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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 the GUID Partition Table (GPT) format and it
       is not designed for large partitions.  In  these  cases  use  the  more
       advanced GNU parted(8).

       Note  that sfdisk does not align partitions to block device I/O limits.
       This functionality is provided by fdisk(8).

   List sizes
       sfdisk -s partition gives the size of partition in blocks.  This may be
       useful  in  connection with programs like mkswap(8).  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
       block devices, 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 device will  list  the  parti-
       tions  on the specified device.  If the device argument is omitted, the
       partitions on all block devices 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

   Create partitions
       The  fourth type of invocation: sfdisk device will cause sfdisk to read
       the specification for the desired partitioning of device from  standard
       input,  and  then  to change the partition tables on that block device.
       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  the  block  device, 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 block device, 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, --version
              Print version number of sfdisk and exit immediately.

       -h, --help
              Print a usage message and exit immediately.

       -T, --list-types
              Print the recognized types (system Id's).

       -s, --show-size
              List the size of a partition.

       -g, --show-geometry
              List  the  kernel's  idea of the geometry of the indicated block
              device(s).

       -G, --show-pt-geometry
              List the geometry of the  indicated  block  devices  guessed  by
              Test whether partitions seem correct.  (See the third invocation
              type above.)

       -i, --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).

       -A, --activate number
              Make the indicated partition(s) active, and all others inactive.

       -c, --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
              two longer 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.

       -u, --unit letter
              Interpret  the  input and show the output in the units specified
              by letter.  This letter can be one of S, C, B or M, meaning Sec-
              tors,   Cylinders,  Blocks  and  Megabytes,  respectively.   The
              default is cylinders, at least when the geometry is known.

       -x, --show-extended
              Also list non-primary extended partitions on output, and  expect
              descriptors for them on input.

       -C, --cylinders cylinders
              Specify  the  number  of cylinders, possibly overriding what the
              kernel thinks.

       -H, --heads heads
              Specify the number of heads, possibly overriding what the kernel
              thinks.

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

              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, --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 relative to the
              actual 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 block device.)

       --IBM, --leave-last
              Certain  IBM  diagnostic  programs  assume that they can use the
              last cylinder on a device 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 block
              device.

       -R, --re-read
              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 block device, sfdisk checks
              that this device 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.)

       --in-order
              Caution, see warning section.  To be documented.

       --not-in-order
              Caution, see warning section.  To be documented.

       --inside-outer

       --onesector
              Caution,  see warning section.  All data partitions are mutually
              disjoint; extended partitions each use one sector  only  (except
              perhaps for the outermost one).

       -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 block device, 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 block device (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 block devices
       (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.)
       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 block device 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-device).

       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.

       A '+' can be specified instead of a number for  size,  which  means  as
       much as possible. This is useful with the -N option.


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

       and 4th are empty.)


CAUTION WARNINGS

       The options marked with caution in the manual page are dangerous.   For
       example not all functionality is completely implemented, which can be a
       reason for unexpected results.


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  block
       device 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 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  device  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 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.'



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 package and  is  available
       from ftp://ftp.kernel.org/pub/linux/utils/util-linux/.


util-linux August 2011 SFDISK(8)



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