Disks and filesystems

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badblocks -v- s /dev/sdb >badblocks.log
Check a device for bad blocks
fdisk /dev/sdb
Manage disk partitions
df -h
Show filesystem block usage in human friendly format
df -i
Show filesystem inode usage
ls -i <file>
Show inode of <file>
rm -i <file>
Remove a file by its inode
mount -t <fstype> -o <options> <devicefile> <mountpoint>
Mount a filesystem (fstype and options can be omitted often)
mount -o loop /path/to/my-iso-image.iso /mnt/iso
Mount an .iso file


Performance

The graphical utility Disks has a benchmark option.

dd if=/dev/zero of=/dev/sdb bs=8k count=10k
Test disk write speed if no filesystems exists
dd if=/dev/zero of=/tmp/output bs=8k count=10k; rm -f /tmp/output
Test disk write speed on existing filesystems

Misc

/dev/null
The unix black hole. Write always succeeds with no effect at all.
/dev/zero
Provides an unlimited amount of null characters (ASCII 0). Can be used for cleaning disks or benchmarking (see #Performance)
Writing to /dev/zero is the same as writing to /dev/null