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I know both are for secure file deletion, but when should one be used instead of the other? Do both overwrite files with 0's?

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Both commands overwrite data using patterns. Which to use in what situations will most certainly be a matter of opinion, I would likely shred a file and scrub a disk, However as that's just my opinion, the following information may be useful to you.

 SHRED:  Note  that shred relies on a very important assumption: that the file system overwrites data in place.  This is the tradi‐
   tional way to do things, but many modern file system designs do not satisfy this assumption.  The following  are  examples  of  file
   systems on which shred is not effective, or is not guaranteed to be effective in all file system modes:

   * log-structured or journaled file systems, such as those supplied with AIX and Solaris (and JFS, ReiserFS, XFS, Ext3, etc.)

   * file systems that write redundant data and carry on even if some writes fail, such as RAID-based file systems

   * file systems that make snapshots, such as Network Appliance's NFS server

   * file systems that cache in temporary locations, such as NFS version 3 clients

   * compressed file systems

   In  the  case  of  ext3 file systems, the above disclaimer applies (and shred is thus of limited effectiveness) only in data=journal
   mode, which journals file data in addition to just metadata.  In both the data=ordered (default)  and  data=writeback  modes,  shred
   works as usual.  Ext3 journaling modes can be changed by adding the data=something option to the mount options for a particular file
   system in the /etc/fstab file, as documented in the mount man page (man mount).

   In addition, file system backups and remote mirrors may contain copies of the file that cannot be removed, and  that  will  allow  a
   shredded file to be recovered later.

Source 'man shred'

Scrub: scrub

Image Source: Software Center

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