Как работает xxd.
По началу есть файлик test_file.txt , его содержимое:

01 00 00 00 02 00 00 00
03 00 00 00 00 00 00 00
02 00 00 00 01 00 00 00



qua@quapc:~/Desktop$ cat test_file.txt
01 00 00 00 02 00 00 00
03 00 00 00 00 00 00 00
02 00 00 00 01 00 00 00
qua@quapc:~/Desktop$ xxd -r -p test_file.txt test_file_a.bin
qua@quapc:~/Desktop$ cat test_file_a.bin



qua@quapc:~/Desktop$ xxd -p test_file_a.bin test_file_b.txt
qua@quapc:~/Desktop$ cat test_file_b.txt
010000000200000003000000000000000200000001000000
qua@quapc:~/Desktop$ xxd -r -p test_file_b.txt test_file_c.bin
qua@quapc:~/Desktop$ cat test_file_c.bin



qua@quapc:~/Deskxxd -p test_file_c.bin test_file_d.txt
qua@quapc:~/Desktop$ cat test_file_d.txt
010000000200000003000000000000000200000001000000