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 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.


   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 stan-
       dard input, and then to change the partition tables on that disk. Thus,
       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).

       -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 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:
              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.  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;
                  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 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  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
       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  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.



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 disk space  after
       any DRDOS partition, I don’t have any other problems with the two coex-
       isting 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.’

       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 hex-
       adecimal  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

       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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