Date: Fri, 18 Dec 2009 16:54:51 +0000 From: Anton Shterenlikht <mexas@bristol.ac.uk> To: Nick Barnes <Nick.Barnes@pobox.com> Cc: glarkin@freebsd.org, mexas@bristol.ac.uk, perryh@pluto.rain.com, freebsd-questions@freebsd.org Subject: Re: editing a binary file Message-ID: <20091218165451.GA89667@mech-cluster241.men.bris.ac.uk> In-Reply-To: <12053.1261154296@thrush.ravenbrook.com> References: <alpine.BSF.2.00.0912180904180.11293@lightning.wonkity.com> <wblock@wonkity.com> <12053.1261154296@thrush.ravenbrook.com>
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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@pluto.rain.com wrote: > > > Greg Larkin <glarkin@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 Mech Eng Dept Bristol University University Walk, Bristol BS8 1TR, UK Tel: +44 (0)117 331 5944 Fax: +44 (0)117 929 4423
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