gpart - guess PC-type hard disk partitions
Options: [-b <backup MBR>][-C c,h,s][-c][-d][-E][-e][-f] [-g][-h][-i][-K
<last-sector>][-k <# of sectors>] [-L] [-l <log file>][-n
<increment>] [-q][-s <sector-size>] [-t
<module-name>][-V][-v] [-W <device>][-w <module-name,
tries to guess which partitions are on a hard disk. If the primary
partition table has been lost, overwritten or destroyed the partitions still
exist on the disk but the operating system cannot access them.
ignores the primary partition table and scans the disk (or disk
image, file) sector after sector for several filesystem/partition types. It
does so by "asking" filesystem recognition modules if they think a
given sequence of sectors resembles the beginning of a filesystem or partition
type. Currently the following filesystem types are known to gpart
(listed by module names) :
- BeOS filesystem type.
- FreeBSD/NetBSD/386BSD disklabel sub-partitioning scheme
used on Intel platforms.
- Linux second extended filesystem.
- MS-DOS FAT12/16/32 "filesystems".
- IBM OS/2 High Performance filesystem.
- Linux LVM physical volumes (LVM by Heinz Mauelshagen).
- Linux swap partitions (versions 0 and 1).
- The Minix operating system filesystem type.
- MS Windows NT/2000 filesystem.
- QNX 4.x filesystem.
- The Reiser filesystem (version 3.5.X, X > 11,
- Sun Solaris on Intel platforms uses a sub-partitioning
scheme on PC hard disks similar to the BSD disklabels.
- Silicon Graphic's journalling filesystem for Linux.
More filesystem guessing modules can be added at runtime (see the -t
option). Please consult the gpart
README file for detailed explanations
on how to create guessing modules. All modules are accompanied by a guessing
weight factor which denotes how "educated" their guesses are
compared to other modules. This weight can be changed if a certain module
keeps on mis-identifying a partition.
Naturally only partitions which have been formatted in some way can be
recognized. If the type of a partition entry in the primary partition table is
changed from x to y while the filesystem is still of type x, gpart
also still guess a type x.
No checks are performed whether a found filesystem is clean or even
consistent/mountable, so it is quite possible that gpart
partitions which existed prior to the current partitioning scheme of the disk.
Especially on large disks old file system headers/superblocks may be present a
long time until they are finally overwritten with user data.
It should be stressed that gpart
does a very heuristic job, never believe
its output without any plausability checks. It can be easily right in its
guesswork but it can also be terribly wrong. You have been warned.
After having found a list of possible partition types, the list is checked for
consistency. For example, a partition which overlaps with the previous one
will be discarded. All remaining partitions are labelled with one of the
following attributes: "primary", "logical",
"orphaned" or "invalid".
A partition labelled "orphaned" is a logical partition which seems ok
but is missed in the chain of logical partitions. This may occur if a logical
partition is deleted from the extended partition table without overwriting the
actual disk space.
An "invalid" partition is one that cannot be accepted because of
various reasons. If a consistent primary partition table was created in this
process it is printed and can be written to a file or device.
If the disk/file to be examined consists of primary partitions only,
has quite a good chance to identify them. Extended partitions on
the other hand can result in a lot of problems.
Extended partitions are realized as a linked list of extended partition tables,
each of which include an entry pointing to a logical partition. The size of an
extended partition depends on where the last logical partition ends. This
means that extended partitions may include "holes", unallocated disk
space which should only be assigned to logical, not primary partitions.
tries to do its best to check a found chain of logical partitions
but there are very many possible points of failure. If "good" fdisk
programs are used to create extended partitions, the resulting tables consist
of a zeroed boot record and the four partition entries of which at least two
should be marked unused. Unfortunately e.g. the fdisk program shipped with
Windows NT does not seem to zero out the boot record area so gpart
to be overly tolerant in recognizing extended partition tables. This tolerance
may result in quite stupid guesses.
If you want to investigate hard disks from other systems you should note down
the geometry (number of cylinders, heads per cylinder and sectors per head)
used for that disk, and tell gpart
about this geometry.
Investigating disks from machines with a different endianness than the scanning
one has not been tested at all, and is currently not recommended.
relies on the OS reporting the correct disk geometry. Unfortunately
sometimes the OS may report a geometry smaller the the actual one (e.g. disks
with more than 1024 or 16384 cylinder).
checks if guessed partitions extend beyond the disk size and marks
those "invalid", but may be mistaken in case the disk size is
calculated from an incorrect geometry. For instance if a disk with the
geometry 1028/255/63 should be scanned, and the OS reports 1024/255/63
should be called like
gpart -C 1028,255,63 <other options>
may be of some help when the primary partition table was lost or
destroyed but it can under no
circumstances replace proper
disk/partition table backups. To save the master boot record (MBR) including
the primary partition table to a file type
dd if=/dev/hda of=mbr bs=512 count=1
exchanging /dev/hda with the block device name of the disk in question. This
should be done for all disks in the system. To restore the primary partition
table without overwriting the MBR type
dd if=mbr of=/dev/hda bs=1 count=64 skip=446
make sure that all parameters are typed as shown and that the
disk device is correct. Failing to do so may result in severe filesystem
corruption. The saved file should be stored in a safe place like a floppy
- -b backupfile
- If the guessed primary partition table seems consistent and
should be written (see the -W option) backup the current MBR into
the specified file.
- -C c,h,s
- Set the disk geometry (cylinders, heads, sectors) for the
scan. This is useful if a disk should be scanned which was partitioned
using a different geometry, if the device is a disk-image or if the
disk geometry cannot be retrieved through the PCs BIOS. No spaces are
allowed between the numbers, unless all three are enclosed in quotes.
