[TAG] 2c tip: filtering in-place

[Ben Okopnik]

On Fri, Mar 19, 2004 at 01:44:22PM -0500, Jay R. Ashworth wrote:
> On Fri, Mar 19, 2004 at 10:39:34AM -0500, Ben Okopnik wrote:
> > On Fri, Mar 19, 2004 at 02:33:21PM +0530, Kapil Hari Paranjape wrote:
> > > I have always thought that filtering files "in-place" was
> > > not possible from the command line...
> > > ...until today---one lives and learns.
> > > 
> > >   dd if=file bs=4k | filter | dd of=file bs=4k conv=notrunc
> > > 
> > > Where "file" is the file you want to filter and "filter" 
> > > is the filtering program you want to apply.
> > 
> > The "buffer" program does exactly the same as the above; the process is
> > called "reblocking".
> > 
> > buffer < foo | filter > foo
> 
> <interrupts self while raising hand to complain>
> 
> Oh, cause buffer reads the entire file before the '>' can stomp it?
> 
> Well, that's not *exactly* the same... :-)
 
Hell, Jay, by that standard of nitpickiness, nothing ever is. I mean,
all those microseconds have passed, many electrons have travelled some
distance in their orbit, we've moved along the direction indicated by
the entropy arrow...

The meaning of "exactly" for us humans always means "close enough".

> Doesn't that still depend on order of evaluation by the shell?  Is that
> defined?

Well, *yeah* - just about as definitively as anything in Bash is.
Otherwise Kapil's method wouldn't work either. Neither would piping
anything through "sort". The left side of the pipe has to terminate
before the right side can do anything with the output; in many cases,
there _is_ no output until just before the left side terminates.

Any conversation that includes "yeah, but csh..." is, in the parlance of
an acquaintance, "right off the bugfuck side of the spectrum."


* Ben Okopnik * okopnik.freeshell.org * Technical Editor, Linux Gazette *
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