Shell Programming and Scripting

printf '%s\n' f0 f1 f2 | 
  awk '{ 
    printf "%c", mystrtonum("0x" $0) 
    }
func mystrtonum(str,        ret, chars, n, i, k, c)
     {
         if (str ~ /^0[0-7]*$/) {
             # octal
             n = length(str)
             ret = 0
             for (i = 1; i <= n; i++) {
                 c = substr(str, i, 1)
                 if ((k = index("01234567", c)) > 0)
                     k-- # adjust for 1-basing in awk
     
                 ret = ret * 8 + k
             }
         } else if (str ~ /^0[xX][0-9a-fA-f]+/) {
             # hexadecimal
             str = substr(str, 3)    # lop off leading 0x
             n = length(str)
             ret = 0
             for (i = 1; i <= n; i++) {
                 c = substr(str, i, 1)
                 c = tolower(c)
                 if ((k = index("0123456789", c)) > 0)
                     k-- # adjust for 1-basing in awk
                 else if ((k = index("abcdef", c)) > 0)
                     k += 9
     
                 ret = ret * 16 + k
             }
         } else if (str ~ /^[-+]?([0-9]+([.][0-9]*([Ee][0-9]+)?)?|([.][0-9]+([Ee][-+]?[0-9]+)?))$/) {
             # decimal number, possibly floating point
             ret = str + 0
         } else
             ret = "NOT-A-NUMBER"
     
         return ret
     }
' |
  dd conv=ascii 2> /dev/null ; echo

$ echo -e "f0\nf1\nf2" | while read val do printf "%c" $(echo -e \\x$val); done | dd conv=ascii 2> /dev/null ; echo
012