Here a solution without xxd or perl:

If the echo builtin of your shell supports it (bash and zsh do, but not dash), you just need to use the right backslash escapes:

echo -ne '\x00\x06\x30\x30\x30\x30\x4e\x43' | nc -l localhost 8181

If you have /bin/echo from GNU coreutils (nearly standard on Linux systems) or from busybox you can use it, too.

With sed you can generate a escaped pattern:

$ echo '0006303030304e43' | sed -e 's/../\\x&/g'
\x00\x06\x30\x30\x30\x30\x4e\x43

Combined:

echo -ne "$(echo '0006303030304e43' | sed -e 's/../\\x&/g')" | nc -l localhost 8181

Convert hex to binary

I have ABC123EFFF.

I want to have 001010101111000001001000111110111111111111 (i.e. binary repr. with, say, 42 digits and leading zeroes).

Short answer:

The new f-strings in Python 3.6 allow you to do this using very terse syntax:

>>> f'{0xABC123EFFF:0>42b}'
'001010101111000001001000111110111111111111'

or to break that up with the semantics:

>>> number, pad, rjust, size, kind = 0xABC123EFFF, '0', '>', 42, 'b'
>>> f'{number:{pad}{rjust}{size}{kind}}'
'001010101111000001001000111110111111111111'

Long answer:

What you are actually saying is that you have a value in a hexadecimal representation, and you want to represent an equivalent value in binary.

The value of equivalence is an integer. But you may begin with a string, and to view in binary, you must end with a string.

Convert hex to binary, 42 digits and leading zeros?

We have several direct ways to accomplish this goal, without hacks using slices.

First, before we can do any binary manipulation at all, convert to int (I presume this is in a string format, not as a literal):

>>> integer = int('ABC123EFFF', 16)
>>> integer
737679765503

alternatively we could use an integer literal as expressed in hexadecimal form:

>>> integer = 0xABC123EFFF
>>> integer
737679765503

Now we need to express our integer in a binary representation.

Use the builtin function, format

Then pass to format:

>>> format(integer, '0>42b')
'001010101111000001001000111110111111111111'

This uses the formatting specification's mini-language.

To break that down, here's the grammar form of it:

[[fill]align][sign][#][0][width][,][.precision][type]

To make that into a specification for our needs, we just exclude the things we don't need:

>>> spec = '{fill}{align}{width}{type}'.format(fill='0', align='>', width=42, type='b')
>>> spec
'0>42b'

and just pass that to format

>>> bin_representation = format(integer, spec)
>>> bin_representation
'001010101111000001001000111110111111111111'
>>> print(bin_representation)
001010101111000001001000111110111111111111

String Formatting (Templating) with str.format

We can use that in a string using str.format method:

>>> 'here is the binary form: {0:{spec}}'.format(integer, spec=spec)
'here is the binary form: 001010101111000001001000111110111111111111'

Or just put the spec directly in the original string:

>>> 'here is the binary form: {0:0>42b}'.format(integer)
'here is the binary form: 001010101111000001001000111110111111111111'

String Formatting with the new f-strings

Let's demonstrate the new f-strings. They use the same mini-language formatting rules:

>>> integer = 0xABC123EFFF
>>> length = 42
>>> f'{integer:0>{length}b}'
'001010101111000001001000111110111111111111'

Now let's put this functionality into a function to encourage reusability:

def bin_format(integer, length):
    return f'{integer:0>{length}b}'

And now:

>>> bin_format(0xABC123EFFF, 42)
'001010101111000001001000111110111111111111'    

Aside

If you actually just wanted to encode the data as a string of bytes in memory or on disk, you can use the int.to_bytes method, which is only available in Python 3:

>>> help(int.to_bytes)
to_bytes(...)
    int.to_bytes(length, byteorder, *, signed=False) -> bytes
...

And since 42 bits divided by 8 bits per byte equals 6 bytes:

>>> integer.to_bytes(6, 'big')
b'\x00\xab\xc1#\xef\xff'

Fast, and works for large strings:

    private String hexToBin(String hex){
        hex = hex.replaceAll("0", "0000");
        hex = hex.replaceAll("1", "0001");
        hex = hex.replaceAll("2", "0010");
        hex = hex.replaceAll("3", "0011");
        hex = hex.replaceAll("4", "0100");
        hex = hex.replaceAll("5", "0101");
        hex = hex.replaceAll("6", "0110");
        hex = hex.replaceAll("7", "0111");
        hex = hex.replaceAll("8", "1000");
        hex = hex.replaceAll("9", "1001");
        hex = hex.replaceAll("A", "1010");
        hex = hex.replaceAll("B", "1011");
        hex = hex.replaceAll("C", "1100");
        hex = hex.replaceAll("D", "1101");
        hex = hex.replaceAll("E", "1110");
        hex = hex.replaceAll("F", "1111");
        return hex;
    }

const char input[] = "..."; // the value to be converted
char res[9]; // the length of the output string has to be n+1 where n is the number of binary digits to show, in this case 8
res[8] = '\0';
int t = 128; // set this to s^(n-1) where n is the number of binary digits to show, in this case 8
int v = strtol(input, 0, 16); // convert the hex value to a number

while(t) // loop till we're done
{
    strcat(res, t < v ? "1" : "0");
    if(t < v)
        v -= t;
    t /= 2;
}
// res now contains the binary representation of the number

As an alternative (this assumes there's no prefix like in "0x3A"):

const char binary[16][5] = {"0000", "0001", "0010", "0011", "0100", ...};
const char digits = "0123456789abcdef";

const char input[] = "..." // input value
char res[1024];
res[0] = '\0';
int p = 0;

while(input[p])
{
    const char *v = strchr(digits, tolower(input[p++]));
    if (v)
        strcat(res, binary[v - digits]);
}
// res now contains the binary representation of the number