shred

Securely delete a file

$ shred [file]

$ shred -u [file]

$ shred -n [5] [file]

$ shred -z [file]

$ shred -v [file]

$ shred -u [file1] [file2] [file3]

$ sudo shred -v /dev/[sdX]

shred [options] file...

shred overwrites files to make them harder to recover, then optionally deletes them. It writes random patterns multiple times over the file data, attempting to destroy information at the physical level.
By default, shred performs 3 passes of random overwrites. Adding -z performs a final pass with zeros, which makes the shredding less obvious (the file looks simply zeroed rather than deliberately destroyed).
Shred can be used on regular files or entire devices (disk partitions). For devices, it overwrites all data on the device.
The -u option removes the file after shredding, including attempting to rename it to disguise the original filename before unlinking.

-u, --remove

Truncate and remove file after overwriting

Overwrite N times (default: 3)

Add final overwrite with zeros to hide shredding

Show progress

Change permissions if necessary to allow writing

Shred only first N bytes (suffixes K, M, G accepted)

Get random bytes from FILE

Do not round file sizes up to full block

Journaling filesystems: On ext3, ext4, ReiserFS, and others, shred may not be effective because the filesystem may write data elsewhere before overwriting. Only the current location is shredded.
Copy-on-write filesystems: On Btrfs, ZFS, and similar, shred is ineffective as old data remains in previous snapshots.
SSD and flash storage: Wear leveling means data isn't overwritten in place. Use manufacturer's secure erase commands or full-disk encryption instead.
RAID: Striping and mirroring complicate secure deletion. Data may remain on individual disks.
For maximum security, use full-disk encryption from the start; then destroying the key makes all data unrecoverable regardless of filesystem or storage type.

rm(1), wipe(1), srm(1), dd(1)