blkid
Identify a filesystem by label, UUID and type
blkid says what a block device holds: its filesystem type, its label and its UUID, all of them
set when mkfs made the filesystem. Those are the
three things mount and /etc/fstab name a device by, and the reason to use
them instead of /dev/sda1 is that they follow the filesystem rather than the slot it happens to
be plugged into.
-o value -s UUID prints one field and nothing else, which is the form for a script. An exit status
of 2 means nothing was recognised, so an unformatted device and a missing one are the same answer.
Results come from a cache under /run unless -c /dev/null says otherwise, and a device whose
label changed since it was last probed is answered from the old reading.
Sample files used on this page
Every example below was run against these files. Recreate them to follow along.
two disk images a container's block devices belong to whoever is running it, so this page brings its own filesystem rather than reporting on those. data.img holds a 32M ext4 labelled tipsdata with a pinned UUID; blank.img is 16M of zeros with nothing on it. blkid reads a file as readily as a device, so most examples here name the image directly. The same pair backs mount and fsck.
total 20912
-rw-r--r-- 1 root root 16777216 Jun 15 10:00 blank.img
-rw-r--r-- 1 root root 33554432 Jun 15 10:00 data.img
What is on this device
Given a device or an image, blkid prints the tags it recognised. TYPE is the filesystem, LABEL and UUID are what an /etc/fstab entry should name it by, and BLOCK_SIZE is what the filesystem was made with.
Identify a filesystem
blkid /srv/images/data.imgThe tags come from the superblock, read without mounting anything. This is the command to run on an unfamiliar disk before deciding what to do with it.
Show output
/srv/images/data.img: LABEL="tipsdata" UUID="4f6b2c18-9a3d-4e07-b5c1-2d8e7a904f31" BLOCK_SIZE="1024" TYPE="ext4"
Ask about something with no filesystem on it
blkid /srv/images/blank.img; echo "exit $?"No output and a status of 2, which is the same answer a device that does not exist gives. Worth pairing with a test -e when a script needs to tell the two apart.
Show output
exit 2
Identify a loop device rather than the file behind it
loop=$(losetup --find --show /srv/images/data.img)
blkid "$loop"The same filesystem under its device name. On real hardware this is the only form there is, since the filesystem has no file standing behind it.
Show output
Your output will differ: the loop device number depends on what else on this machine has one attached
/dev/loop0: LABEL="tipsdata" UUID="4f6b2c18-9a3d-4e07-b5c1-2d8e7a904f31" BLOCK_SIZE="1024" TYPE="ext4"
Show one tag instead of all of them
blkid -s TYPE /srv/images/data.img-s keeps the named tag and drops the rest, with the device name still in front of it. -s can be repeated to keep several.
Show output
/srv/images/data.img: TYPE="ext4"
Keep two tags and drop the rest
blkid -s LABEL -s UUID -o export /srv/images/data.imgRepeating -s accumulates rather than replacing, so this asks for the two identifiers and nothing about the filesystem's shape. DEVNAME is always included under -o export.
Show output
DEVNAME=/srv/images/data.img
LABEL=tipsdata
UUID=4f6b2c18-9a3d-4e07-b5c1-2d8e7a904f31
Output for a script
The default format is for reading. -o value prints the values alone, -o export prints assignments a shell can source, and both are stable in a way the default layout is not.
Get a UUID with nothing around it
blkid -o value -s UUID /srv/images/data.img-o value with a single -s is the form to put in a command substitution. Without the -s it prints every value on its own line, which is rarely what a script wants.
Show output
4f6b2c18-9a3d-4e07-b5c1-2d8e7a904f31
Get the label
blkid -o value -s LABEL /srv/images/data.imgA label is chosen by whoever made the filesystem and nothing stops two disks sharing one, so it is the friendlier identifier and the less reliable one.
Show output
tipsdata
Get the filesystem type
blkid -o value -s TYPE /srv/images/data.imgThe answer mount -t wants, and the one to branch on in a script that handles more than one filesystem. An empty answer with status 2 means nothing recognised it.
Show output
ext4
Build an fstab line from the device
u=$(blkid -o value -s UUID /srv/images/data.img)
echo "UUID=$u /mnt/data ext4 loop 0 0"The reason -o value -s exists. Generating the line rather than typing the UUID keeps a transposed character from taking the machine down at its next boot.
Show output
UUID=4f6b2c18-9a3d-4e07-b5c1-2d8e7a904f31 /mnt/data ext4 loop 0 0
Print assignments a shell can read
blkid -o export /srv/images/data.img-o export is shaped for eval or for sourcing, and it names the device as DEVNAME. The tag names match what udev puts in the environment, which is where the format comes from.
Show output
DEVNAME=/srv/images/data.img
LABEL=tipsdata
UUID=4f6b2c18-9a3d-4e07-b5c1-2d8e7a904f31
BLOCK_SIZE=1024
TYPE=ext4
Print a table with the mount state
blkid -o list /srv/images/data.img-o list adds where the filesystem is mounted, which the other formats do not carry. The blank line under the header is part of the format rather than a missing row.
Show output
device fs_type label mount point UUID
/srv/images/data.img
ext4 tipsdata (not mounted) 4f6b2c18-9a3d-4e07-b5c1-2d8e7a904f31
Searching
-L and -U go the other way: given a label or a UUID, they name the device carrying it. -t filters a list of devices down to the ones matching a tag.
