I’ve long been enjoying using sfdisk to manipulate my disk partitions, especially for creating disk partitions.
Creating disk partitions with sfdisk is super easy. The followings are the notes I jotted down back in the old days when HD were still called hda instead of sda. Replace hdX with sdX, and everything are still as good as new, as far as MBR type of disks are concerned.
Input lines have fields <start>,<size>,<type>… - see sfdisk.8.
sfdisk reads lines of the form
<start> <size> <id> <bootable> <c,h,s> <c,h,s>
Usually no <start> is given, and input lines start with a comma.
Example 1
1) One big partition:
sfdisk /dev/hda << EOF
;
EOF
or,
echo ';' | sfdisk /dev/sdc
(If there was garbage on the disk before, you may get error messages
like: ERROR: sector 0 does not have an msdos signature
and /dev/hda: unrecognized partition. This does not matter
if you write an entirely fresh partition table anyway.)
The output will be:
Old situation:
...
New situation:
Units = cylinders of 208896 bytes, blocks of 1024 bytes, counting from 0
Device Boot Start End #cyls #blocks Id System
/dev/hda1 0+ 1023 1024- 208895+ 83 Linux native
Successfully wrote the new partition table
hda: hda1
Writing and rereading the partition table takes a few seconds - don’t be alarmed if nothing happens for six seconds or so.
To create a single “W95 FAT32 (LBA)” partition, try:
echo ',,c;' | sfdisk /dev/sdd
Example 2
2) Three primary partitions: two of size 50MB and the rest:
sfdisk /dev/hda -uM << EOF
,50
,50
;
EOF
The result:
New situation:
Units = megabytes of 1048576 bytes, blocks of 1024 bytes, counting from 0
Device Boot Start End MB #blocks Id System
/dev/hda1 0+ 50- 51- 51203+ 83 Linux native
/dev/hda2 50+ 100- 51- 51204 83 Linux native
/dev/hda3 100+ 203 104- 106488 83 Linux native
Successfully wrote the new partition table
hda: hda1 hda2 hda3
/dev/hda1 is one block (in fact only half a block) shorter than
/dev/hda2 because its start had to be shifted away from zero in
order to leave room for the Master Boot Record (MBR).
Example 3
3) A 1MB OS2 Boot Manager partition, a 50MB DOS partition, and three extended partitions (DOS D:, Linux swap, Linux):
sfdisk /dev/hda -uM << EOF
,1,a
,50,6
,,E
;
,20,4
,16,S
;
EOF
The result:
Device Boot Start End MB #blocks Id System
/dev/hda1 0+ 1- 2- 1223+ a OS/2 Boot Manager
/dev/hda2 1+ 51- 51- 51204 6 DOS 16-bit FAT >=32M
/dev/hda3 51+ 203 153- 156468 5 Extended
/dev/hda4 0 - 0 0 0 Empty
/dev/hda5 51+ 71- 21- 20603+ 4 DOS 16-bit FAT <32M
/dev/hda6 71+ 87- 17- 16523+ 82 Linux swap
/dev/hda7 87+ 203 117- 119339+ 83 Linux native
Successfully wrote the new partition table
hda: hda1 hda2 hda3 < hda5 hda6 hda7 >
All these rounded numbers look better in cylinder units:
% sfdisk -l /dev/hda
Device Boot Start End #cyls #blocks Id System
/dev/hda1 0+ 5 6- 1223+ a OS/2 Boot Manager
/dev/hda2 6 256 251 51204 6 DOS 16-bit FAT >=32M
/dev/hda3 257 1023 767 156468 5 Extended
/dev/hda4 0 - 0 0 0 Empty
/dev/hda5 257+ 357 101- 20603+ 4 DOS 16-bit FAT <32M
/dev/hda6 358+ 438 81- 16523+ 82 Linux swap
/dev/hda7 439+ 1023 585- 119339+ 83 Linux native
Explanations
But still - why does /dev/hda5 not start on a cylinder boundary? Because it is contained in an extended partition that does. Of the chain of extended partitions, usually only the first is shown. (The others have no name under Linux anyway.) But these additional extended partitions can be made visible:
% sfdisk -l -x /dev/hda
Device Boot Start End #cyls #blocks Id System
/dev/hda1 0+ 5 6- 1223+ a OS/2 Boot Manager
/dev/hda2 6 256 251 51204 6 DOS 16-bit FAT >=32M
/dev/hda3 257 1023 767 156468 5 Extended
/dev/hda4 0 - 0 0 0 Empty
/dev/hda5 257+ 357 101- 20603+ 4 DOS 16-bit FAT <32M
- 358 1023 666 135864 5 Extended
- 257 256 0 0 0 Empty
- 257 256 0 0 0 Empty
/dev/hda6 358+ 438 81- 16523+ 82 Linux swap
- 439 1023 585 119340 5 Extended
- 358 357 0 0 0 Empty
- 358 357 0 0 0 Empty
/dev/hda7 439+ 1023 585- 119339+ 83 Linux native
- 439 438 0 0 0 Empty
- 439 438 0 0 0 Empty
- 439 438 0 0 0 Empty
Why the empty 4th input line? The description of the extended partitions starts after that of the four primary partitions. You force an empty partition with a “,0” input line, but here all space was divided already, so the fourth partition became empty automatically.
How did I know about 4,6,a,E,S? Well, E,S,L stand for Extended, Swap and Linux. The other values are hexadecimal and come from the table:
% sfdisk -T
Id Name
0 Empty
1 DOS 12-bit FAT
2 XENIX root
3 XENIX usr
4 DOS 16-bit FAT <32M
5 Extended
6 DOS 16-bit FAT >=32M
7 OS/2 HPFS or QNX or Advanced UNIX
8 AIX data
9 AIX boot or Coherent
a OS/2 Boot Manager
...
Ref:
/usr/share/doc/util-linux/examples/sfdisk.examples.gz