition table from disk)
are performed before exiting when the partition table has been updated.

.SH "DOS mode and DOS 6.x WARNING"
.B Note that all this is deprecated.  You don't have to care about things like
.B geometry and cylinders on modern operating systems.  If you really want
.B DOS-compatible partitioning then you have to enable DOS mode and cylinder
.B units by using the '-c=dos -u=cylinders' fdisk command-line options.

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 \fBfdisk\fR or \fBcfdisk\R to change the
size of a DOS partition table entry, then you must also use
.BR dd "(1) to " "zero the first 512 bytes"
of that partition before using DOS FORMAT to format the partition.  For
example, if you were using \fBfdisk\fR to make a DOS partition table entry for
/dev/sda1, then (after exiting \fBfdisk\fR and rebooting Linux so that the
partition table information is valid) you would use the command "dd
if=/dev/zero of=/dev/sda1 bs=512 count=1" to zero the first 512 bytes of the
partition.

.B fdisk
usually obtains the disk geometry automatically.  This is not necessarily the
physical disk geometry (indeed, modern disks do not really have anything like a
physical geometry, certainly not something that can be described in the simplistic
Cylinders/Heads/Sectors form), but it is the disk geometry that MS-DOS uses for
the partition table.

Usually all goes well by default, and there are no problems if Linux is the
only system on the disk.  However, if the disk has to be shared with other
operating systems, it is often a good idea to let an fdisk from another
operating system make at least one partition.  When Linux boots it looks at the
partition table, and tries to deduce what (fake) geometry is required for good
cooperation with other systems.

Whenever a partition table is printed out in DOS mode, a consistency check is
performed on the partition table entries.  This check verifies that the
physical and logical start and end points are identical, and that each
partition starts and ends on a cylinder boundary (except for the first
partition).

Some versions of MS-DOS create a first partition which does not begin
on a cylinder boundary, but on sector 2 of the first cylinder.
Partitions beginning in cylinder 1 cannot begin on a cylinder boundary, but
this is unlikely to cause difficulty unless you have OS/2 on your machine.

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 fdisk or Linux cfdisk programs.

.SH AUTHORS
.MT kzak@redhat.com
Karel Zak
.ME
.br
.MT dave@gnu.org
Davidlohr Bueso
.ME
.br
.PP
The original version was written by
Andries E. Brouwer, A. V. Le Blanc and others.

.SH ENVIRONMENT
.IP "Setting LIBFDISK_DEBUG=0xffff enables debug output."

.SH "SEE ALSO"
.BR cfdisk (8),
.BR sfdisk (8),
.BR mkfs (8),
.BR partx (8)

.SH AVAILABILITY
The fdisk command is part of the util-linux package and is available from
ftp://ftp.kernel.org/pub/linux/utils/util-linux/.