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= 1. Basic `zzuf` usage =

`zzuf`’s behaviour is configured through the command line. A comprehensive list of flags and their meaning is given in the `zzuf` manual page. Just run '''`man zzuf`''' on your system to see it.

== 1.1. Launching `zzuf` ==

Let’s start with a simple command that reads data from a file. We choose `hd`, the hexadecimal dump command, so that we get a chance to observe what exactly happens to the data.

This is how to tell `hd` to read 32 bytes from `/dev/zero`:

{{{
% hd -vn 32 /dev/zero
00000000  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000010  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000020
%
}}}

Now let’s fuzz `hd`’s input using `zzuf`. It’s completely straightforward: just prepend `zzuf` to the commandline.

{{{
% zzuf hd -vn 32 /dev/zero
00000000  00 00 02 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000010  00 00 00 00 00 02 00 00  00 00 00 00 00 00 00 00  |................|
00000020
%
}}}

We see that two `00` values have been changed to `02`s. `zzuf` '''intercepted''' `hd`'s opening of `/dev/zero` and automatically '''corrupted''' the bits it read at random. Let’s do it again:

{{{
% zzuf hd -vn 32 /dev/zero
00000000  00 00 02 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000010  00 00 00 00 00 02 00 00  00 00 00 00 00 00 00 00  |................|
00000020
%
}}}

We get exactly the same output. This is a very important property of `zzuf`: its behaviour is '''reproducible'''.

== 1.2. Invoking different programs ==

Let’s fuzz the `cat` utility instead of `hd`, but read the final output with `hd` nonetheless:

{{{
% zzuf cat /dev/zero | hd -vn 32
00000000  00 00 02 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000010  00 00 00 00 00 02 00 00  00 00 00 00 00 00 00 00  |................|
00000020
%
}}}

Now instead of calling `hd`, let’s try `od`, the octal dumper:
{{{
% zzuf od -vN 32 /dev/zero
0000000 000000 000002 000000 000000 000000 000000 000000 000000
0000020 000000 000000 001000 000000 000000 000000 000000 000000
0000040
%
}}}

If you understand octal dumps as fluently as hexadecimal dumps, you noticed that the data has been fuzzed exactly like with `hd`.

This is another very important property of `zzuf`: '''data is fuzzed the same way regardless of the fuzzed application'''.

== 1.3. The fuzzing ratio ==

The '''fuzzing ratio''' is the proportion of bits that `zzuf` changes. It is specified with the '''`-r` flag'''. The default fuzzing ratio is 0.004, meaning "fuzz 0.4% of the bits". 32 bytes is 256 bits, and 0.4% of 256 bits is approximately 1. `zzuf` should have fuzzed 1 bit, but since it fuzzes bits at random, 2 bits is not surprising.

Let’s try fuzzing more bits, for instance 5% of the bits, using '''`-r` 0.05''':

{{{
% zzuf -r 0.05 hd -vn 32 /dev/zero
00000000  00 01 00 00 00 00 44 00  04 80 00 40 21 00 0a 20  |......D....@!.. |
00000010  40 20 00 04 00 00 02 00  00 00 00 00 00 00 00 00  |@ ..............|
00000020
%
}}}

We see that 15 bits have been changed. 5% of 256 bits is 12.8, so here again the behaviour is as expected.

Now let’s fuzz fewer bits, for instance 0.1%, using '''`-r` 0.001''':

{{{
% zzuf -r 0.001 hd -vn 32 /dev/zero
00000000  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000010  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000020
%
}}}

No bits have been changed, because 0.1% of 256 is 0.256, so there were few chances that the bits would be changed at all.

Very high fuzzing ratios can be specified, for instance 50%, using '''`-r` 0.5''':

{{{
% zzuf -r 0.5 hd -vn 32 /dev/zero  
00000000  c0 a0 20 b0 ad 40 07 c2  8a 14 30 1b 83 21 1a 69  |.. ..@....0..!.i|
00000010  11 28 05 07 30 00 70 01  43 08 62 c8 6d 45 e4 1a  |.(..0.p.C.b.mE..|
00000020
%
}}}

== 1.4. The random seed ==

`zzuf`’s behaviour is reproducible, but we might not be satisfied with the output. Or we may simply want to fuzz in several different ways, but still using the same fuzzing ratio. This is done by changing the '''random seed''' with the '''`-s` flag'''. The random seed is the initial value of `zzuf`’s random number generator. The default seed is 0, so let’s try with other values:

{{{
% zzuf -s 2 hd -vn 32 /dev/zero
00000000  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000010  00 00 00 00 80 00 00 00  00 00 00 00 00 00 00 00  |................|
00000020
% zzuf -s 79432 hd -vn 32 /dev/zero  
00000000  00 00 00 00 00 00 00 20  00 00 00 00 00 00 00 00  |....... ........|
00000010  00 00 00 00 00 02 00 00  00 00 00 00 00 00 00 00  |................|
00000020
%
}}}

As can be seen, each seed value initiates a different behaviour of the random number generator.

== 1.5. Creating fuzzed files ==

It is possible to fuzz files directly, without calling applications at all.

To do so, simply call `zzuf` with no application argument. It will fuzz its standard input by default:

{{{
% cat /dev/zero | zzuf | hd -vn32          
00000000  00 00 02 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000010  00 00 00 00 00 02 00 00  00 00 00 00 00 00 00 00  |................|
00000020
%
}}}

`zzuf` can be used to create files. Again, the behaviour is entirely reproducible:

{{{
% dd if=/dev/zero bs=1 count=32 | zzuf > output.file
32+0 records in
32+0 records out
32 bytes (32 B) copied, 9.1129e-05 s, 351 kB/s
% hd -v output.file
00000000  00 00 02 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000010  00 00 00 00 00 02 00 00  00 00 00 00 00 00 00 00  |................|
00000020
%
}}}

This may be used if a given application is not supported by `zzuf`, but it is especially useful to generate files that reproduce `zzuf`’s behaviour without requiring `zzuf` at all.

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