This worked for me.

$ echo -n 54657374696e672031203220330 | xxd -r -p
Testing 1 2 3$

echo -n prevents a newline from being added to the end

-r tells it to convert hex to ascii as opposed to its normal mode of doing the opposite

-p tells it to use a plain format.

I used to do this with xxd:

echo -n 5a | xxd -r -p

But then I realised that in Debian/Ubuntu, xxd is part of vim-common and hence might not be present in a minimal system. To also avoid Perl (IMHO also not part of a minimal system), I ended up using sed, xargs, and printf like this:

echo -n 5a | sed 's/\([0-9A-F]\{2\}\)/\\\\\\x\1/gI' | xargs printf

Mostly, I only want to convert a few bytes and it's okay for such tasks. The advantage of this solution over the one of ghostdog74 is, that this can convert hex strings of arbitrary lengths automatically. xargs is used because printf doesnt read from standard input.

Your input file looks like it was produced using something like this:

hexdump -e '"%08_ad  "  8/1 "%04x "' -e '"" 0/0 "" "\n"' original_file

Unfortunately, xxd -r can't deal with decimal offsets.

Here is a short Gnu AWK program to give you the output you're looking for:

gawk '{printf "%s  |", $0; for (f=2; f<=9; f++) { c = strtonum("0x" $f); if (c >= 32 && c <= 126) printf "%c",c; else printf "."}; printf "|\n"}' input_file

If you're using an AWK other than gawk, you can use the strtonum() function here.

Here's another way to do the same thing as the gawk script above:

cut -c 11- input_file | sed 's/\<00//g' | xxd -r -p | hexdump -e '"%08_ad  "  8/1 "%04x " ""' -e '"  |" 8/1 "%_p" "|\n"'

If, instead, you want to convert your input file to text:

cut -c 11- input_file | xxd -r -p

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.

There's a printf tool that simulates the C function; normally it's at /usr/bin/printf, but a lot of shells implement built-ins for it as well. You can use %02x to get the hex representation of a character, but you need to make sure you pass a string that includes the character in single-quotes (Edit: It turns out just a single-quote at the beginning is sufficient):

printf "%%%02x\n" "'-"   # Outputs %2d

You can make a shell function for convenience:

function hex() {
    printf "%%%02x\n" "'$1"
}

hex -   # Outputs %2d
hex _   # Outputs %5f
hex .   # Outputs %2e

Assuming exactly eight hexadecimals per line, here is a pure bash solution:

#!/bin/bash

while read a
do
   # echo $a
   printf "\x${a:0:2}\x${a:2:2}\x${a:4:2}\x${a:6:2}\n"
done < infile

If infile contains:

215f6674
41424344
61626364

the output from this script is:

!_ft
ABCD
abcd