- Check/compare mode (implies the -q quiet option).
After the scan is done, the resulting primary partition table is compared
to the existing one. The return code of gpart then contains the
number of differences (0 if they are identical except for the boot/active
flag which cannot be guessed). This option has no effect if -d is
given on the command line.
- Do not start the guessing loop. Useful if the partition
table should be printed (in combination with the -v option) without
actually scanning for partitions.
- Do not try to identify extended partition tables. If there
are extended partitions on the given device gpart will most
certainly complain about too many primary partitions because there can be
only four primary partitions. Existing logical partitions will be listed
as primary ones.
- Do not skip disk read errors. If this option is given, and
short disk reads or general disk read errors (EIO) are encountered,
gpart will exit. If not given, the program tries to continue.
- Full scan. When a possible partition is found, gpart
normally skips all sectors this entry seems to occupy and continues the
scan from the end of the last possible partition. The disk scan can take
quite a while if this option is given, be patient.
- Do not try to get the disk geometry from the OS. If the
device is no block or character device but a plain file this option
should be supplied. If the file to be scanned is an image of a disk, the
geometry can be given by the -C option.
- Show some help.
- Run interactively. Each time a possible partition is
identified the user is asked for confirmation.
- -K last sector
- Scan only up to the given sector or the end of the file or
device whichever comes first.
- -k sectors
- Skip given number of sectors before the scan. Potentially
useful if a partition is looked for at the end of a large disk.
- List available filesystem/partition type modules and their
weights, then exit.
- -l logfile
- Log output to the given file (even if -q was
- -n increment
- Scan increment: number of sectors or "s" for
single sector increment, "h" for an increment of sectors per
head (depends on geometry) or "c" for cylinder increment.
The increment also influences the condition where extended partition tables
are searched: if the scan increment is "s" (i.e. 1) extended
partition tables are required to be on a head boundary, otherwise they
must be on a cylinder boundary.
If the disk geometry could not be retrieved and no geometry was given on the
command line, the default increment is one sector.
- Quiet/no output mode. However if a logfile was specified
(see -l option) all output is written to that file. This option
overrides the -i interactive mode.
- -s sector size
- Preset medium sector size. gpart tries to find out
the sector size but may fail in doing so. Probed sector sizes are 2^i,
i=9..14 (512 to 16384 bytes). The default medium sector size is 512
- Show version number.
- Be verbose. This option can be given more than once
resulting in quite a lot of information.
- -W device
- Write partition table. If a consistent primary partition
table has been guessed it can be written to the specified file or device.
The supplied device can be the same as the scanned one.
Additionally the guessed partition entries can be edited. No checks are
performed on the entered values, thus the resulting table is allowed to be
highly inconsistent. Please beware.
Warning: The guessed partition table should be checked very carefully
before writing it back. You can always write the guessed partition table
into a plain file and write this into sector 0 using dd(1) (see
section PRECAUTIONS above).
- -w module name,weight
- Put the given module at the head of the module chain and
assign a new weight to that module. All modules are given an initial
weight of 1.0. Again no spaces are allowed.
Default settings are "-n h".
- To scan the first IDE hard disk under Linux using default settings type
- To print the primary partition table of the third IDE drive without
starting the scan loop in FreeBSD type
gpart -vvd /dev/wd2
- If lilo(8)
was installed in the master boot record (MBR) of a
hard disk it saves the contents of the first sector in a file called
/boot/boot.<major/minor>. To list the partitions contained in such a
file type e.g.
gpart -vdg /boot/boot.0300
If the partition table contains an extended partition, gpart
complain about invalid extended partition tables because the extended entry
points to sectors not within that file.
- Usually the first primary partition starts on the second head. If
cannot identify the first partition properly this may not be the
can be told to start the scan directly from sector one of
the disk, using the sector-wise scan mode:
gpart -k 1 -n s /dev/hdb
- Suppose gpart
identifies an NTFS partition as FAT on a certain
disk. In this situation the "ntfs" module should be made the first
module to be probed and given a weight higher than the usual weight of 1.0:
gpart -w ntfs,1.5 /dev/hdb
To list the available modules and their weights use the -L
- After having checked the output of gpart
at least thrice, the
primary partition table can be written back to the device this way:
gpart -W /dev/sdc /dev/sdc
This of course may be extremely dangerous to your health and social security, so
- A hard disk with 63 sectors per head is scanned in steps of 63 sectors.
To perform the scan on every second head while skipping the first 1008 sectors
gpart -k 1008 -n 126 /dev/sda
- If you want to see how easily gpart
can be mislead, and how many
probable partition starts are on a disk, search the whole disk really sector
by sector, writing all output to a logfile:
gpart -vvfn s -ql /tmp/gpart.log /dev/sd2
will not be able to produce an educated guess of the
primary partition table in this mode. The logfile however may contain enough
hints to manually reconstruct the partition table.
Hard disk block devices. The naming scheme of
hard disk block devices is OS dependent, consult your system manuals for more
There are many error message types, all of them should be self-explanatory.
Complain if they are not.
is beta software, so expect buggy behaviour.
only accepts extended partition links with one logical
partition. There may be fdisk
variants out there creating links with up
to three logical partition entries but these are not accepted.
- Support big-endian architectures.
- Test on 64-bit architectures.
- Look for boot manager partitions (e.g. OS/2 BM).
- Think about reconstructing logical partition chains.
Please send bug reports, suggestions, comments etc. to