sfdisk


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(8) 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 par-
       titions 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.

       it  is  possible  to use sfdisk from a shell script. When sfdisk deter-
       mines 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 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).

       -G or --show-pt-geometry
              List the geometry of the indicated disks guessed by  looking  at
              the partition table.

       -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  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).

       -A number
              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 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 kernel
              thinks.

       -S sectors
              Specify the number of sectors, possibly overriding what the ker-
              nel 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:
              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.   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 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  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;

       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  vari-
       ous 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 fol-
       lowed  by whitespace; initial and trailing whitespace is ignored.  Num-
       bers 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/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
              ;
              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  parti-
       tions  inside a extended partition, instead of four, and terminate with
       end-of-file (^D).  (And sfdisk will assume that your input line  repre-
       sents  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 sec-
       tor  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.


       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 particular.  This efdisk sets the system type to hexadeci-
       mal  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 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
       There are too many options.

       There is no support for non-DOS partition types.


SEE ALSO
       cfdisk(8), fdisk(8), mkfs(8), parted(8), partprobe(8), kpartx(8)

AVAILABILITY
       The sfdisk command is part of the util-linux-ng package and  is  avail-
       able from ftp://ftp.kernel.org/pub/linux/utils/util-linux-ng/.



Linux                          1 September 1995                      SFDISK(8)
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