There was an upload recently on generating a pseudo-random file when /dev/[u]random does NOT exist.
This does not need /dev/random, /dev/urandom or $RANDOM either...
(I assume $RANDOM relies on the /dev/[u]random device in some way.)
This code uses hexdump just because I like hexdump for ease of use, but od or xxd could well be used with some minor code alteration instead...
The random file is NOT great, and it IS SSLLOOWW, but hey - IT WORKS...
I found this problem interesting, although it may not be of much use...
Code:
#!/bin/bash --posix
# random.sh
> /tmp/binary.file
count=1
random_file_size=1024
hex_value="00"
dec_value=0
seed=17
subscript=9999
file_size=$(echo -n $(wc -c < /bin/bash))
# This could produce an error if inputted value is not a number!
read -p "Enter a pseudo-seed from 9999 to 32767:- " -e subscript
if [ "$subscript" == "" ]; then subscript=9999; fi
if [ $subscript -le 9999 ]; then subscript=9999; fi
if [ $subscript -ge 32767 ]; then subscript=32767; fi
# This is the DEMO working loop and in this case generates a 256 byte "random" file.
for count in $( seq 1 1 $random_file_size )
do
hex_value=$(hexdump -n1 -s$subscript -v -e '1/1 "%02x"' /bin/bash)
dec_value=$(hexdump -n1 -s$subscript -v -e '1/1 "%u"' /bin/bash)
subscript=$[ ( $subscript + $dec_value + $seed ) ]
if [ $subscript -ge $file_size ]; then subscript=9999; fi
echo -e -n '\x'"$hex_value" >> /tmp/binary.file
done
hexdump -C /tmp/binary.file
With "not great" you probably mean that the periodicity is low: there are pseudo-seeds where the periodicity is less than 30.
The quality of the output of the algorithm is also very sensitive to the value of seed (line 10).
The quality of the output of the algorithm is also very sensitive to the value of seed (line 10).
I was well aware of this that is why I used a variable instead of a fixed value inside line 25.
The idea was to use the last byte read as the next "seed" but I didn't pursue it any further. "seed" must always
have a value greater than or equal to 1 and less than or equal to decimal value 256 added to the current byte in use.
(I assumed you guys would see the flaw and correct as you need.)
However if the current byte value would be zero then "seed" would have to be a minimum value of "1".
(This is taken care of with the limitation of fixing "seed" in the script shown.)
Not because of anything to do with binary zero in shell scripting per-se but it would just stop there until the
loop finished and give that character value at that point continuously ( <- if that sentence is lucid enough. ).
Bazza...
EDIT: Made more lucid.
This is a simple way of making it a little better without the complexity of storing the previous and current
bytes read. (Line 26.)
Code:
if [ $subscript -ge $file_size ]; then subscript=$[ ( $dec_value + 9999 ) ]; fi
Last edited by wisecracker; 09-21-2013 at 06:27 PM..
Reason: Rewrite to make more lucid...
I am using the below to random generate a password but I need to have 2 numeric characters and 6 alphabetic chars
head /dev/urandom | tr -dc A-Za-z0-9 | head -c 8 ; echo ''
6USUvqRB
------ Post updated at 04:43 PM ------
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