The danger you are trying to mitigate is filesystem journal, i.e.
shred is not effective on filesystems that have a journal (e.g. ext3, ext4, reiserfs).
Assuming you are not using any unionfs for persistence (apparently you can do it in Tails although I never tried), everything is stored in a
The linux documentation on
tmpfs does not detail whether is performs journaling. Yet,
tmpfs is based on
ramfs, the same filesystem that is used in
initramfs, and that filesystem does not have a journal. Therefore it is (more-or-less) safe to assume that
tmpfs does not have a journal as well.
On a filesystem without a journal
shred will perform the overwrite of the file, making it difficult to recover with analytical tools (pretty much impossible to recover from a RAM dump). Since everything happens in memory pages, and the inodes of a
tmpfs simply point to memory pages, using
shred is much better because it will be able to write to these memory pages.
The above certainly works in this way on Tails, and on Knoppix. It will likely work in a similar fashion on almost all Linux distros on LiveCDs, including Kali Linux yet there is a caveat.
This works for files! Memory will also contain application memory, see Gilles' answer on application memory. Seriously, look at that answer, it opens an important point.
Also a distro based on Ubuntu Linux (that may or may not include Kali Linux* since its predecessor, Backtrack, was based on Ubuntu) will mount any swap it finds on the machine it boots, which may leave a much worse attack vector! Persistent data on the device itself!
Another caveat with Kali Linux, is that it comes with
metasploit and boots the
postgres database for use with
metasploit. Postgres has its own journalling (which is file based not filesystem based), which you may want to shred as well (i.e. shred the postgres files not just delete the data through
* Kali is not based on Ubuntu, it is based on Debian, yet I am not confident whether it dropped all its configuration scripts from the time it was called Backtrack and was based on Ubuntu