Bytes and Sort

Press this button to create a simple file to manipulate. This will create a file "/home/caine/bindat".

Use xxd to examine bindat. What is the hex value stored at offset 0x61? Format your answer in hex with a leading 0x, e.g. 0xff.
Byte at 0x61:

Assuming a block size of 512, what is byte 6 of block 1 (where the first block is called block 0)?
Byte at block 1 offset 0x06:

If you do

echo -ne "\x65"

For instance, to set the byte offset 5 of file1.dd to 0x55, you could do:

echo -ne "\x55" | dd of=file1.dd seek=5 bs=1 count=1 conv=notrunc

Demonstrate this technique by copying bindat to a file "bincopy", and setting byte offset 29 to 0x9f.

Challenge question... try this question for a maximum of 10 minutes. If you cannot get it dont worry, as it is a little tricky!

Copy bindat to bincopy2, and change bincopy2 so that byte offsets 0x310 to 0x330 inclusive are set to 0x55.

Hints: Remember 0x310 to 0x330 INCLUSIVE. Remember there is an offset 0x000. Check what you are doing with xxd into less. And dd uses decimals not hex... Finally "count=" is the maximum size transferred, so make sure there is sufficient data comming from the echo to make this work... For instance:

echo -ne "\x55"                  -- This supplies 1 byte of data, or count=1
echo -ne "\x55\x55"              -- This supplies 2 bytes, so could do up to count=2

The next few questions check your understanding of the sort command. Sort can read from a pipe or from a specified filename. The basic way sort works is to sort on a column where the columns are seperated by spaces, so a file names.txt like:

gordon  russell
jim     jackson
bob     ludwiniak

sort names.txt

sort -k 2,2 names.txt

To practice, press the button below to create a file "stuff.txt" in /home/caine.

Sort stuff.txt into alphanumeric order based on column 1 (the md5 value). Save this into /home/caine/sort1.txt.

Now sort stuff.txt by column 2, and save the output as sort2.txt.

Introduce the "-u" flag to sort, which removes any duplicate entries (such as /bin/ls in out example file). The 'u' stands for 'unique'. So now sort stuff.txt by column 2, but make sure all rows are unique. Save the output to sort3.txt.

In these examples the character which split column 1 from column 2 was the space character. This is called the column delimiter. Sometimes columns will be seperated with other characters, such as "," for comma seperated files. So for instance if names.txt was:

gordon,russell
jim,jackson
bob,ludwiniak

sort -t"," names.txt

sort -t"," -k 2,2 names.txt

Sort /etc/passwd (delimited by the ":" character) by column 5 and save that output into sort4.txt.

If you are sortinga column which is composed of numbers, you need to decide if you want alphanumeric or numeric ordering. In alphanumeric ordering, "10" comes before "9" as the first character of "10" has a lower ascii value than "9". If you want numeric ordering where "10" is higher than "9", you must specify this in the "-k" command after each column number using the trailing character "n". So to sort column 10 numerically you need to do: "-k 10n,10n"

Firstly sort /etc/passwd on column 3 alphanumerically, saving the output to sort5.txt. Confirm that the last line of sort5.txt has a 9 in column 3.

Repeat the sort, but this time sort numerically and save the output into sort6.txt. Confirm that the last line of sort6.txt has the number 65534 in column 3.