BLKID(8)

NAME
blkid - locate/print block device attributes

SYNOPSIS
blkid -L label | -U uuid blkid [-dghlv] [-c file] [-o format] [-s tag]
[-t NAME=value] [device ...]
blkid -p [-O offset] [-o format] [-S size] [-s tag]
[-n list] [-u list] device ...
blkid -i [-o format] [-s tag] device ...

DESCRIPTION
The blkid program is the command-line interface to working with the libblkid(3) library. It can determine the type of content (e.g. filesystem or swap) that a block device holds, and also the attributes (tokens, NAME=value pairs) from the content metadata (e.g. LABEL or UUID fields).

It is recommended to use lsblk(8) command to get information about block devices, or lsblk --fs to get an overview of filesystems, or findmnt(8) to search in already mounted filesystems.

    lsblk(8) provides more information, better control on output formatting, easy to use in scripts and it does not require root permissions to get actual information. blkid reads information directly from devices and for non-root users it returns cached unverified information. blkid is mostly designed for system services and to test libblkid functionality.

When device is specified, tokens from only this device are displayed. It is possible to specify multiple device arguments on the command line. If none is given, all devices which appear in /proc/partitions are shown, if they are recognized.

blkid has two main forms of operation: either searching for a device with a specific NAME=value pair, or displaying NAME=value pairs for one or more specified devices.

For security reasons blkid silently ignores all devices where the probing result is ambivalent (multiple colliding filesystems are detected). The low-level probing mode (-p) provides more information and extra return code in this case. It's recommended to use wipefs(8) to get a detailed overview and to erase obsolete stuff (magic strings) from the device.

OPTIONS
The size and offset arguments may be followed by the multiplicative suffixes like KiB (=1024), MiB (=1024*1024), and so on for GiB, TiB, PiB, EiB, ZiB and YiB (the "iB" is optional, e.g. "K" has the same meaning as "KiB"), or the suffixes KB (=1000), MB (=1000*1000), and so on for GB, TB, PB, EB, ZB and YB.

-c cachefile
    Read from cachefile instead of reading from the default cache file (see the CONFIGURATION FILE section for more details). If you want to start with a clean cache (i.e. don't report devices previously scanned but not necessarily available at this time), specify /dev/null.

-d  Don't encode non-printing characters. The non-printing characters are encoded by ^ and M- notation by default. Note that the -o udev output format uses a different encoding which cannot be disabled.

-g  Perform a garbage collection pass on the blkid cache to remove devices which no longer exist.

-h  Display a usage message and exit.

-i  Display information about I/O Limits (aka I/O topology). The 'export' output format is automatically enabled. This option can be used together with the -p option.

-k  List all known filesystems and RAIDs and exit.

-l  Look up only one device that matches the search parameter specified with the -t option. If there are multiple devices that match the specified search parameter, then the device with the highest priority is returned, and/or the first device found at a given priority. Device types in order of decreasing priority are: Device Mapper, EVMS, LVM, MD, and finally regular block devices. If this option is not specified, blkid will print all of the devices that match the search parameter.

-L label
    Look up the device that uses this filesystem label; this is equal to -l -o device -t LABEL=label. This lookup method is able to reliably use /dev/disk/by-label udev symlinks (dependent on a setting in /etc/blkid.conf). Avoid using the symlinks directly; it is not reliable to use the symlinks without verification. The -L option works on systems with and without udev. Unfortunately, the original blkid(8) from e2fsprogs uses the -L option as a synonym for -o list. For better portability, use -l -o device -t LABEL=label and -o list in your scripts rather than the -L option.

-n list
    Restrict the probing functions to the specified (comma-separated) list of superblock types (names). The list items may be prefixed with "no" to specify the types which should be ignored. For example:

      blkid -p -n vfat,ext3,ext4 /dev/sda1

    probes for vfat, ext3 and ext4 filesystems, and

      blkid -p -n nominix /dev/sda1

    probes for all supported formats except minix filesystems. This option is only useful together with -p.

