Linux – Convert a list of decimal values in a text file into hex format

You can do this using bsdtar:

ire@localhost: bsdtar -cvf pax.tar --format=pax @gnu.tar
ire@localhost:file gnu.tar
gnu.tar: POSIX tar archive (GNU)
ire@localhost:file pax.tar
pax.tar: POSIX tar archive

@archive is the magic option. From the manpage:

@archive
     (c and r mode only) The specified archive is opened and the
     entries in it will be appended to the current archive.  As a sim-
     ple example,
       tar -c -f - newfile @original.tar
     writes a new archive to standard output containing a file newfile
     and all of the entries from original.tar.  In contrast,
       tar -c -f - newfile original.tar
     creates a new archive with only two entries.  Similarly,
       tar -czf - --format pax @-
     reads an archive from standard input (whose format will be deter-
     mined automatically) and converts it into a gzip-compressed pax-
     format archive on stdout.  In this way, tar can be used to con-
     vert archives from one format to another.

This is related to the generalised strnum handling in version 4.2 of GAWK.

Input values which look like numbers are treated as strnum values, represented internally as having both string and number types. “0123” qualifies as looking like a number, so it is handled as a strnum. strtonum is designed to handle both string and number inputs; it looks for numbers first, and when it encounters an input number, returns the number without transformation:

NODE *
do_strtonum(int nargs)
{
        NODE *tmp;
        AWKNUM d;

        tmp = fixtype(POP_SCALAR());
        if ((tmp->flags & NUMBER) != 0)
                d = (AWKNUM) tmp->numbr;
        else if (get_numbase(tmp->stptr, tmp->stlen, use_lc_numeric) != 10)
                d = nondec2awknum(tmp->stptr, tmp->stlen, NULL);
        else
                d = (AWKNUM) force_number(tmp)->numbr;

        DEREF(tmp);
        return make_number((AWKNUM) d);
}

Thus “0123” becomes the number 123, and strtonum returns that directly.

“0x123” doesn’t look like a number (by the rules defined in the link given above), so it is handled as a string and processed as you’d expect by strtonum.

A number is defined as follows in AWK:

The input string is decomposed into two parts: an initial, possibly empty, sequence of white-space characters (as specified by isspace()) and a subject sequence interpreted as a floating-point constant.

The expected form of the subject sequence is an optional '+' or '-' sign, then a non-empty sequence of digits optionally containing a <period>, then an optional exponent part. An exponent part consists of 'e' or 'E', followed by an optional sign, followed by one or more decimal digits.

The sequence starting with the first digit or the <period> (whichever occurs first) is interpreted as a floating constant of the C language, and if neither an exponent part nor a <period> appears, a is assumed to follow the last digit in the string. If the subject sequence begins with a <hyphen-minus>, the value resulting from the conversion is negated.