You can use the search replace functionality of sed:

echo "00124a5b" | sed 's/../\\x&/g'
\x00\x12\x4a\x5b

The two dots search for any two characters in the stream. The \\x& replaces the match with a \x followed by the match. Adding the g on end tells sed to continue the search/replace.

I would check out this tutorial for sed: http://www.grymoire.com/Unix/Sed.html There are several other tutorials on that site for other helpful commands.

I used the -r and -p switches for xxd:

$ echo '0006303030304e43' | xxd -r -p | nc -l localhost 8181

Thanks to inspiration from @Gilles' answer, here's a Perl version:

$ echo '0006303030304e43' | perl -e 'print pack "H*", <STDIN>' | nc -l localhost 8181

What about this —

$ echo 00: 0123456789abcdef | xxd -r | xxd -g 8 -e | xxd -r | od -tx1
0000000    ef  cd  ab  89  67  45  23  01
0000010

According to man xxd:

• -e

  Switch to little-endian hexdump. This option treats byte groups as words in little-endian byte order. The default grouping of 4 bytes may be changed using -g. This option only applies to hexdump, leaving the ASCII (or EBCDIC) representation unchanged. The command line switches -r, -p, -i do not work with this mode.

• -g bytes | -groupsize bytes

  Separate the output of every bytes bytes (two hex characters or eight bit-digits each) by a whitespace. Specify -g 0 to suppress grouping. Bytes defaults to 2 in normal mode, 4 in little-endian mode and 1 in bits mode. Grouping does not apply to postscript or include style.

xxd -r -p

Example:

sudo apt install xxd
printf 123456789ABCDEF0 | xxd -r -p | od -A n -t x1 -v

gives:

 12 34 56 78 9a bc de f0

od shows raw bytes as human readable bytes, which means that xxd -r -p did what we wanted and converted the human readable 123456789ABCDEF0 into corresponding raw bytes.

Tested on Ubuntu 19.04, xxd V1.10.

Should you want only the hex strings:

$ echo '0Y0*†HÎ*†HÎ=B¬`9E>ÞÕ?ÐŽ·‹ñ6ì­Â‰&ÉÐL_cüsyxoú¢'|od -vt x1|awk '{$1="";print}'
 30 59 30 2a e2 80 a0 48 c3 8e 2a e2 80 a0 48 c3
 8e 3d 42 c2 ac 60 39 45 3e c3 9e c3 95 3f c3 90
 c5 bd c2 b7 e2 80 b9 c3 b1 36 c3 ac c2 ad c3 82
 e2 80 b0 26 c3 89 c3 90 4c 5f 63 c3 bc 73 79 78
 6f c3 ba c2 a2 0a

You can avoid the awk part by just using od -vt x1 -A n. Thanks @Stefan van den Akker.

As @user786653 suggested, use the xxd(1) program:

xxd -r -p input.txt output.bin

I came here seeing three answers thinking that I'd have nothing to add, and that this would be an exercise in how many people can post the same 1-liner in the first minute of a question being asked. But I find people using some new-fangled hexdump tool. That command is way longer than 2 letters; it alludes to some base other than The One True Base (base 8); and it's even apparent from its name what it does. Clearly this is not the Unix way.

So here's the joy of od ("octal dump").

First GNU, as you will find on your Linux Mint:

od --format=x1 --read-bytes=10 foo

Now BSD, where the irony is that it's actually the same program as hexdump:

od -t x1 -N 10 foo

Use hexdump(1)

$ hexdump -x /usr/bin/hexdump 
0000000    feca    beba    0000    0300    0001    0700    0080    0300
0000010    0000    0010    0000    5080    0000    0c00    0000    0700
0000020    0000    0300    0000    00a0    0000    b06f    0000    0c00
0000030    0000    1200    0000    0a00    0100    0010    0000    107c
0000040    0000    0c00    0000    0000    0000    0000    0000    0000
0000050    0000    0000    0000    0000    0000    0000    0000    0000

...

The reason is because hexdump by default prints out 16-bit integers, not bytes. If your system has them, hd (or hexdump -C) or xxd will provide less surprising outputs - if not, od -t x1 is a POSIX-standard way to get byte-by-byte hex output. You can use od -t x1c to show both the byte hex values and the corresponding letters.

If you have xxd (which ships with vim), you can use xxd -r to convert back from hex (from the same format xxd produces). If you just have plain hex (just the '4161', which is produced by xxd -p) you can use xxd -r -p to convert back.

If you want to get the hex values of some string, then this works:

$ echo "testing some values"$'\157' | xxd
0000000: 7465 7374 696e 6720 736f 6d65 2076 616c  testing some val
0000010: 7565 736f 0a                             ueso.

If you just need the "plain" string:

$ echo "testing some values"$'\157' | xxd -p
74657374696e6720736f6d652076616c7565736f0a

If you need to "reverse" an hex string, you do:

$ echo "74657374696e6720736f6d652076616c7565736f0a" | xxd -r -p
testing some valueso

If what you need is the character representation (not hex), you could do:

$ echo "testing:"$'\001\011\n\bend test'  | od -vAn -tcx1
  t   e   s   t   i   n   g   : 001  \t  \n  \b   e   n   d    
 74  65  73  74  69  6e  67  3a  01  09  0a  08  65  6e  64  20
  t   e   s   t  \n
 74  65  73  74  0a

Or:

$ echo "testing:"$'\001\011\n\bend test'  | od -vAn -tax1
  t   e   s   t   i   n   g   : soh  ht  nl  bs   e   n   d  sp
 74  65  73  74  69  6e  67  3a  01  09  0a  08  65  6e  64  20
  t   e   s   t  nl
 74  65  73  74  0a