Shell Programming and Scripting

Some limits of classic awk (maybe you need this):
100 fields
3000 chars per input record
3000 chars per output record
1024 chars per field
3000 chars per printf string
400 chars max literal string
15 open files

Also browsing 'comp.lang.awk' [on Google] can yield some results as well.
comp.lang.awk

If you are ONLY interested in the first 80 chars on the FIRST line, you can easily use either the sed OR the nawk solution(s) posted.

For the efficiency you might want to modify either one of them to quit AFTER the first line is processed.

The only simple generic solution to this entire problem (for files that exceed the limits posed by awk, for example) is to fire up an HEX editor and make the change manually. You could write a C program to do it, or use Perl/PHP (note this is UNTESTED Perl code - just found from Googling around...)

Code:

#!/usr/bin/perl
# replace byte in binary file
($file, $offset, $byte) = @ARGV;
$byte || die "Usage: $0 FILE OFFSET BYTE\n";
$byte = pack('C', eval("$byte"));
$offset = eval("$offset");
@file = split('', `cat $file`);
die "No such offset $offset in $file\n" if !defined($file[$offset]);
$file[$offset] = $byte;
open(FILE, ">$file") || die "Cannot overwrite $file: $!\n";
print FILE join('', @file);
close(FILE);

Even the sed solution will also be operating on a character set not a byte set, as will the original nawk solution you posted.

I've tried the sed 's/./A/24' myfile on a very old version of sed (shipped with UNICOS 9.0) and it seems to work okay, as it does with newer seds too.

Cheers
ZB