[SOLVED] How to Write Block of Random Data to File using Bash

How to Write Block of Random Data to File using Bash

I need to write a 5MB block of data obtained from /dev/urandom to a partition, at a specific location in the partition. I then need to check that the write executed correctly. I was previously successfully doing this in C++, but now I would like to implement it in a bash script.

My C++ code consisted of:

create 10MB array of data populated from /dev/urandom (RANDOM_ARRAY)
open partition with open()
use lseek() to navigate to desired position in partition
use write() to write the array into the partition
close and reopen partition, use lseek to navigate back to desired position
use read() to read 5MB at this position and populate another array with this data (WRITTEN_ARRAY)
compare each element in (RANDOM_ARRAY) with (WRITTEN_ARRAY)

Im not experienced with writing bash scripts but this is what I've got so far, although it doesn't seem to work:

random_data="$(cat /dev/urandom | head -c<5MB>)"

printf $random_data | dd of=<partition_path> bs=1 seek=<position_in_partition> count=<5MB>

file_data=$(dd if=<partition_path> bs=1 skip=<position_in_partition> count=<5MB>)

if [ "$random_data" == "$file_data" ]
then
 echo "data write successful"
fi

Thanks to the helpful commenters my script now looks something like this:

# get 10MB random data 
head -c<10MB> /dev/urandom > random.bin
# write 10MB random data to partition
dd if=random.bin of=<partition_location>
# copy the written data
dd if=<partition_location> count=<10MB/512 bytes> of=newdata.bin
# compare 
cmp random.bin newdata.bin

At this point cmp returns that the first char is different. Looking at a verbose output of cmp and turns out all values in newdata.bin are 0.

Solution

Here's a simpler approach which just saves the data in a temporary file.

#!/bin/sh

set -e

random_data=$(mktemp -t ideone.XXXXXXXX) || exit
trap 'rm -rf "$t"' EXIT

dd if=/dev/urandom bs=10240 count=512 of="$random_data"

dd if="$random_data" of=<partition_path> bs=1 seek=<position_in_partition> count=<5MB>

if dd if=<partition_path> bs=1 skip=<position_in_partition> count=<5MB> |
    cmp "$random_data"
then
 echo "data write successful"
fi