Find the device with a given label
losetup --find --show /srv/images/data.img >/dev/null
blkid -L tipsdata; echo "exit $?"The lookup mount performs for LABEL=tipsdata, and running it by hand is the way to check an fstab entry resolves before rebooting on it. The answer is the device rather than the image behind it, because that is what a mount would be given.
Show output
Your output will differ: the loop device number depends on what else on this machine has one attached
/dev/loop0
exit 0
Find the device with a given UUID
losetup --find --show /srv/images/data.img >/dev/null
blkid -U 4f6b2c18-9a3d-4e07-b5c1-2d8e7a904f31; echo "exit $?"Same lookup against the identifier that cannot collide. -U takes the UUID exactly, so a partial one finds nothing rather than matching a prefix.
Show output
Your output will differ: the loop device number depends on what else on this machine has one attached
/dev/loop0
exit 0
Search for a label nothing has
blkid -L nosuchlabel; echo "exit $?"Status 2 and no output. An fstab entry naming a label in this state drops the machine into an emergency shell at boot, which is what findmnt --verify exists to catch first.
Show output
exit 2
Filter several devices by tag
blkid -t TYPE=ext4 /srv/images/data.img /srv/images/blank.img; echo "exit $?"-t keeps only the devices whose tag matches, so the blank image is absent from the answer rather than reported as empty. Given no device list it searches every block device on the machine.
Show output
/srv/images/data.img: LABEL="tipsdata" UUID="4f6b2c18-9a3d-4e07-b5c1-2d8e7a904f31" BLOCK_SIZE="1024" TYPE="ext4"
exit 0
Restrict which filesystem types are considered
blkid -n ext4 /srv/images/data.img; echo "exit $?"-n limits the probe to the types you name, and -n noext4 inverts it. Useful against a device carrying the remains of an older filesystem, where the probe can find more than one signature.
Show output
/srv/images/data.img: LABEL="tipsdata" UUID="4f6b2c18-9a3d-4e07-b5c1-2d8e7a904f31" BLOCK_SIZE="1024" TYPE="ext4"
exit 0
The cache, and probing past it
blkid keeps what it has seen in /run/blkid/blkid.tab and will answer from there. -c /dev/null forces a fresh read of the device, and -p bypasses the cache entirely and reports what the probe itself found.
Read the device rather than the cache
blkid -c /dev/null -o value -s UUID /srv/images/data.img-c names a cache file and /dev/null is the way to say there is not one. Worth having in any script that has just changed a filesystem, since the cached answer describes the device as it was.
Show output
4f6b2c18-9a3d-4e07-b5c1-2d8e7a904f31
Change a UUID and read it back
tune2fs -U 99999999-8888-7777-6666-555555555555 /srv/images/data.img >/dev/null
blkid -c /dev/null -o value -s UUID /srv/images/data.imgtune2fs -U rewrites the identifier in the superblock, which breaks every /etc/fstab entry and every bootloader configuration naming the old one. Do the fstab edit first.
Show output
99999999-8888-7777-6666-555555555555
Probe low level instead of reporting tags
blkid -p -s TYPE -o value /srv/images/data.img-p skips the cache and probes the device itself, as udev does. It also refuses to run on a mounted filesystem, since the answer could change underneath it.
Show output
ext4
See everything the probe found
blkid -p -o udev /srv/images/data.img-o udev prints the full set of properties in the form udev stores them, including the size and block counts the ordinary output leaves out. This is where to look when a filesystem is recognised but something about it is wrong.
Show output
ID_FS_LABEL=tipsdata
ID_FS_LABEL_ENC=tipsdata
ID_FS_UUID=4f6b2c18-9a3d-4e07-b5c1-2d8e7a904f31
ID_FS_UUID_ENC=4f6b2c18-9a3d-4e07-b5c1-2d8e7a904f31
ID_FS_VERSION=1.0
ID_FS_FSBLOCKSIZE=1024
ID_FS_BLOCK_SIZE=1024
ID_FS_FSLASTBLOCK=32768
ID_FS_FSSIZE=33554432
ID_FS_TYPE=ext4
ID_FS_USAGE=filesystem
Probe a device with nothing on it
blkid -p /srv/images/blank.img; echo "exit $?"Status 2 from the probe as well as from the cached form, so neither can tell an empty device from one whose filesystem the probe does not recognise.
Show output
exit 2
Signatures on the device
What blkid reads is a signature written at a known offset. wipefs lists those signatures and removes them, which is the honest way to make a device look empty.
List the signatures on a device
wipefs /srv/images/data.imgA single signature, at offset 0x438, which is where ext2, ext3 and ext4 all keep their magic number. Listing is what wipefs does without -a, so this is safe to run on anything.
Show output
DEVICE OFFSET TYPE UUID LABEL
data.img 0x438 ext4 4f6b2c18-9a3d-4e07-b5c1-2d8e7a904f31 tipsdata
Erase the signature and watch blkid lose it
wipefs -a /srv/images/data.img
blkid -c /dev/null /srv/images/data.img; echo "exit $?"The filesystem is unrecognisable after two bytes went, though every block of data is still there. Run it before reusing a disk, and on anything whose old signature is still getting it mounted by accident.
Show output
/srv/images/data.img: 2 bytes were erased at offset 0x00000438 (ext4): 53 ef
exit 2