Linux sfdisk command

Updated: 04/26/2017 by Computer Hope

About sfdisk

sfdisk is a partition table manipulator for Linux. It is more powerful (and dangerous, if used incorrectly) than fdisk and cfdisk, and unlike those two programs, it can be run non-interactively.

Description

sfdisk has four (main) uses: it can list the size of a partition, list the partitions on a device, check the partitions on a device, and - this one is very dangerous! - it can 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.

Listing Partition Sizes

The following command:

sfdisk -s partition

gives the size of partition in blocks. This may be useful in connection with programs like mkswap. Here partition is usually something like /dev/hda1 or /dev/sdb12, but may also be an entire disk, like /dev/xda. The following example command, and output, shows the size of partition /dev/hda9:

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

Listing partitions

The second type of invocation:

sfdisk -l device

will list the partitions on the specified device. If the device argument is omitted, the partitions on all hard disks are listed. The following example lists the partitions on device /dev/hdc:

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 or more. To see the exact values, ask for a listing with sectors as the unit (using the "-u S" option).

Checking Partitions

The third type of invocation:

sfdisk -V device

will apply various consistency checks to the partition tables on device. It either prints "OK", or complains. The -V option can be used together with -l. For example, in a shell script you could use

sfdisk -V -q device

which would only return the status of device.

Creating 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 disk. Thus it is possible to use sfdisk from a shell script. When sfdisk determines 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, you 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 command 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.

sfdisk syntax

sfdisk [options] device
sfdisk -s [partition]

Options

sfdisk accepts the following command-line 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 disk(s).
-G, --show-pt-geometry List the geometry of the indicated disks guessed by looking at the partition table.
-l, --list List the partitions of a device.
-d, --dump Dump the partitions of a device in a format that is usable as input to /fBsfdisk/fR. For example, the following sequence of commands:

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, --verify 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, the following sfdisk command, and the listed input:

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 Sectors, 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 kernel thinks.
-f, --force Do what I say, even if it is stupid.
-q, --quiet Suppress warning messages.
-L, --Linux Do not complain about things irrelevant for Linux.
-D, --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, --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 disk.
--IBM, --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, --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 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.
--in-order THIS IS A DANGEROUS OPTION. Not yet been documented.
--not-in-order THIS IS A DANGEROUS OPTION. Not yet been documented.
--inside-outer THIS IS A DANGEROUS OPTION. Affects chaining order.
--not-inside-outer THIS IS A DANGEROUS OPTION. Affects chaining order.
--nested THIS IS A DANGEROUS OPTION. Every partition is contained in the surrounding partitions and is disjoint from all others.
--chained THIS IS A DANGEROUS OPTION. Every data partition is contained in the surrounding partitions and disjoint from all others, but extended partitions may lie outside (insofar as allowed by all_logicals_inside_outermost_extended).
--onesector THIS IS A DANGEROUS OPTION. 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 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.

Technical Details And Background 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 > 8 GB). In fact, due to the wasteful representation (that uses a byte for the number of heads, which is typically 16), problems already start with 0.5 GB. However Linux does not use these fields, and problems can arise only at boot time, before Linux has been started. For more details, see your 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 contains 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 various 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 followed by whitespace; initial and trailing whitespace is ignored. Numbers 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 computes 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.

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

sfdisk examples

sfdisk /dev/hdc << EOF
0,407
,407
;
;
EOF

This command (and the listed input) partitions /dev/hdc just as indicated above.

sfdisk /dev/hdb << EOF
,3,L
,60,L
,19,S
,,E
,130,L
,130,L
,130,L
,,L
EOF

This command (and the listed input) will partition /dev/hdb into two Linux partitions of 3 and 60 cylinders, 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 partitions inside an extended partition, instead of four, and terminate with end-of-file (^D). And sfdisk will assume that your input line represents the first of four, that the second one is extended, and the 3rd and 4th are empty.

cfdisk — A more user-friendly version of the fdisk disk partitioning utility.
fdisk — A disk partitioning utility.
mkfs — Build a Linux file system, usually a hard disk partition.
parted — A disk partition manipulation program.
partprobe — Inform the operating system about changes to the partition table.