editing a binary file

[Anton Shterenlikht]

On Fri, Dec 18, 2009 at 04:38:16PM +0000, Nick Barnes wrote:
> At 2009-12-18 16:33:49+0000, Warren Block writes:
> > perryh at pluto.rain.com wrote:
> > > Greg Larkin <glarkin at freebsd.org> wrote:
> > > > ...
> > > > > truncate -4 myfile should get rid of the last four bytes.  Maybe
> > > > > there's a similar efficient way to truncate the start of a file.
> > > >
> > > > This should do it:
> > > >
> > > > dd if=oldfile of=newfile bs=1 skip=4
> > > 
> > > Or, perhaps marginally more efficient:
> > > 
> > > dd if=oldfile of=newfile bs=4 skip=1
> > 
> > It would be nice to avoid the file copy, but maybe there's no way to do 
> > that.  The small buffer size for dd will probably make copies of 
> > multi-gig files slow.  This might be faster:
> > 
> > tail -c +5 myfile > outfile
> > truncate -4 outfile
> > 
> > (Has anyone mentioned that you can edit binary files interactively with 
> > vi yet?  No?  Well, it's horrific and surely has interesting failure 
> > modes.  And there are probably disadvantages also.)
> 
> All very interesting, but the OP is wanting to lose all the Fortran
> record markers, not just the first (and last) four bytes of the file.
> The record markers precede and follow each record, and give the
> record's length.  The size and enddian-ness of the record marker
> itself depends on the Fortran implementation.

actually the file consists of just one (potentially very long)
record, so erasing the first and the last 4 bytes is all I need.

I haven't had a chance to look at the proposed solutions, but
many thanks for all your advice.

-- 
Anton Shterenlikht
Room 2.6, Queen's Building
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