sed question...

[Gary Kline]

On Tue, Sep 25, 2007 at 09:31:04AM +0300, Nikos Vassiliadis wrote:
> On Tuesday 25 September 2007 06:07, Howard Goldstein wrote:
> > Gary Kline wrote:
> > > 	My earlier post about deleting the first N lines was answered by
> > > 	this one-liner site {below}.   I wasn't including any
> > > 	redirection; doing so finally resolved the problem.  Now I need
> > > 	to delete every line from the 19th or so to the last line.
> 
> sed -e 18q
> that is, quit after processing line 18.


	This quits after line 18, as you say.  Given a file of 100 lines,
	I was everything from line 81,100d.  Which is what "#method 1"
	does.  But trying to parse this from man sed is more than
	difficule.  And I have yet to find "ba" in the man page.  That is
	why I asked for some insights rather that to be told to "go read
	the man page"; to me, that's dismissing the issue rather than
	addressing it.

> 
> > > 	Question one, can anybody explain the following syntax?  What do
> > > 	"P", "D" "ba" represent, in other words?
> 
> The manual page explains sed in a very good way. For sure, better
> than I could describe it here. You'd better read it.
> 
> > >
> > >
> > >  # delete the last 10 lines of a file
> > >  sed -e :a -e '$d;N;2,10ba' -e 'P;D'   # method 1
> > >  sed -n -e :a -e '1,10!{P;N;D;};N;ba'  # method 2
> > >
> > >
> > > 	Question two, can sed do its thing inline?
> 
> Yes.
>  -i extension
>      Edit files in-place, saving backups with the specified extension.
>      If a zero-length extension is given, no backup will be saved.  It
>      is not recommended to give a zero-length extension when in-place
>      editing files, as you risk corruption or partial content in situ-
>      ations where disk space is exhausted, etc.
> 

	Right.  I always do a perl -pi.bak [...] mostly out of habit.
	With sed, redirection saved the new output, leaving the original
	in ``.''  FWIW, I was using the sed on my Ubuntu server.  It is 
	different from the BSD sed that I've used now/then since 1978.
	The linux sed man page is just slightly more readable that the
	BSD.  Probably newer.

	gary


> >
> > Wouldn't it be easier to use  head -n 18 ?
> 
> No, it's the same. Some sed operation are trivial to read/write,
> others aren't.
> 
> HTH
> 
> Nikos

-- 
  Gary Kline  kline at thought.org   www.thought.org  Public Service Unix
      http://jottings.thought.org   http://transfinite.thought.org