$ which xxd
/usr/bin/xxd

$ xxd --version
xxd V1.10 27oct98 by Juergen Weigert


user@comp ~
$ echo xyz>afile

user@comp ~
$ xxd -p afile
78797a0a


user@comp ~
$ (echo ef bb bf; xxd -p afile) | xxd -rp >a.a

user@comp ~
$ (echo ef bb bf; xxd -p file.csv) | xxd -r -p >a.b

user@comp ~
$ cat a.a
▒▒xyz

user@comp ~
$ cat a.b
xyz

user@comp ~
$ xxd -p a.a
000000000000000000000000000000000000000000000000000000000000
000000000000000000000000000000000000000000000000000000000000
000000000000000000000000000000000000000000000000000000000000
000000000000000000000000000000000000000000000000000000000000
000000000000000000000000000000000000000000000000000000000000
000000000000000000000000000000000000000000000000000000000000
000000000000000000000000000000000000000000000000000000000000
0000000000000000000000000000000000000000000000000000000000bb
bf78797a0a

user@comp ~
$ xxd -p a.b
efbbbf78797a0a


user@comp ~
$