-o format
    Use the specified output format. Note that the order of variables and devices is not fixed. See also option -s. The format parameter may be:

    full
        print all tags (the default)

    value
        print the value of the tags

    list
        print the devices in a user-friendly format; this output format is unsupported for low-level probing (-p or -i). This output format is DEPRECATED in favour of the lsblk(8) command.

    device
        print the device name only; this output format is always enabled for the -L and -U options

    udev
        print key="value" pairs for easy import into the udev environment; the keys are prefixed by ID_FS_ or ID_PART_ prefixes The udev output returns the ID_FS_AMBIVALENT tag if more superblocks are detected, and ID_PART_ENTRY_* tags are always returned for all partitions including empty partitions. This output format is DEPRECATED.

    export
        print key=value pairs for easy import into the environment; this output format is automatically enabled when I/O Limits (-i option) are requested

-O offset
    Probe at the given offset (only useful with -p). This option can be used together with the -i option.

-p  Switch to low-level superblock probing mode (bypassing the cache). Note that low-level probing also returns information about partition table type (PTTYPE tag) and partitions (PART_ENTRY_* tags).

-s tag
    For each (specified) device, show only the tags that match tag. It is possible to specify multiple -s options. If no tag is specified, then all tokens are shown for all (specified) devices. In order to just refresh the cache without showing any tokens, use -s none with no other options.

-S size
    Override the size of device/file (only useful with -p).

-t NAME=value
    Search for block devices with tokens named NAME that have the value value, and display any devices which are found. Common values for NAME include TYPE, LABEL, and UUID. If there are no devices specified on the command line, all block devices will be searched; otherwise only the specified devices are searched.

-u list
    Restrict the probing functions to the specified (comma-separated) list of "usage" types. Supported usage types are: filesystem, raid, crypto and other. The list items may be prefixed with "no" to specify the usage types which should be ignored. For example:

      blkid -p -u filesystem,other /dev/sda1

    probes for all filesystem and other (e.g. swap) formats, and

      blkid -p -u noraid /dev/sda1

    probes for all supported formats except RAIDs. This option is only useful together with -p.

-U uuid
    Look up the device that uses this filesystem uuid. For more details see the -L option.

-V  Display version number and exit.

RETURN CODE
If the specified token was found, or if any tags were shown from (specified) devices, 0 is returned. If the specified token was not found, or no (specified) devices could be identified, an exit code of 2 is returned. For usage or other errors, an exit code of 4 is returned. If an ambivalent probing result was detected by low-level probing mode (-p), an exit code of 8 is returned.

CONFIGURATION FILE
The standard location of the /etc/blkid.conf config file can be overridden by the environment variable BLKID_CONF. The following options control the libblkid library:

SEND_UEVENT=<yes|not>
    Sends uevent when /dev/disk/by-{label,uuid,partuuid,partlabel}/ symlink does not match with LABEL, UUID, PARTUUID or PARTLABEL on the device. Default is "yes".

CACHE_FILE=<path>
    Overrides the standard location of the cache file. This setting can be overridden by the environment variable BLKID_FILE. Default is /run/blkid/blkid.tab, or /etc/blkid.tab on systems without a /run directory.

EVALUATE=<methods>
    Defines LABEL and UUID evaluation method(s). Currently, the libblkid library supports the "udev" and "scan" methods. More than one method may be specified in a comma-separated list. Default is "udev,scan". The "udev" method uses udev /dev/disk/by-* symlinks and the "scan" method scans all block devices from the /proc/partitions file.

AUTHOR
blkid was written by Andreas Dilger for libblkid and improved by Theodore Ts'o and Karel Zak.

ENVIRONMENT

Setting LIBBLKID_DEBUG=0xffff enables debug output.

SEE ALSO
libblkid(3), findfs(8), wipefs(8)

AVAILABILITY
The blkid command is part of the util-linux package and is available from ftp://ftp.kernel.org/pub/linux/utils/util-linux/.